Texas Instruments bq25570 User manual

User's Guide
SLUUAA7A–July 2013–Revised August 2014
User's Guide for bq25570 Battery Charger Evaluation
Module for Energy Harvesting
This user’s guide describes the bq25570 evaluation module (EVM), how to perform a stand-alone
evaluation and how to allow the EVM to interface with the system and host. The boost charger output is
configured to deliver up to 4.2-V maximum voltage to its output, VSTOR, using external resistors. This
voltage will be applied to the storage element as long as the storage element voltage at VBAT is above
the internally programmed undervoltage of 2.0 V. The integrated buck converter provides up to 1.8 V and
100 mA at VOUT. The VBAT_OK indicator toggles high when VSTOR ramps up to 3.0 V and toggles low
when VSTOR ramps down to 2.8 V.
Contents
1 Introduction ................................................................................................................... 2
1.1 EVM Features....................................................................................................... 2
1.2 General Description ................................................................................................ 2
1.3 Design and Evaluation Considerations .......................................................................... 3
1.4 EVM Schematic..................................................................................................... 4
1.5 EVM I/O Connections .............................................................................................. 5
2 EVM Performance Specification Summary............................................................................... 7
3 Test and Measurment Summary........................................................................................... 7
3.1 Test Setups and Results........................................................................................... 8
4 Bill of Materials and Board Layout ....................................................................................... 17
4.1 Bill of Materials .................................................................................................... 17
4.2 EVM Board Layout................................................................................................ 18
5 PCB Layout Guideline ..................................................................................................... 20
List of Figures
1 EVM Schematic.............................................................................................................. 4
2 Test Setup for Measuring Boost Charger Efficiency .................................................................... 9
3 Charger Efficiency versus Input Voltage.................................................................................. 9
4 Charger Efficiency versus Input Current ................................................................................ 10
5 Test Setup for Measuring Buck Converter Efficiency ................................................................. 11
6 Buck Converter Efficiency versus Output Current .................................................................... 11
7 Test Setup for Performing Load Transient on Buck Output .......................................................... 12
8 50-mA Load Transient on VOUT ........................................................................................... 12
9 Charger Operational Waveforms During 50-mA Load Transient .................................................... 13
10 Buck Operational Waveforms During 50-mA Load Transient ........................................................ 14
11 Test Setup for Charging a Super Capacitor from Buck Output ...................................................... 15
12 Charging a Super Cap from VOUT ........................................................................................ 15
13 EVM PCB Top Assembly.................................................................................................. 18
14 EVM PCB Top Layer ...................................................................................................... 18
15 EVM PCB Bottom Layer................................................................................................... 19
List of Tables
1 I/O Connections and Configuration for Evaluation of bq25570 EVM ................................................. 5
2 Bill of Materials ............................................................................................................. 17
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Introduction www.ti.com
1 Introduction
1.1 EVM Features
• Evaluation module for bq25570
• Ultra-low power boost charger and buck converter with battery management for energy harvester
applications
• Resistor-programmable settings for over voltage providing flexible battery management
• Programmable push-pull output indicator for battery status (VBAT_OK)
• Test points for key signals available for testing purpose – easy probe hook-up.
• Jumpers available – easy to change settings
1.2 General Description
The bq25570 is an integrated energy harvesting Nano-Power management solution that is well suited for
meeting the special needs of ultra-low power applications. The product is specifically designed to
efficiently acquire and manage the microwatts (µW) to miliwatts (mW) of power generated from a variety of
high output impedance (HiZ) DC sources like photovoltaic (solar) or thermal electric generators; or with an
AC/DC rectifier, a piezoelectric generator. The bq25570 implements a highly efficient, pulse-frequency
modulated (PFM) boost converter/charger targeted toward products and systems, such as wireless sensor
networks (WSN) which have stringent power and operational demands. Assuming a depleted storage
element has been attached, the bq25570 DC-DC boost converter/charger that requires only microwatts of
power to begin operating in cold start mode. Once the boost converter output, VSTOR, reaches ~1.8 V
and can now power the converter, the main boost converter can now more efficiently extract power from
low voltage output harvesters such as thermoelectric generators (TEGs) or single and dual cell solar
panels. For example, assuming the HiZ input source can provide at least 5 µW typical and the load on
VSTOR (including the storage element leakage current) is less than 1 µA of leakage current, the boost
converter can be started with VIN_DC as low as 330 mV typical, and once VSTOR reaches 1.8 V, can
continue to harvest energy down to VIN_DC ≃120 mV. The integrated PFM buck converter is also
powered from VSTOR and, assuming enough input power is available, provides up to 100 mA from the
VOUT pin. The VOUT voltage is externally programmed to slightly less than the VSTOR voltage.
HiZ DC sources have a maximum output power point (MPP) that varies with ambient conditions. For
example, a solar panel's MPP varies with the amount of light on the panel and with temperature. The MPP
is listed by the harvesting source manufacturer as a percentage of its open circuit (OC) voltage. Therefore,
the bq25570 implements a programmable maximum power point tracking (MPPT) sampling network to
optimize the transfer of power into the device. The bq25570 periodically samples the open circuit input
voltage every 16 seconds by disabling the boost converter for 256 ms and stores the programmed MPP
ratio of the OC voltage on the external reference capacitor (C2) at VREF_SAMP. Typically solar cells are
at their MPP when loaded to ~70–80% of their OC voltage and TEGs at ~50%. While the storage element
is less than the user programmed maximum voltage (VBAT_OV), the boost charger loads the harvesting
source until VIN_DC reaches the MPP (voltage at VREF_SAMP). This results in the boost charger
regulating the input voltage of the converter until the output reaches VBAT_OV, thus transferring the
maximum amount of power currently available per ambient conditions to the output.
The battery undervoltage, VBAT_UV, threshold is checked continuously to ensure that the internal battery
FET, connecting VSTOR to VBAT, does not turn on until VSTOR is above the VBAT_UV threshold (2.0
V).The over voltage (VBAT_OV) setting initially is lower than the programmed value at startup (varies on
conditions) and is updated after the first ~32 ms. Subsequent updates are every ~64 ms. The VBAT_OV
threshold sets maximum voltage on VSTOR and the boost converter stops switching when the voltage on
VSTOR reaches the VBAT_OV threshold. The open circuit input voltage (VIN_OC) is measured every ~16
seconds in order for the Maximum Power Point Tracking (MPPT) circuit to sample and hold the input
regulation voltage. This periodic update continually optimizes maximum power delivery based on the
harvesting conditions.
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www.ti.com Introduction
The bq25570 was designed with the flexibility to support a variety of energy storage elements. The
availability of the sources from which harvesters extract their energy can often be sporadic or time-
varying. Systems will typically need some type of energy storage element, such as a re-chargeable
battery, super capacitor, or conventional capacitor. The storage element will make certain constant power
is available when needed for the systems. In general, the storage element also allows the system to
handle any peak currents that can not directly come from the input source. It is important to remember
that batteries and super capacitors can have significant leakage currents that need to be included with
determining the loading on VSTOR.
To prevent damage to a customer’s storage element, both maximum and minimum voltages are monitored
against the internally programmed under-voltage (VBAT_UV) and user programmed over-voltage
(VBAT_OV) levels.
To further assist users in the strict management of their energy budgets, the bq25570 toggles a user
programmable battery good flag (VBAT_OK), checked every 64 ms, to signal the microprocessor when
the voltage on an energy storage element or capacitor has risen above (OK_HYST threshold) or dropped
below (OK_PROG threshold) a pre-set critical level. To prevent the system from entering an undervoltage
condition or if starting up into a depleted storage element, it is highly recommended to isolate the system
load from VSTOR by 1) setting VBAT_OK equal to the buck converter's enable signal VOUT_EN and 2)
using an NFET to invert the BAT_OK signal so that it drives the gate of PFET, which isolates the system
load from VSTOR.
For details, see the bq25570 data sheet (SLUSBH2).
1.3 Design and Evaluation Considerations
This user's guide is not a replacement for the data sheet. Reading the data sheet first will help in
understanding the operations and features of this IC. In this document, “battery” or "VBAT" will be used
but one could substitute any appropriate storage element.
System Design Tips
Compared to designing systems powered from an AC/DC converter or large battery (for example, low
impedance sources), designing systems powered by HiZ sources requires that the system load-per-unit
time (for example, per day for solar panel) be compared to the expected loading per the same time unit.
Often there is not enough real time input harvested power (for example, at night for a solar panel) to run
the system in full operation. Therefore, the energy harvesting circuit collects more energy than being
drawn by the system when ambient conditions allow and stores that energy in a storage element for later
use to power the system. See SLUC461 for an example spreadsheet on how to design a real solar-panel-
powered system in three easy steps:
1. Referring the system rail power back to VSTOR
2. Referring the required VSTOR power back to bq255xx input power
3. Computing the minimum solar panel area from the input power requirement
As demonstrated in the spreadsheet, for any boost converter, you must perform a power balance, POUT /
PIN = (VSTOR × ISTOR) / (VIN × IIN)= ηwhere ηis the estimated efficiency for the same or very similar
configuration in order to determine the minimum input power needed to supply the desired output power.
This IC is a highly efficient charger for a storage element such as a battery or super capacitor. The main
difference between a battery and a super capacitor is the capacity curve. The battery typically has little or
no capacity below a certain voltage, where as the capacitor does have capacity at lower voltages. Both
can have significant leakage currents that will appear as a DC load on VSTOR/VBAT.
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VIN
VIN
GND
VREF_SAMP
VSTOR
GND
VSTOR
VBAT
GND
VBAT
GND
BAT_OK
GND
VOUT
GND
GND
VBAT
/EN
GND
VSTOR
VOUT_EN
JP1 to R3-R5
VOC_SAMP
50% 80%
VRDIV
VSTOR
GND
GND
1
VOUT
GND
GND
LBOOST
LBUCK
BAT_OK
VOUT_EN
1
Not Installed
1
4.2V (Adj. up to 5.25V), 100mA
4.2V (Adj. up to 5.25V)
0.1V-4.0V
1.8V (Adj. 1.3V-5.05V), 50mA
1
C1
4.7uF
TP1
J2
C7
0.1uF
JP5
C2
0.01uF
R1
7.5M
R2
5.76M
TP2
C5
4.7uF J5
C4
0.1uF
J4
J8
C6
100u
TP4
TP5
R3
4.99M
R4
10M
JP4
JP1
J13
R10
8.66M
R8
5.36M
R7
6.98M
R6
887k
R9
4.22M
TP7
C3
22uF
J11
JP2
JP3
L2
10 uH
L1
22uH
J6
J7
J9
J10
J12
R5
4.99M
J1
J3 +C9
+C8
+C10
1VSS
2VIN_DC
3VOC_SAMP
4VREF_SAMP
5EN
6VOUT_EN 7VBAT_OV
8VRDIV 9NC
10 OK_HYST
11
OK_PROG
12
VOUT_SET
13
VBAT_OK
14
VOUT
15
VSS
16
LBUCK
17
NC
18
VBAT
19
VSTOR
20
LBOOST
21
PWPD
U1 BQ25570RGR
BQ25570RGR
TP3
TP6
TP8
TP9
JP6
VIN1
VIN1
VSTOR
VOC_SAMP
VBAT
VSTOR
VOC_SAMP VBAT
VRDIV
VRDIV
Introduction www.ti.com
1.4 EVM Schematic
Figure 1 is the schematic for this EVM.
Figure 1. EVM Schematic
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www.ti.com Introduction
1.5 EVM I/O Connections
Table 1. I/O Connections and Configuration for Evaluation of bq25570 EVM
Headers and Description Comments/Recommended Setting
Terminals
J1–VIN Input source (+) If VIN_DC is higher than VSTOR and VSTOR is equal to VBAT_OV, the input VIN_DC is pulled to ground
through a small resistance to stop further charging of the attached battery or capacitor. It is critical that if
J2 - VIN/GND Input source terminal block this case is expected, the impedance of the source attached to VIN_DC be higher than 20 Ωand not a
J3–GND Input source return (–) low impedance source.
J4 - VSTOR Boost charger output (+) Buck converter input
J5 - VSTOR/GND Boost charger output terminal block
J6 - GND Boost charger return (-)
J7– VBAT Rechargeable storage element connection (+)
J8 - VBAT/GND Rechargeable storage element terminal block
J9–GND Rechargeable storage element connection return (–)
J10 – VOUT Buck converter output (+)
J11 - VOUT/GND Buck converter output terminal block
J12 -GND Buck converter output (-)
J13 – BAT_OK Battery Status Indicator (+/-)
Test Points
TP1 Input source (+)
TP2 Boost charger switching node
TP3 Buck converter switching node
TP4 Boost charger output, VSTOR (+)
TP5 Rechargeable storage element connection, BAT_SEC (+)
TP6 Buck converter output, VOUT (+)
TP7 VRDIV node CAUTION Providing an additional low impedance current path in parallel with the feedback resistors , for
example, with a 10 MΩscope probe attached, will degrade regulation accuracy.
TP8 Output return (-)
TP9 Input return (-)
Jumpers
JP1 – VOC_SAMP VOC_SAMP = external resistors sized to configure the IC Uninstalled (NOTE: Do not install if JP4 shunt is installed)
to regulate VIN to 75% of VOC_SAMP.
JP2 - /EN /EN = GND enables the IC. /EN=VSTOR disables the IC. /EN=GND
JP3 - VOUT_EN VOUT_EN = VSTOR enables the buck converter when VOUT_EN=VSTOR (NOTE: Do not install if JP6 shunt is installed)
VSTOR is up VOUT_EN = GND disables the buck
converter
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Introduction www.ti.com
Table 1. I/O Connections and Configuration for Evaluation of bq25570 EVM (continued)
Headers and Description Comments/Recommended Setting
Terminals
JP4 - VOC_SAMP VOC_SAMP = 80% configures the IC to regulate VIN to JP4 = 80% (NOTE: Do not install if JP1 shunt is installed)
80% of OCV. VOC_SAMP = 50% configures the IC to
regulate VIN to 50% of OCV.
JP5 - VREF_SAMP to VREF_SAMP = GND Uninstalled (NOTE: Providing an additional leakage path for the VREF_SAMP capacitor for example,
GND through a 10 MΩscope probe attached to VREF_SAMP, will degrade input voltage regulation
performance).
JP6 - VBAT_OK to BAT_OK=VOUT_EN configures the buck converter to be Uninstalled (NOTE: Do not install if JP3 shunt is installed)
VOUT_EN enabled only when VSTOR is greater than the VBAT_OK
threshold per the resistors (2.786V on the EVM)
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www.ti.com EVM Performance Specification Summary
2 EVM Performance Specification Summary
See Data Sheet “Recommended Operating Conditions” for component adjustments. For details about the
resistor programmable settings, see bq25570 data sheet (SLUSBH2).
MIN NOM MAX UNIT
VIN(DC) DC input voltage into VIN_DC 0.13 4.0 V
VIN_Start-up(DC) DC minimum start-up voltage into depleted storage element, no load attached 330 mV
to VSTOR or VOUT and IBATLEAK <=1uA
VBAT_OV Battery Over Voltage Threshold –min and max values include +/- 2% set point 4.04 4.18 4.32 V
accuracy and +/-1% resistor tolerance but excludes effects of output ripple
VOUT Buck Converter Output Voltage for IVOUT < 95 mA - min and max values 1.75 1.8 1.85 V
include +/- 2% set point regulation accuracy and +/-1% resistor tolerance but
excludes effects of output voltage ripple, line regulation and load regulation
OK_HYST indication toggles high when VSTOR ramps up - min and max 2.70 2.79 2.88 V
values include +/- 2% set point accuracy and +/-1% resistor tolerance
VBAT_OK OK_PROG indication toggles low when VSTOR ramps down - min and max 2.89 2.99 3.09 V
values include +/- 2% set point accuracy and +/-1% resistor tolerance
MPPT Maximum Power Point Tracking, Resistor Programmed % of Open Circuit 80%
Voltage
CBAT A 100 µF low leakage ceramic capacitor is installed on the EVM as the 100 µF
minimum recommended equivalent battery capacitance.
See SLUC484 spreadsheet tool to assist with modifying the MPPT, VBAT_OV, VBAT_OK and VOUT
resistors for your application.
CAUTION
If changing the board resistors or the capacitor on VREF_SAMP (C2), it is
important to remember that residual solder flux on a board has a resistivity in
the 1-20 MΩrange. Therefore, flux remaining in parallel with changed 1-20 MΩ
resistors can result in a lower effective resistances, which will produce different
operating thresholds than expected. Similarly flux remaining in parallel with the
VREF_SAMP capacitor provides an additional leakage path, which results in
the input voltage regulation set point drooping during the 16-s MPPT cycle.
Therefore, it is highly recommended that boards be throughly cleaned twice,
once after removing the old components and again after installing the new
components. If possible, the boards should be cleaned until the wash solution
measures ionic contamination greater than 50 MΩ.
3 Test and Measurment Summary
Test Setup Tips
Energy harvesting power sources are high impedance sources. A source-meter configured as a current
source with voltage compliance set to the harvester's open circuit voltage is the best way to simulate the
harvester. When simulating a HiZ energy harvester with low output impedance lab power supply, it is
necessary to simulate the harvester's impedance with a physical resistor between the supply, VPS, and VIN
of the EVM. When the MPPT sampling circuit is active, VIN = VPS = the harvester open circuit voltage
(VOC) because there is no input current to create a drop across the simulated impedance (that is, open
circuit); therefore, VPS should be set to the intended harvester's open circuit voltage. When the boost
converter is running, it draws only enough current until the voltage at VIN_DC droops to the MPPT's
sampled voltage that is stored at VREF_SAMP.
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R1
D1
C A
BAT+
GND
Test and Measurment Summary www.ti.com
The battery (storage element) can be replaced with a simulated battery. Often electronic 4 quadrant loads
give erratic results with a “battery charger” due to the charger changing states (fast-charge to termination
and refresh) while the electronic load is changing loads to maintain the “battery” voltage. The charging and
loading get out of phase and create a large signal oscillation which is due to the 4 quadrant meter. A
simple circuit can be used to simulate a battery and works well and can quickly be adjusted for voltage. It
consists of load resistor (~10 Ω, 2 W) to pull the output down to some minimum storage voltage (sinking
current part of battery) and a lab supply connected to the BAT pin via a diode. The lab supply biases up
the battery voltage to the desired level. It may be necessary to add more capacitance across R1.
3.1 Test Setups and Results
3.1.1 Boost Charger Efficiency
The test setup is shown in Figure 2. The specific equipment used for the test results in Figure 3 and
Figure 4 is listed below:
1. VIN_DC was connected to a Keithley 2420 source-meter configured as a current source with voltage
compliance (clamp) set to the open circuit voltage.
2. VSTOR was connected a Keithley 2420 source-meter configured as a voltage source set to the
VSTOR voltage. The current sunk by the source-meter was the output current of the charger
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0
10
20
30
40
50
60
70
80
90
100
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
Efficiency (%)
Input Voltage (V)
VSTOR = 2.0 V
VSTOR = 3.0 V
VSTOR = 5.5 V
IIN = 100 PA
Source/Sink
Meter
Configured
As Current
Source
SM1
SM1
Source (Sink)
Meter
Configured
As Voltage
Source
+
-
SM2
SM2
www.ti.com Test and Measurment Summary
Figure 2. Test Setup for Measuring Boost Charger Efficiency
Figure 3. Charger Efficiency versus Input Voltage
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20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100
Efficiency (%)
Input Current (mA)
VSTOR = 1.8 V
VSTOR = 3.0 V
VSTOR = 5.5 V
VIN = 0.5 V
Test and Measurment Summary www.ti.com
Figure 4. Charger Efficiency versus Input Current
Because the boost converter regulates input voltage instead of output voltage, uses PFM switching,
operates at very low currents and has MPPT, efficiency cannot be measured using the same test setup as
for an output regulating, higher power, fixed frequency PWM switching boost converter. The VSTOR
output must be held at a fixed voltage (below VBAT_OV threshold) by an external source that is capable
of sinking current, with that sunk current being the measured output current. In addition to filtering bursts
of current due to PFM switching and the ripple voltage voltage on VIN_DC due to input voltage regulation,
the series input current meter and input voltage meter must be set to filtering, or averaging, or both, which
will result in longer than usual measurement times, but not longer than the 16 s MPPT sample time.
Measurements for both VIN and IN will be most accurate when taken at the midpoint of the 16 s MPPT
period. Remote sensing by the source-meters is possible but, on the input side, the source-meter output
regulation loop and the charger MPPT input regulation loop may interfere with each other and cause the
input voltage to oscillate. Adding a large capacitor across VIN_DC and GND will eliminate this oscillation
but the capacitor's leakage current will inflate the input current measurement and result in lower efficiency.
See SLUA691 for a detailed explanation on how to take these and other measurements with source-
meters.
3.1.2 Buck Converter Efficiency
The test setup is shown in Figure 5. The specific equipment used for the test results in Figure 6 is listed
below:
1. VSTOR was connected to a low impedance power supply with a series current meter to measure
current. The current meter must be able to measure currents in the nA range and may require manual
range adjustments so that the range is always ≤10 X the expected current for best accuracy. The
voltage meter measures the input voltage as close to the IC VSTOR pin as possible.
2. VOUT was connected to a resistor box with a series current meter to measure the current. (NOTE: The
current meter must be able to measure currents in the nA range and may require manual range
adjustments so that the range is always ≤10 X the expected current for best accuracy.) The voltage
meter measured the VOUT voltage as close to the IC VOUT pin as possible
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40
50
60
70
80
90
100
0.001 0.01 0.1 1 10 100
Efficiency (%)
Output Current (mA)
VSTOR = 2.1V
VSTOR = 3.6V
VSTOR = 5.5V
VOUT = 1.8V, TA= 25oC
+ VM1 -
-
VM2
+
+ CM2 -
+ PS1 -
+
CM1
-
www.ti.com Test and Measurment Summary
Figure 5. Test Setup for Measuring Buck Converter Efficiency
Figure 6. Buck Converter Efficiency versus Output Current
The buck converter is powered from VSTOR; therefore, to measure its efficiency alone, VIN_DC should
be left floating, and the input power supply applied to VSTOR. To filter bursts of current due to PFM
switching, the series input current meter from the VSTOR supply must be set to the highest level of
filtering and/or averaging, which will result in longer than usual measurement times.
Alternatively, these measurements can be taken with source-meters instead of discrete power supply,
resistor load box and meters. The source-meter on VSTOR is configured as a voltage source. The source-
meter on OUT can be configured as either a current source that sinks current (i.e., negative current) or as
a voltage source with voltage at least 100 mV below the lowest expected regulation voltage point.
11
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+
2.0V
-
100 :
100 :
4.2V coin cell
36 :
Test and Measurment Summary www.ti.com
3.1.3 Buck Converter Load Transient
The test setup is shown in Figure 7. The specific equipment used for the test results in Figure 8 is listed
below:
1. VIN_DC was connected to a low impedance power supply by a series 100-Ωresistor. JP4 sets the
MPPT voltage to 50% of VIN_OC.
2. VOUT was connected to a switch with a series resistor that switches in a 36 Ωresistor.
3. VBAT was connected a 3.2-V charged 4.2-V coin cell.
4. VSTOR, VOUT and VIN_DC was monitored by oscilloscope voltage scope probes attached to TP4,
TP6, and TP1 respectively, and GND. IOUT was measured with a current probe.
Figure 7. Test Setup for Performing Load Transient on Buck Output
Figure 8. 50-mA Load Transient on VOUT
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3.1.4 Charger Operation During Load Transient
The test setup is shown in Figure 7. The specific equipment used for the test results in Figure 9 is listed
below:
1. VIN_DC, VBAT and VOUT are configured as explained in Section 3.1.3.
2. The boost charger inductor current (IL) was measured by using an oscilloscope current probe across a
current loop that was inserted in series with inductor L1.
3. VSTOR's ripple voltage was measured using an oscilloscope voltage probe placed directly across the
VSTOR capacitor (C5). The scope probe's standard ground lead was replaced with very short lead.
4. VIN and the LBOOST pin (switching node of the boost charger) were measured by oscilloscope
voltage probes connected to TP1 and TP2.
Figure 9. Charger Operational Waveforms During 50-mA Load Transient
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Test and Measurment Summary www.ti.com
3.1.5 Buck Converter Operation During Load Transient
The test setup is shown in Figure 7. The specific equipment used for the test results in Figure 10 is listed
below:
1. VIN_DC, VBAT and VOUT are configured as explained in Section 3.1.3.
2. The buck converter inductor current (IL) was measured by using an oscilloscope current probe across
a current loop that was inserted in series with inductor L2.
3. VSTOR's ripple voltage was measured using an oscilloscope voltage probe placed directly across the
VSTOR capacitor (C5). VOUT's ripple voltage was measured using an oscilloscope voltage probe
placed directly across the VOUT capacitor (C3). Both scope probes' standard ground leads were
replaced with very short lead.
4. The LBUCK pin's ripple voltage (switching node of the buck converter) was measured by a
oscilloscope voltage probe connected to TP3.
Figure 10. Buck Operational Waveforms During 50-mA Load Transient
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IOUT =1.0 mA
COMP=1.2 V
Keithley 2420
120mF
www.ti.com Test and Measurment Summary
Charging a Super Capacitor from Buck Converter Output
The test setup is shown in Figure 11. The specific equipment used for the test results in Figure 12 is listed
below:
1. VIN_DC was connected to a Keithley 2420 source-meter configured as a 1.0-mA current source with
1.2-V voltage compliance.
2. VOUT was connected to a 120 mF super capacitor. There were no other loads on VSTOR, VBAT or
VOUT.
3. VIN_DC, VSTOR and VOUT were measured with oscilloscope voltage probes connected at TP1, TP4
and TP6.
Figure 11. Test Setup for Charging a Super Capacitor from Buck Output
Figure 12. Charging a Super Cap from VOUT
The benefit of charging of the super capacitor on VOUT instead of VBAT is faster charge time due to the
charger spending less time in less efficient cold start mode.
Tips for other Tests and Measurements
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Test and Measurment Summary www.ti.com
The quiescent current during main boost operation, which is basically the current from the battery to the
IC, is measured at the VSTOR pin. If a source-meter is not available to make the measruement, connect a
100-kΩresistor to VSTOR and connect a 3-V supply from the other end of this resistor to the ground of
the EVM. A 10-MΩmeter can be used to measure the voltage drop across the resistor and calculate the
current. No other connections should be made to the EVM and the measurement should be taken after
steady state conditions are reached (may take a few minutes). The reading should be much less than 100
nA.
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www.ti.com Bill of Materials and Board Layout
4 Bill of Materials and Board Layout
This section contains the bill of materials (BOM) and the PCB board layouts for the bq25570 EVM.
4.1 Bill of Materials
Table 2 lists the BOM for the EVM.
Table 2. Bill of Materials
Count RefDes Value Description Size Part Number MFR
1 C1 4.7uF Capacitor, Ceramic Chip, 6.3V, X7R, 10% 805 C0805C475K9RACTU Kemet
1 C2** 0.01u** Capacitor, Ceramic, 50V, X7R, 10% 0603 GRM188R71H103KA01D Murata
1 C3 22uF Capacitor, Ceramic Chip, 6.3V, X5R, 10% 805 JMK212BJ226MG-T Taiyo Yuden
2 C4 C7 0.1uF Capacitor, Ceramic Chip, 6.3V, X5R, 10% 603 06036D104KAT2A AVX
1 C5 4.7uF Capacitor, Ceramic Chip, 10V, X7R, 10% 805 LMK212B7475KG-T Taiyo Yuden
1 C6 100uF Capacitor, Ceramic Chip, 6.3V, X5R, 20% 1812 GRM43SR60J107ME20L Murata
0 C8-10 DNP Capacitor, Electrolytic, Snap Mt., vvV 7343 (D) n/a n/a
9 J1 J3-4 J6-7 J9-10 J12-13 PEC02SAAN Header, Male 2-pin, 100mil spacing, 0.100 inch x 2 PEC02SAAN Sullins
4 J2 J5 J8 J11 ED555/2DS Terminal Block, 2-pin, 6-A, 3.5mm 0.27 x 0.25 inch ED555/2DS OST
3 JP1 JP5 JP6 PEC02SAAN Header, Male 2-pin, 100mil spacing, 0.100 inch x 2 PEC02SAAN Sullins
3 JP2-4 PEC03SAAN Header, Male 3-pin, 100mil spacing, 0.100 inch x 3 PEC03SAAN Sullins
1 L1 22uH Inductor, SMT, 0.65A, 360mΩ4.0mmx4.0mmx1.80mm LPS4018-223M Coilcraft
Inductor SMT, 0.36A, 410mΩ3.8mmx3.8mmx1.65mm 744031220
1 L2 10 uH Inductor, SMT, 1.4A, 216mΩ2.0mm x 2.5 mm 1239AS-H-100N Toko
Inductor SMT, 250mA, 500mΩ2.5mm x 2.0mm x 1.00mm 74479888310 Wurth Elektronik
Inductor SMT, 500mA, 390mΩ2.8mm x 2.8mm x 1.35mm 744029100 Wurth Elektronik
Inductor SMT, 500mA, 500mΩ2.5mm x 2.0mm x 1.2mm 74479889310 Wurth Elektronik
1 R1 7.5M Resistor, Chip, 1/16W, 1% 603 CRCW06037M50FKEA Vishay Dale
1 R10 8.66M Resistor, Chip, 1/16W, 1% 603 CRCW06038M66FKEA Vishay Dale
1 R2 5.76M Resistor, Chip, 1/16W, 1% 603 CRCW06035M76FKEA Vishay Dale
2 R3 R5 4.99M Resistor, Chip, 1/16W, 1% 603 CRCW06034M99FKEA Vishay Dale
1 R4 10M Resistor, Chip, 1/16W, 1% 603 CRCW060310M0FKEA Vishay Dale
1 R6 887k Resistor, Chip, 1/16W, 1% 603 CRCW0603887KFKEA Vishay Dale
1 R7 6.98M Resistor, Chip, 1/16W, 1% 603 CRCW06036M98FKEA Vishay Dale
1 R8 5.36M Resistor, Chip, 1/16W, 1% 603 CRCW06035M36FKEA Vishay Dale
1 R9 4.22M Resistor, Chip, 1/16W, 1% 603 CRCW06034M22FKEA Vishay Dale
4 TP1 TP4-6 5002 Test Point, White, Thru Hole Color Keyed 0.100 x 0.100 inch 5002 Keystone
0 TP2-3 TP7 DNP Test Point, 0.020 Hole 0.100 x 0.100 inch STD STD
2 TP8-9 5001 Test Point, Black, Thru Hole Color Keyed 0.100 x 0.100 inch 5001 Keystone
1 U1 BQ25570RGR IC, Ultra Low Power Harvester Charger + Buck IC VQFN BQ25570RGR TI
4 Shunt, 100-mil, Black 0.1 929950-00 3M
1 -- PCB, 2.5212 in x 2.6039 in PWR206 Any
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+
+
+
1
VIN
GND
GND
VOC_SAMP
GND
/EN
BAT_SEC
VSTOR
GND
VOUT_EN
BAT_OK
VOUT
VBAT
GND
GND
GND
VSTOR
VREF_SAMP
PWR206 REV A
80%
50%
GND
VRDIV
LBUCK
LBOOST
BAT_OK
2013
bq25570EVM-206
C1
C3
C5
C10
C8
C9
C2
C7
C4
C6
J1
J6 J7
J9
J4
J10
J13
J12
J3
J11
J2
J5 J8
JP1
JP5
JP6
JP2
JP4
JP3
L1 L2
R1
R10
R2
R3
R4
R5
R6
R7
R8
R9
TP1
TP4
TP5
TP6
TP2
TP3
TP7
TP8
TP9
U1
Bill of Materials and Board Layout www.ti.com
4.2 EVM Board Layout
Figure 13 through Figure 15 are the board layouts for this EVM.
Figure 13. EVM PCB Top Assembly
Figure 14. EVM PCB Top Layer
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Figure 15. EVM PCB Bottom Layer
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PCB Layout Guideline www.ti.com
5 PCB Layout Guideline
As for all switching power supplies, the PCB layout is an important step in the design, especially at high
peak currents and high switching frequencies. If the layout is not carefully done, the boost
converter/charger and buck converter could show stability problems as well as EMI problems. Therefore,
use wide and short traces for the main current path and for the power ground paths. The input and output
capacitors as well as the inductors should be placed as close as possible to the IC. For the boost
converter / charger, first priority are the output capacitors, including the 0.1uF bypass capacitor (CBYP),
followed by CSTOR, which should be placed as close as possible between VSTOR, pin 19, and VSS, pin
1. Next, the input capacitor, CIN, should be placed as close as possible between VIN_DC, pin 2, and
VSS, pin 1. Last in priority is the boost converter inductor, L1, which should be placed close to LBOOST,
pin 20, and VIN_DC, pin 2. For the buck converter, the output capacitor COUT should be placed as close
as possible between VOUT, pin 14, and VSS, pin 15. The buck converter inductor (L2) should be placed
as close as possible beween the switching node LBUCK, pin 16, and VOUT, pin 14. It is best to use vias
and bottom traces for connecting the inductors to their respective pins instead of the capacitors.
To minimize noise pickup by the high impedance voltage setting nodes (VBAT_OV, OK_PROG,
OK_HYST, VOUT_SET), the external resistors should be placed so that the traces connecting the
midpoints of each divider to their respective pins are as short as possible. When laying out the non-power
ground return paths (for example from resistors and CREF), it is recommended to use short traces as well,
separated from the power ground traces and connected to VSS pin 15. This avoids ground shift problems,
which can occur due to superimposition of power ground current and control ground current. The
PowerPad should not be used as a power ground return path.
The remaining pins are either NC pins, that should be connected to the PowerPad as shown below, or
digital signals with minimal layout restrictions.
In order to maximize efficiency at light load, the use of voltage level setting resistors > 1MΩis
recommended. However, during board assembly, contaminants such as solder flux and even some board
cleaning agents can leave residue that may form parasitic resistors across the physical resistors and/or
from one end of a resistor to ground, especially in humid, fast airflow environments. This can result in the
voltage regulation and threshold levels changing significantly from those expected per the installed resistor
values. Therefore, it is highly recommended that no ground planes be poured near the voltage setting
resistors. In addition, the boards must be carefully cleaned, possibly rotated at least once during cleaning,
and then rinsed with de-ionized water until the ionic contamination of that water is well above 50 MOhm. If
this is not feasible, then it is recommended that the sum of the voltage setting resistors be reduced to at
least 5X below the measured ionic contamination.
Revision History
Changes from Original (July 2013) to A Revision ........................................................................................................... Page
• Changed contents in the bill of materials............................................................................................. 17
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
20 Revision History SLUUAA7A–July 2013–Revised August 2014
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