MAX16821A_09 Datasheet, PDF (20/24 Pages) Maxim Integrated Products – High-Power Synchronous HBLED Drivers with Rapid Current Pulsing

High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Boost:
R_CF ≤
V_RAMP × f_SW × L
A_V × R_S × (V_LED − V_IN) × g_m
The crossover frequency of the inner current loop is
given by:
f_C
=
R_S
V_RAMP
×
V_LED
2×π×L
×
34.5
×
g_m
× R_CF
For adequate phase margin at crossover, place the zero
formed by R_CF and C_CZ at least 3 to 5 times below the
crossover frequency. The pole formed by R_CF and C_CP
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.
V_LED
R9
R10
C11
C10 _R9
R7
C9
C8
R6
V_CC
R4
C3
R3
ON/OFF
14
I.C.
15 OVI
13
12
11
OUTV RT/SYNC EN
10
MODE
9
CLKOUT
8
SGND
N.C. 7
16 CLP
DH 6
17 EAOUT
18 EAN
19 DIFF
MAX16821A
MAX16821B
MAX16821C
LX 5
R5
BST 4
DL 3
20 CSN
N.C. 2
21 CSP
PGND 1
SGND SENSE- SENSE+ SGND
IN
V_CC
V_DD
22
23
24
25
26
27
28
V_IN
V_IN
7V TO 28V
C2
Q1
V_LED
L1
C4
PWM DIM
Q2
LED
STRING
Q3
D2
R2
R1
C7
C6
C5
Figure 10. Low-Side Buck LED Driver with PWM Dimming (Patent Pending)
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