38V Step-Up LED Driver

8

SGM3725

SG Micro Corp

www.sg-micro.com

FUNCTIONAL DESCRIPTION

The SGM3725 uses a constant-frequency current-mode

boost converter architecture to control the LED current by

regulating the feedback voltage. Please refer to the

Functional Block Diagram above for an explanation of

SGM3725 operation. The beginning of each cycle turns

on the Power MOSFET. A slope compensation ramp is

added to the output of the current sense amplifier and the

result is fed into the positive input of the comparator

(COMP). When this voltage goes above the output

voltage of the error amplifier (Gm), the Power MOSFET is

turned off. The voltage at the output of the Gm block

amplifies the difference between the reference voltage

and the feedback voltage (FB), so that FB voltage can be

regulated to the reference voltage.

The SGM3725 has built-in soft-start to limit the inrush

current during start-up and to limit the amount of

overshoot on the output. Protection features in the

SGM3725

include

over-voltage

protection

(OVP),

cycle-by-cycle current limit protection and thermal

shutdown. OVP protects in the event where an LED fails

open, which forces the feedback voltage to zero. This

causes the boost converter to operate in maximum duty

cycle mode, ramping up the output voltage. Switching will

stop when the output reaches the OVP threshold. The

OVP feature protects the IC from damaging itself by

exceeding the voltage rating on SW/VOUT pins.

The single wire control interface accepts a series of

pulses into the CTRL pin to program the reference

voltage. The number of rising edges is detected internally

and decoded as illustrated in Table 1. The register

contains 5-bits, yielding 32 different current levels. Using

the simplest control method, output current can be

connected between FB and GND. Please see LED

Maximum Current Setting section in this document to

determine the proper resistor value. The next level of

control is achieved by injecting a series of pulses into the

CTRL pin to program the reference voltage at 32 levels.

The number of rising edges is detected internally and

decoded to the current level as illustrated in Table 1. The

reference voltage levels are evenly spaced among these

32 steps.

The first rising edge on the CTRL pin takes the current

be changed with each additional qualified pulse. The

current stays unchanged as long as the CTRL pin stays

high.

Table 1. Current Setting

DATA

LED Current Ratio

DATA

LED Current Ratio

DATA

LED Current Ratio

DATA

LED Current Ratio

1

32/32

9

24/32

17

16/32

25

8/32

2

31/32

10

23/32

18

15/32

26

7/32

3

30/32

11

22/32

19

14/32

27

6/32

4

29/32

12

21/32

20

13/32

28

5/32

5

28/32

13

20/32

21

12/32

29

4/32

6

27/32

14

19/32

22

11/32

30

3/32

7

26/32

15

18/32

23

10/32

31

2/32

8

25/32

16

17/32

24

9/32

32

1/32