Texas Instruments LP8728EVM User manual

Using the LP8728EVM Evaluation Module
User's Guide
Literature Number: SNVU231
August 2013

Chapter 1
SNVU231–August 2013
Introduction
The Texas Instruments' LP8728EVM evaluation module (EVM) helps designers evaluate the operation
and performance of this device. The LP8728EVM uses the LP8728’s four buck converters to provide 3.3
V, 2.65 V, 1.8 V, and 1.25 V output voltages. Information about output voltage and current ratings of the
LP8728 can also be found in the LP8728 datasheet.
In order to facilitate ease of testing and evaluation of this circuit, the LP8728EVM contains screw-in
terminals for the VIN and all four VOUT connections. Dual connectors are provided for easy four-wire
connection.
For evaluation purposes, the LP8728EVM has been tested over a 4.5 V to 5.5 V input range. Users are
cautioned to evaluate their specific operating conditions and choose components with the appropriate
voltage ratings before designing in LP8728 into a final product.
Figure 1-1. LP8728EVM
2Introduction SNVU231–August 2013
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LP8728
FB_B1
SW_B1 VOUT1
FB_B2
SW_B2 VOUT2
FB_B3
SW_B3
VOUT3
FB_B4
SW_B4 VOUT4
1.5µH
1.5µH
AGND GND_B1 GND_B2
GND_B3
GND_B4
EN_B1
EN_B2
EN_B3
EN_B4
PG_B1
PG_B2
PG_B3
PG_B4
Micro
Controller
DEFSEL
AVDD
BYP
VIN VIN
VIN_B1
VIN
VIN_B2
1.5µH
1.5µH
VIN
VIN
VIN_B3
VIN_B4
VDDIO
1µF
1µF 10µF
10µF
10µF
10µF
10µF
10µF
10µF
10µF
Chapter 2
SNVU231–August 2013
Description of LP8728
The LP8728 is a quad output Power Management Unit, optimized for low power FPGAs, microprocessors,
and DSPs. This device integrates four highly efficient step-down DC/DC converters into one package. All
the converters run at fixed 3.3 MHz switching frequency. Buck converters switching is phase shifted to
minimize the input current spiking.
2.1 Features
• Four High-Efficiency Step-Down DC/DC Converters
– Max output current 1.0 A
– Forced PWM operation
– Soft-start control
– VOUT1 = 3.3 V
– VOUT2 - 1.25 V
– VOUT3 = 1.8 V or 2.65 V (pin selectable)
– VOUT4 = 1.8 V
• Separate Enable Inputs for each Converter Control
• Separate Power Good Outputs for each Converter
• Output Over-Current and Input Over-Voltage Protection
• Over-Temperature Protection
• Under-Voltage Lockout (UVLO)
• 28-pin 0.5 mm Pitch QFN Package
2.2 Applications
• Automotive Systems
Figure 2-1. Typical Application
3
SNVU231–August 2013 Description of LP8728
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Chapter 3
SNVU231–August 2013
LP8728EVM Setup
Figure 3-1 shows the LP8728EVM with input and output connectors and main components on the board.
Input power supply is connected to VIN connectors. Two connectors are provided for easy 4-wire
connection. A force line should be connected to connector X1 terminals VIN and GND. Sense wires
should be connected to connector X2 terminals VINS and GNDS. If a two-wire connection is used, wires
should be connected to connector X1 VIN and GND only. Input voltage needs to be set between 4.5 V to
5.5 V.
Two connectors are also provided for each buck output. This is useful, for example, when connecting the
source meter in 4-wire mode for accurate IOUT vs VOUT measurement. Two output terminals are
connected in parallel so either one can be used for sense or force lines.
Connector X3 terminals VDDIOX and GND can be used to connect separated pull-up voltage for the
EN_Bx and DEFSEL pins. If separated pull-up voltage is not desired, input pins can be pulled up to VIN
voltage. Pull-up voltage is selected by changing the shunt position on the J2 terminal. Connecting shunt
between VDDIO and VDDIOX selects VDDIOX as the pull-up voltage. Connecting a shunt between
VDDIO and AVDD selects VIN as the pull-up voltage.
Extra test points are placed close to the buck output capacitors for convenient probe points for an
oscilloscope. Similar test points are also placed close to each input voltage pin (VIN_Bx and AVDD).
Figure 3-1. Evaluation Board Connectors and Setup
4LP8728EVM Setup SNVU231–August 2013
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Chapter 4
SNVU231–August 2013
Board Layout
Figure 4-1. Layer 1 Top (Signal)
Figure 4-2. Layer 2 (GND) (Picture in reverse colors)
5
SNVU231–August 2013 Board Layout
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Figure 4-3. Layer 2 (GND / VIN)
Figure 4-4. Layer 4 Bottom (Signal)
6Board Layout SNVU231–August 2013
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Figure 4-5. Close-up of the LP8728 and Main External Components
7
SNVU231–August 2013 Board Layout
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Chapter 5
SNVU231–August 2013
Board Stackup
Figure 5-1. Evaluation Board Stackup
Details:
• 4-layer board FR4
• Top layer 1 - copper, 35µm
• Prepreg 0.1 mm
• Internal Layer 2, 35 µm
• Core 0.7 mm
• Ground plane Layer 3, 35 µm
• Prepreg 0.1 mm
• Bottom Layer 4 - copper, 35 µm
• Surface finish immersion gold
8Board Stackup SNVU231–August 2013
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Chapter 6
SNVU231–August 2013
Controls
The LP8728 has four enable pins (EN_Bx). When any of the enable pins are pulled high, the device
activates and starts up corresponding the buck converter (EN_B1 controls buck1, EN_B2 controls buck2,
etc). When all EN_Bx pins are low, the device goes to shutdown mode.
DEFSEL is used to select buck 3 output voltage. If the DEFSEL pin is pulled high, buck 3 output voltage is
set to 2.65 V. If DEFSEL pin is set low, buck 3 output voltage is set to 1.8 V
EN_Bx pins and the DEFSEL pin have internal pull-down resistors (500 kΩ) which will set the state to low
when the EN input is floating.
Power good outputs indicate the status of each buck converter. Once buck output voltage reaches 96%
(typ) of the desired output voltage, the power good pin is pulled high. If output voltage drops below 93%
(typ) of desired output voltage due to, for example, an overload condition, the corresponding power good
pin is set low.
6.1 LP8728EVM Control Interface
Figure 6-1. LP8728EVM Control Pins
All LP8728 digital input and out signals are routed to 20-pin connector (J3) pins. Pins closer to PCB edge
go straight to the LP8728 digital pins. These pins can be used to connect LP8728 pins to any external
source (signal generators, external controllers, etc). For simple enable / disable operation pull-up resistors
are connected to adjacent pins of the digital inputs. This allows enabling the bucks by setting shunts
between the J3 pins.
Indicator LEDs are provided for power good signals PG_Bx. LEDs are connected between VIN and
PG_Bx pin so the LED illuminates when power good signal is low (buck is disabled or output voltage out
of regulation). Indicator LEDs can be disconnected from the PG_Bx pins by removing the shunts form
connector J3.
9
SNVU231–August 2013 Controls
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Chapter 7
SNVU231–August 2013
Evaluation Board Schematic
Figure 7-1. Evaluation Board Schematic
10 Evaluation Board Schematic SNVU231–August 2013
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