Texas Instruments AN-1907 LM3423 User Manual

AN-1907 LM3423 Buck-Boost Configuration Evaluation

1 Introduction

This evaluation board has been designed to demonstrate the LM3423 low-side controller as a step­up/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

• VINrange 4.5V to 35V

• Low side PWM fast dimming

• 2 to 8 series connected LEDs (VO< 35V)

• UVLO set at 8.4V

• I

• 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

= 1A

LED

Available features that can be configured on the standard evaluation board by the user for are listed
below:

User's Guide

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Board

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1

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Board Connections and Configuration

• 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

V

IN

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).

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Power supply (Positive) primary connection

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

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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

LM3423 TSSOP 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 R
voltage that is sensed by HSP (pin 19) and HSN (pin 20). HSP and HSN are the inputs to a high side

(i.e. R9) in series with the LED load is used to convert the LED current, I

SENSE

sense amplifier that is used in combination with a resistor tied to CSH (pin 4) and an error amplifier to
program a desired I

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LED

current.

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Figure 1. Top View

LM3423 Pin Connection

LED

, into a

3

V

SNS

= V

REF

x

R17

©

§
¹

·

R7

I

LED

=

R9

©

§
¹

·

V

SNS

=

R9

©

§
¹

·

V

REF

x

R17

©

§
¹

·

R7

V

CSH

= (R17 x I

CSH

) = V

SNS

x

R7

©

§
¹

·

R17

I

CSH

=

V

SNS

R7

©

§
¹

·

I

LED

=

1.24V
R9

©

§
¹

·

R7

R17

©

§
¹

·

x

+

-

1.24V

19

R7

20

+

-

4

2

HSP

HSN

CHS

COMP

I

CHS

+

-

V

SNS

+

-

R8

R9

V

OUT

C5

R21

V

CHS

I

LED

R17

Board Features

This establishes a current gain determined by a resistor ratio consisting of R17 and R7 along with R9 as
described in the equation:

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Figure 2. High-Side Sensing Circuit

Substituting in the resistor values as listed in the board schematic gives a fixed I

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 V
understanding the relationship between V
high side amplifier in forces its input terminals to equal potential. Because of this, the V
forced across R
R7 (R

) until V

HSP

. Another way to view this is that the amplifier’s output transistor pulls current through

HSP

HSP

The current flowing down to the CSH pin is given by,

and the voltage at the CSH pin is then given by,

The CSH voltage is the sense voltage gained up by the ratio of R17 to R7. In addition, the control

4

system’s error amplifier regulates the CSH voltage to V
derived,

The above equations show how current in the LED relates to the regulated voltage V
approximately 1.25V for the LM3423.

AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013

= V

REF

HSN

current of 1A.

LED

. Understanding how average LED current is regulated comes down to

and V

CSH

and this happens when the voltage across R7 is equal to V

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, because V

SNS

. Using equation 14, the following equations are

REF

SNS

and R

set the LED current. The

SNS

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SNS

.

SNS

, which is

REF

voltage is

(1)

(2)

(3)

(4)

VO x I

LED

VIN x I

IN

©

§
¹

·

= 0.85

©

§

¹

·

P

IN

P

O

= K

fSW =

25

C1 x R

1

©

§
¹

·

= 588 kHz

D =

t

ON

tON + t

OFF

©

§
¹

·

= tON x f

SW

|

D

V

O

VIN + V

O

©

§
¹

·

|

D

¶

D

©

§
¹

·

V

IN

V

O

©

§
¹

·

t

S

'

TD

S

DT

L

Âi

L

i

L(t)

I

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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.

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
V

= 18V

IN-MIN

V
V
I
Buck Boost Convertion Ratio:

IN-NOM

IN-MAX

LED

= 24V

= 36V

= 1A

Board Features

Figure 3. Inductor Current Waveform

Therefore:

D @ V
D @ V

tON= @ V
tON= @ V

IN-MAX

IN-MIN

= 0.436

= 0.609

IN-MAX

IN-MIN

= 0.742 µs

= 1.05 µs

Calculate average input current: The average input current is equal to the average inductor current.

Assume efficiency = 85%

IIN= 915 mA @ VIN= 36V

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(5)

(6)

(7)

(8)

(9)

5

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ICL =

0.245V
R6

©

§
¹

·

+

-

OC

0.245V

17

R6

14

U1

Q5

V

OUT

V

IN

I

SW

-+

L = V

IN

= V

IN

D

'i x f

SW

©

§
¹

·

dt
di

©

§
¹

·

VIN = L

di
dt

©

§

¹

·

Board Features

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

Therefore:

Inductor value @ V

IN-MIN

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.

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(10)

(11)

≊ 33 µH

Jumper Operation

J2 Open Jumper
J3 Close Jumper

J4A, J4B Open Jumper

R6 Populate

The current limit (ICL) is calculated by the equation:

Substituting in the resistor value as listed in the board schematic gives a current limit ICLof approximately

4.1A.

6

AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013

Figure 4. External R

Table 4. External R

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SENSEISW

SENSE

Current Sense

Resistor Configuration

(12)

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ICL =

0.245V
R

DS(ON)

©

§
¹

·

+

-

OC

17

U1

14

Q5

V

IN

I

SW

V

OUT

0.24V

-+

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Board Features

MOSFET switch current can also be sensed directly across the R
need for a sense resistor (see Table 5).

Figure 5. MOSFET R

Sensing Configuration

DS(ON)

of MOSFET Q5, eliminating the

DS(ON)

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:

Substituting in the resistor values as listed in the board schematic and an R
(SUD40N10-25) gives a current limit ICLof 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.

Table 5. MOSFET R

Jumper Operation

J2 Close Jumper
J3 Open Jumper

J4A, J4B Close Jumper

R6 No Load

Sensing Configuration

DS(ON)

of 0.025Ω for Q5

DS(ON)

(13)

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T

dim

DT

dim

t

I

LED(t)

I

F

Board Features

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.

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Figure 6. Illustration of Current through LED Stack with PWM Dimming

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. VINUVLO, 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 VINby 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.

(r

x C

D-TOTAL

OUT

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.

) = (R21 x C5) (14)

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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+ R
Now that we have calculated the dynamic resistance of our LED string, we can calculate the

compensation resistor and capacitor values (C5 and R21).

Figure 7. Dynamic Resistance

= 5(250 mV) + 100 mΩ = 1.35Ω

SNS

Board Features

C

= 330 µF and rD= 1.35Ω

OUT

rD-TOTAL x C

= 1.95Ω x 220 µF = 430E-6

OUT

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

V

OVP_UP

± 1.24V

©

§
¹

·

R20 =

V

HYST

23 PA

+

-

-+

1.24V

7

OVP

R20

R22

C18

Q3

RPD

V

O

V

IN

OVP

COMP

Board Features

Calculating OVP hysterisis and set points:
Step 1: Determine V
Step 2: Calculate R20

HYST

, V

HYST

= (V

Figure 8. OVP Circuit

OVP_UP

– V

OVP_DN

)

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(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).

10

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+

-

-+

V

IN

I

UVLO

R13

R25

8

23 PA

i

X

+

-

-+

V

IN

I

UVLO

R13

R25

8

23 PA

i

X

V

IN

I

UVLO

V

HYST

t

V

UVLO_UP

V

UVLO_DOWN

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Board Features

Figure 9. UVLO Circuit During Start-Up

The UVLO threshold and hysteresis can be programmed completely independent of each other. UVLO
hysteresis is accomplished with an internal 23 µA current source that is switched on and off into the
impedance of the UVLO set-point resistor divider. When the UVLO pin exceeds 1.24V, the current source
is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the

1.24V threshold, the current source is turned off, causing the voltage at the UVLO pin to fall. The UVLO
hysteresis range can be user adjusted using the gain resistor R26.

Figure 10. UVLO Circuit During Normal Operation

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11

+

-

-+

V

IN

I

UVLO

R13

R26

R25

8

23 PA

i

X

R25 =

R13 x 1.24V

V

INUV_UP

- 1.24V

©

§
¹

·

R13 =

V

HYST

23 PA

Evaluation Board Test Procedure

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Step 1: Choose VINvoltage where converter starts to operate (V
converter shuts down (V

UV_DN

). V

HYST

= (V

UVLO_UP

- V

UV_DN

)

) and choose VIN voltage where

UV_UP

Step 2: Solve for resistor value R13 with the following equation.

Solve for resistor value R25 wit the following equation:

Example Calculation of UVLO with Hysterisis:

• VINstart-up = V

• VINshut down = V

• I

• V

= 23 µA

UVLO

= 8.45V – 8.2 = 0.25V

HYST

UV_UP

UV_DN

= 8.45V

= 8.2V

• R13 ≊ 10 kΩ

• R25 ≊ 1.74 kΩ
If a small amount of hysteresis is desired and VINis large, resistor R26 may need to be populated.

(17)

(18)

Figure 11. UVLO Circuit with R26 Populated for Small Hysteresis and Large Input Voltage

6 Evaluation Board Test Procedure

Proper Board Connections

Be sure to choose the correct wire size when connecting the source supply and load. Monitor the current
into and out of the unit under test (UUT). Monitor the voltages directly at the board terminals, as resistive
voltage drops along the wires may decrease measurement accuracy. The LM3423 evaluation board has
two pairs of positive and negative inputs connectors which allows for Kelvin connections to be made from
the power supplies to the evaluation board. These precautions are especially important during
measurement of conversion efficiency.

12

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1

2

6

3

4

5

8

7

20

19

15

18

17

16

13

14

OVP

CSH

COMP

EN

RCT

AGND

OVP

nDIM DDRV

GATE

Vcc

PGND

IS

RPD

HSP

HSN

RPD

RPD

J3

TP12

V

CC

V

IN

LED

(+)

R8

C13

LED

(-)

R

6

R25

R9

R7

C12

TP7

C2

U

1

V

IN

C3

TP2

J

1

R17

R21

R13

C5

L1

CONN

2

C17

C1

R14

C8

V

IN

V

IN

P

GND

V

IN

TP8

R27

V

O

D1

C16

C7

C11

C4

R30

Q9

BNC

TP6

R31

V

IN

J5

V

CC

R23

R3

Q7

Q4

V

O

R26

CONN

1

LM3423

R29

C14

Q6

Z1

9

10

12

11

TIMR

FLT

LRDY

DPOL

V

IN

TP9R2

TP11

C6

TP5

R15

J6

V

IN

J4B

Q5

J4A

R5

D2

J2

TP10

R20

OVP

RPD

V

IN

C18

R22

Q3

V

O

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7 LM3432 Evaluation Board Schematic

LM3432 Evaluation Board Schematic

Figure 12. LM3423 Evaluation Board: All features and external components shown

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13

Bill of Materials

8 Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 Buck-Boost controller, TSSOP TI LM3423
C1 0.1 µF 10% 25V Vishay VJ0805Y104KXXCW1BC
C2 2.2 µF, 25V Panasonic ECJ-2FB1E225K
C3 Capacitor 0805 1200 pF, 100V Murata GRM2195C2A122JA01D

C4, C11 Capacitor 1210 10 µF, 25V Panasonic ECJ-4YB1E106M

C5 Capacitor 0805 0.022 µF, 50V Panasonic ECJ-2VB1H223K
C6 Capacitor 0805 0.01 µF, 50V Panasonic ECJ-2VB1H103K

C7, C8 Capacitor 330 µF, 35V 5mm Panasonic ECA-1VM331

C12, C13, C18 Capacitor 0805 47 pF, 50V Panasonic ECJ 2VC1H470J
C14, C16, C17 Capacitor 1206 0.1 µF, 50V Murata GRM319R71H104KA01D

D1 D-Pak 12A, 100V Vishay 12CWQ10FN
D2 SOT-23 200 mA, 100V Fairchild MMBD914L

VIN, GND, LED+, LED- Connector Keystone 575-8

J1-J6 Jumper Molex 22-28-4023

L1 22 µH Coilcraft DO5040H

Q1, Q5 N-channel MOSFET TO-252 Vishay SUD40N10-25-E3

Q3 SOT-23 200mA, 40V Fairchild MMB3904
Q6 SOT-23 200mA, 40V Fairchild MMB2907

Q7, Q9 N channel MOSFET SOT23 Fairchild 2N7002

R2, R3, R15 Resistor 0805 100 kΩ Vishay CRCW08051003F

R5 0Ω
R6 Resistor 2512 0.06Ω Vishay WSL2512R0600FEA

R7, R8 Resistor 0805 1 kΩ Vishay CRCW08051001F

R9 Resistor 1812 0.1 Panasonic ERJL12KF10CU

R13, R31, R23 Resistor 0805 10k Vishay CRCW08051002F

R14 Resistor 0805 35.7k Vishay CRCW08053572F
R17, R22 Resistor 0805 12.4 kΩ Vishay CRCW08051242F
R21, R26 Resistor 0805 4.99k Vishay CRCW08054991F

R20 Resistor 0805 562 kΩ Vishay CRCW08055623F

R25 Resistor 1206 1.74k Vishay CRCW12061741F

R27 Resistor 0805 10Ω Vishay CRCW080510R0F
R29, R30 Resistor 1206 2Ω Yageo RC1206JR-072RL

Z1 Zenner diode 10V 225 mW Vishay MMBZ5240-V

Test Points Connector Keystone 1502-2

www.ti.com

Lead

40A, 100V

200mA, 60V

14

AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

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OVP

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GATE

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PGND

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HSP

HSN

RPD

RPD

J3

TP12

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CC

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(+)

R8

C13

LED

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R25

TP10

R20

R9

R7

C12

TP7

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LM3421

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TP2

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R17

R21

R13

C5

L1

CONN

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C17

C1

R14

C8

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IN

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P

GND

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TP8

R27

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D1

C16

C7

C11

C4

R30

Q9

BNC

TP6

R31

V

IN

J5

V

CC

R23

R3

Q7

Q4

Q4

OVP

RPD

V

IN

C18

R22

Q3

V

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R26

CONN

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www.ti.com

LM3421 Buck-Boost Design Example (High Side Current Sense & High Speed Dimming)

9 LM3421 Buck-Boost Design Example (High Side Current Sense & High Speed

Dimming)

Figure 13. LM3421 Design Example: High Side Current Sense with High Speed Dimming

SNVA376A–December 2008–Revised May 2013 AN-1907 LM3423 Buck-Boost Configuration Evaluation Board

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

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PGND

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RPD

HSP

HSN

R26

RPD

RPD

J3

RPD

TP12

V

CC

V

IN

LED

(+)

R8

C13

LED

(-)

J5

V

IN

R

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Q

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R25

TP10

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IN

C18

R22

Q3

R20

V

O

R9

R7

C12

TP7

C2

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V

IN

LM3421

C3

TP2

J

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R17

R21

R13

C5

L1

CONN

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C17

C1

R14

C8

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TP8

R27

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C7

C11

C4

R30

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LM3421 Buck-Boost Design Example (High Side Current Sense)

10 LM3421 Buck-Boost Design Example (High Side Current Sense)

www.ti.com

Figure 14. LM3421 Design Example: High Side Current Sense

16

AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

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