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

SNVA376A–December 2008–Revised May 2013

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).

2

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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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