Texas Instruments AN-1907 LM3423 User manual

User's Guide
SNVA376A–December 2008–Revised May 2013
AN-1907 LM3423 Buck-Boost Configuration Evaluation
Board
1 Introduction
This evaluation board has been designed to demonstrate the LM3423 low-side controller as a step-
up/step-down (buck-boost) regulator to deliver constant current to high power LEDs. A complete circuit
schematic and bill of materials for the evaluation board are included at the end of this document. The
printed circuit board consists of two layers of two ounce copper on FR4 material. The LM3423 evaluation
board is designed so that all options available can be evaluated and tested. Most applications will only
require a few options therefore jumpers can be placed, or removed as needed. A schematic of the full
featured LM3423 evaluation board and its bill of materials is provided in this document. Simplified design
examples with schematics and a bill of materials follow.
2 Device Description
The LM3423 is a high voltage, low-side NFET controller with an adjustable output current sense voltage.
Output voltage regulation is based on peak current-mode control, which eases the design of loop
compensation while providing inherent input voltage feed-forward compensation. The LM3423 includes a
high-voltage start-up regulator that operates over a wide input range of 4.5V to 75V. The PWM controller
is designed for high speed capability including a switching frequency range to 2.0 MHz. Additional features
include “zero” current shutdown, error amplifier, precision reference, logic-compatible DIM input suitable
for fast PWM dimming of the output, cycle-by-cycle current limit, LED ready flag, fault flag, programmable
fault timer, and thermal shutdown.
Standard Evaluation Board Operating Configuration
• fSW = 600 kHz
• Over-voltage protection set at 56V
• VIN range 4.5V to 35V
• Low side PWM fast dimming
• 2 to 8 series connected LEDs (VO< 35V)
• UVLO set at 8.4V
• ILED = 1A
Available features that can be configured on the standard evaluation board by the user for are listed
below:
• Fixed or programmable LED current
• High-speed PWM high-side or low-side dimming
• User programmable over-voltage protection (OVP)
• Under-voltage lock-out (UVLO) protection
• Fault protection
• Soft-start
• Hysteretic current-mode control
All trademarks are the property of their respective owners.
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Board Connections and Configuration www.ti.com
•Higher Input and / or Output Voltage Modifications
– Although the standard LM3423 evaluation board is designed to operate at input and output voltages
up to 35V, the device is capable of operating with input and output voltages up to 75V. Operation
up to 75V can be achieved by changing the voltage ratings of the input capacitors (C1, C8, C17),
output capacitors (C4, C7, C11, C16), and transistors Q4, Q5, Q7. For output voltages greater than
35V the OVP resistors R20 and R22 will need to be adjusted.
3 Board Connections and Configuration
Connecting the evaluation board to a power supply and load is accomplished through banana-plug type
connectors (refer to Table 1).
Table 1. LM3423 Eval Board Connectors
Connector Designation Function or Use
VIN Power supply (Positive) primary connection
GND Power supply (Negative) primary connection
LED+ Connect to anode of LED.
LED- Connect to cathode of LED.
Configuration of the evaluation board is accomplished through the use of on-board jumpers (refer to
Table 2).
Table 2. LM3423 Evaluation Board Jumpers
Jumper Designation Function or Use Notes
J1 Enable (EN)
OPEN: Disables LM3423.
CLOSED: Enables LM3423.
J2 Current Limit (IS)
OPEN: Disables MOSFET RDS(ON) current sensing "Q5".
CLOSED: Enables MOSFET RDS(ON) current sensing "Q5".
J3 Current Limit (IS)
OPEN: Disables external sense resistor MOSFET current sensing
"Q5".
CLOSED: Enables external sense resistor MOSFET current
sensing "Q5".
J4A, J4B Current Limit (IS): Must be used in conjunction with jumper J2.
OPEN: Enables sensing MOSFET switch current across sense
resistor "R6".
CLOSED: Disables sensing MOSFET switch current across sense
resistor "R6".
J5 PWM Dimming
OPEN: Enables high-side PWM dimming.
CLOSED: Disables high-side PWM dimming.
J6 Fault Timer (FLT)
OPEN: External capacitor programs fault condition time to set flag
(FLT).
CLOSED: Disables fault timer and flag (FLT).
Test points in the form of clip-on pegs are available to the user for making measurements on the LM3423
evaluation board (see Table 3).
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1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
VIN
EN
COMP
CSH
RCT
AGND
OVP
nDIM
FLT
TIMR LRDY
DPOL
PGND
GATE
VCC
IS
RPD
HSP
HSN
DDRV
www.ti.com LM3423 TSSOP Pin Connection
Table 3. LM3423 Evaluation Board Test Points
Test Point Designation Function or Use
TP1 Test point for "LED+" connector (LED anode).
TP2 Test point for "LED-" connector (LED cathode).
TP3 Test point for regulated output voltage.
TP5 Test point for L-RDY pin.
TP6 Test point for "PWM Dimming" input signal.
TP7 Test point for IS pin.
TP8 Test point for nDIM pin.
TP9 Test point for FLT pin.
TP10 Test point for GROUND.
TP11 Test point for TIMR pin.
TP12 Test point for switch-node.
4 LM3423 TSSOP Pin Connection
Figure 1. Top View
LM3423 Pin Connection
5 Board Features
This evaluation board has all the necessary connections and jumpers to evaluate the LM3423 controller in
a boost converter topology with the following operating features and options:
5.1 Setting Average LED Current
The LM3423 uses peak current-mode control to regulate the boosted output voltage. An external current
sense resistor RSENSE (i.e. R9) in series with the LED load is used to convert the LED current, ILED, into a
voltage that is sensed by HSP (pin 19) and HSN (pin 20). HSP and HSN are the inputs to a high side
sense amplifier that is used in combination with a resistor tied to CSH (pin 4) and an error amplifier to
program a desired ILED current.
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VSNS = VREF x R17
©
§¹
·
R7
ILED = R9
©
§¹
·
VSNS =R9
©
§¹
·
VREF xR17
©
§¹
·
R7
VCSH = (R17 x ICSH) = VSNS x R7
©
§¹
·
R17
ICSH = VSNS
R7
©
§¹
·
ILED = 1.24V
R9
©
§¹
·
R7
R17
©
§¹
·x
+
-
1.24V
19
R7
20
+
-
42
HSP
HSN
CHS COMP
ICHS
+
-VSNS
+
-
R8
R9
VOUT
C5
R21
VCHS
ILED
R17
Board Features www.ti.com
Figure 2. High-Side Sensing Circuit
This establishes a current gain determined by a resistor ratio consisting of R17 and R7 along with R9 as
described in the equation:
(1)
Substituting in the resistor values as listed in the board schematic gives a fixed ILED current of 1A.
5.2 Setting the Current Sense Voltage
By substituting in different resistor values, the LED average current can be user adjusted. The LM3423
controller uses a high-side sense amplifier to regulate LED average current. The CSH pin is regulated by
the error amplifier to be VREF. Understanding how average LED current is regulated comes down to
understanding the relationship between VCSH and VSNS, because VSNS and RSNS set the LED current. The
high side amplifier in forces its input terminals to equal potential. Because of this, the VSNS voltage is
forced across RHSP. Another way to view this is that the amplifier’s output transistor pulls current through
R7 (RHSP) until VHSP = VHSN and this happens when the voltage across R7 is equal to VSNS.
The current flowing down to the CSH pin is given by,
(2)
and the voltage at the CSH pin is then given by,
(3)
The CSH voltage is the sense voltage gained up by the ratio of R17 to R7. In addition, the control
system’s error amplifier regulates the CSH voltage to VREF. Using equation 14, the following equations are
derived,
(4)
The above equations show how current in the LED relates to the regulated voltage VREF, which is
approximately 1.25V for the LM3423.
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VOx ILED
VIN x IIN
©
§¹
·= 0.85
©
§¹
·
PIN
PO= K
fSW = 25
C1x R1
©
§¹
·= 588 kHz
D = tON
tON + tOFF
©
§¹
·= tON x fSW
|
DVO
VIN + VO
©
§¹
·
|D¶
D
©
§¹
·
VIN
VO©
§¹
·
t
S
'TD
S
DT
L
Âi
L
i
L(t)
I
www.ti.com Board Features
The selection of resistors is not arbitrary, for matching and noise performance, the CSH current should be
set to be around 100 µA. This current does not flow in the LEDs and will not affect either off state LED
current or the regulated LED current. CSH current can be above or below this value, but high side
amplifier offset characteristics and jitter performance may be affected slightly.
5.3 Inductor Selection
The inductor should be chosen so that the current ripple (ΔiL) is between 20% and 40% of the average
current (<IL>) through the inductor.
Figure 3. Inductor Current Waveform
The worst case ripple is seen when the input voltage is at its lowest magnitude. This is true if we can say
that the output voltage stays relatively constant.
Design Example:
VO≊28V
VIN-MIN = 18V
VIN-NOM = 24V
VIN-MAX = 36V
ILED = 1A
Buck Boost Convertion Ratio:
(5)
Therefore:
(6)
D@VIN-MAX = 0.436
D@VIN-MIN = 0.609
(7)
tON =@VIN-MAX = 0.742 µs
tON =@VIN-MIN = 1.05 µs
Calculate average input current: The average input current is equal to the average inductor current.
(8)
Assume efficiency = 85%
(9)
IIN = 915 mA @ VIN = 36V
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ICL = 0.245V
R6
©
§¹
·
+
-
OC
0.245V
17
R6
14
U1
Q5
VOUT VIN
ISW
-+
L = VIN = VIN D
'i x fSW
©
§¹
·
dt
di
©
§¹
·
VIN = L di
dt
©
§¹
·
Board Features www.ti.com
IIN = 1830 mA @ VIN = 18V
Set inductor current ripple for 30% of average current.
ΔIIN = 915 mA x 0.30 = 275 mA
ΔIIN = 1830 mA x 0.30 = 550 mA
(10)
Therefore:
(11)
Inductor value @ VIN-MIN ≊33 µH
5.4 Peak Current Limit
Due to its peak current-mode control architecture, the LM3423 has inherent cycle-to-cycle current limit
control. Inductor current flowing through the low-side power MOSFET (Q5) is sensed as a voltage
between IS (pin 17) and PGND (pin 14). This voltage is fed into an internal comparator which establishes
the peak current allowed during each switching cycle.
Two methods of switch current sensing are available on the evaluation board. The first is accomplished
through the use of an external sense resistor which allows for higher accuracy in sensing the peak current.
For the LM3423 evaluation board, the sense resistor R6 can be utilized using the jumper configuration as
described in Table 4.
Figure 4. External RSENSE ISW Current Sense
Table 4. External RSENSE Resistor Configuration
Jumper Operation
J2 Open Jumper
J3 Close Jumper
J4A, J4B Open Jumper
R6 Populate
The current limit (ICL) is calculated by the equation:
(12)
Substituting in the resistor value as listed in the board schematic gives a current limit ICL of approximately
4.1A.
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ICL = 0.245V
RDS(ON)
©
§¹
·
+
-
OC
17
U1
14
Q5
VIN
ISW
VOUT
0.24V
-+
www.ti.com Board Features
MOSFET switch current can also be sensed directly across the RDS(ON) of MOSFET Q5, eliminating the
need for a sense resistor (see Table 5).
Figure 5. MOSFET RDS(ON) Sensing Configuration
Table 5. MOSFET RDS(ON) Sensing Configuration
Jumper Operation
J2 Close Jumper
J3 Open Jumper
J4A, J4B Close Jumper
R6 No Load
The trade-off will be less accuracy and performance flexibility for reduced component count, and
increased efficiency. The current limit (ICL) using this sense method is calculated by the equation:
(13)
Substituting in the resistor values as listed in the board schematic and an RDS(ON) of 0.025Ωfor Q5
(SUD40N10-25) gives a current limit ICL of approximately 10A.
5.5 PWM Dimming
The average LED forward current is often controlled or reduced with a pulse-width modulated (PWM)
signal. By reducing the average LED current, light from the LEDs is reduced.
This dimming method allows the converter to operate the LEDs at a specific peak output current level (iL),
which is usually a set point determined by the LED manufacturer. This allows the LED to illuminate with a
consistent light color while still having the ability to reduce its lumens output.
The dimming frequency should be fast enough so that the ON and OFF blinking of the LEDs is not
perceived by the human eye. Usually the dimming frequency should be greater than 120 Hz, but less than
5 kHz for best results.
The LM3423 evaluation board implements PWM dimming by placing a series connected MOSFET in
series with the LED stack. The PWM signal is applied to this MOSFET, and the LED current is interrupted
when the MOSFET turns OFF.
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Tdim DTdim
t
ILED(t)
IF
Board Features www.ti.com
Figure 6. Illustration of Current through LED Stack with PWM Dimming
The LM3423 evaluation board can be configured for either high-side PWM dimming or low-side PWM
dimming. The definition of high side dimming is when a MOSFET that interrupts the forward current
through the LED stack is placed on the anode side of the LED stack. Low side dimming places the
MOSFET on the cathode side of the LED stack. The PWM dimming signal should be applied to either the
BNC connector or test point TP6.
Dimming on the low-side (cathode) of the LED load is enabled using the jumper configuration described in
Table 6.
Table 6. Low-Side PWM Dimming Configuration
Jumper Operation
J5 Open Jumper
5.6 Shutdown Operation
The LM3423 can be configured for either a very low quiescent current shut down (“Zero Current” IQ < 1
µA), or the standard enable/disable configuration (IQ< 3 mA).
“Zero Current” is achieved by tying the bottom resistor of all external resistor dividers (i.e. VIN UVLO, OVP)
to the RPD Pin 18. Bias currents in the resistor dividers are essentially eliminated during shutdown. The
evaluation board is designed using the “zero” shutdown feature.
5.7 Fault Protection Flag
The LM3423 can be configured with fault protection by using the fault flag indicator FLT (pin 9).
When a fault condition is detected, the FLT pin will go high (pulled up to VIN by resistor R2).
5.8 Compensation
The LM3423’s error amplifier (EA) is a transconductance type amplifier, which allows for easy single-pin
compensation. When a capacitor is used on the output of the converter to reduce LED ripple current, a
two pole system results. To offset one of the two poles, and guarantee loop stability, a zero is introduced
at the output of the EA. This takes the form of a resistor in series with a compensation capacitor (R21 and
C5). The value of the EA resistor and capacitor is calculated to give the same RC time constant as the
output capacitor and the dynamic resistance (RD) of the LED string.
(rD-TOTAL x COUT) = (R21 x C5) (14)
5.9 LED Dynamic Resistance
When the load is an LED or string of LEDs, the load resistance is replaced with the dynamic resistance
(rD) and the current sense resistor. LEDs are PN junction diodes, and their dynamic resistance shifts as
their forward current changes. Dividing VFby IFleads to incorrect results that are 5 to 10 times higher than
the true rDvalue.
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Figure 7. Dynamic Resistance
1 Amp is a typical driving current for 3W LEDs, and the calculation below shows how the dynamic
resistance of a 5W white InGaN was determined at 1A:
ΔVF= 3.85V - 3.48V = 370 mV
ΔIF= 1.5A - 0A = 1.5A
rD=ΔVF/ΔIF= 370 mV / 1.5A = 250 mΩ
Dynamic resistances combine in series and parallel like linear resistors, hence for a string of 'n' series-
connected LEDs the total dynamic resistance would be:
rD-TOTAL = n x rD+ RSNS = 5(250 mV) + 100 mΩ= 1.35Ω
Now that we have calculated the dynamic resistance of our LED string, we can calculate the
compensation resistor and capacitor values (C5 and R21).
COUT = 330 µF and rD= 1.35Ω
rD-TOTAL x COUT = 1.95Ωx 220 µF = 430E-6
Choose C5 to equal 100 nF, therefore R21 equals 4.32 kΩ
5.10 Overvoltage Protection
An over-voltage protection (OVP) with programmable hysteresis feature is available on the LM3423 to
protect the device from damage when the boosted output voltage goes above a maximum value.
The OVP threshold is set up by the resister divider network of R22 and R20 which is referenced to the
regulated output voltage (VO). The OVP threshold and hysteresis can be programmed completely
independent of each other. OVP hysteresis is accomplished with an internal 23 µA current source that is
switched on and off into the impedance of the OVP set-point resistor divider. When the OVP pin exceeds
1.24V, the current source is activated to instantly raise the voltage at the OVP pin. When the OVP pin
voltage falls below the 1.24V threshold, the current source is turned off, causing the voltage at the OVP
pin to fall.
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R22 = R20 x 1.24V
VOVP_UP ±1.24V
©
§¹
·
R20 = VHYST
23 PA
+
-
-+
1.24V
7
OVP
R20
R22
C18
Q3
RPD
VO
VIN
OVPCOMP
Board Features www.ti.com
Figure 8. OVP Circuit
Calculating OVP hysterisis and set points:
Step 1: Determine VHYST, VHYST = (VOVP_UP – VOVP_DN)
Step 2: Calculate R20
(15)
The VOOVP release point (which includes the OVP hysteresis) is described by the equation:
(16)
The evaluation board is already configured with OVP, and the VOOVP threshold is programmed on the
evaluation board to 55V with 13V of hysteresis. OVP will therefore release when VOreaches 42V (R20 =
562 kΩ, R22 = 12.4 kΩ).
5.11 Under-Voltage Protection
The LM3423 can be configured for under-voltage lockout (UVLO) protection with hysteresis using the
dimming input nDIM (pin 8) and a resistor divider from input voltage to ground. UVLO protects the power
devices during power supply startup and shutdown to prevent operation at voltages less than the minimum
operating input voltage. The UVLO threshold is set up by the resister divider network of R13 and R25 (see
Figure 9).
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