Boost:

The crossover frequency of the inner current loop is

given by:

For adequate phase margin at crossover, place the zero

is added to eliminate noise spikes riding on the current

waveform and is placed at the switching frequency.

PWM Dimming

Even though the MAX16821A/MAX16821B/MAX16821C

do not have a separate PWM input, PWM dimming can

be easily achieved by means of simple external circuitry.

See Figures 10 and 11.

f

R

V

V

L

gR

C

S

RAMP

LED

mCF

.

=×

××

××

×

2

34 5

π

R

Vf

L

AR

V

V

g

CF

RAMP

SW

V

S

LED

IN

m

(

)

≤

××

××

−

×

High-Power Synchronous HBLED

Drivers with Rapid Current Pulsing

20

______________________________________________________________________________________

1

2

3

4

5

6

8

7

21

20

19

18

17

16

15

9

10

11

12

13

14

22

23

24

25

26

27

28

Q1

L1

C4

LED

STRING

C2

D2

7V TO 28V

C11

C10

C9

C8

R9

R5

R10

R3

R4

ON/OFF

R7

R6

PGND

N.C.

N.C.

DL

BST

LX

DH

I.C.

OUTV

RT/SYNC

EN

MODE

CLKOUT

SGND

SGND

SENSE-

SENSE+

SGND

IN

OVI

CLP

EAOUT

EAN

DIFF

CSN

CSP

MAX16821A

MAX16821B

MAX16821C

C3

C7

C6

C5

R1

R2

R9

Q2

PWM DIM

Q3

Figure 10. Low-Side Buck LED Driver with PWM Dimming (Patent Pending)