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MAX16821A Datasheet, PDF (15/25 Pages) Maxim Integrated Products – High-Power Synchronous HBLED Drivers with Rapid Current Pulsing
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High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
SEPIC LED Driver
Figure 7 shows the MAX16821A/MAX16821B/
MAX16821C configured as a SEPIC LED driver. While
buck topologies produce an output always lower than
the input, and boost topologies produce an output
always greater than the input, a SEPIC topology allows
the output voltage to be greater than, equal to, or less
than the input. In a SEPIC topology, the voltage across
C3 is the same as the input voltage, and L1 and L2 have
the same inductance. Therefore, when Q1 turns on (on-
time), the currents in both inductors (L1 and L2) ramp
up at the same rate. The output capacitor supports the
output voltage during this time. When Q1 turns off (off-
time), L1 current recharges C3 and combines with L2 to
provide current to recharge C1 and supplies the load
current. Since the voltage waveform across L1 and L2
are exactly the same, it is possible to wind both induc-
tors on the same core (a coupled inductor). Although
voltages on L1 and L2 are the same, RMS currents can
be quite different so the windings may require a differ-
ent gauge wire. Because of the dual inductors and seg-
mented energy transfer, the efficiency of a SEPIC
converter is lower than the standard buck or boost con-
figurations. As in the boost driver, the current-sense
resistor connects to ground, allowing the output voltage
of the LED driver to exceed the rated maximum voltage
of the MAX16821A/MAX16821/MAX16821C.
VLED
R8
R9
C10
C9 R7
R6
C8
C7
R5
VCC
R4
C2
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
BST 4
DL 3
20 CSN
N.C. 2
21 CSP
SGND SENSE- SENSE+ SGND
IN
22
23
24
25
26
VIN
PGND 1
VCC
VDD
27
28
VIN
7V TO 28V
L1
C3
D1
VLED
Q1
L2
C1
LED
STRING
R2
R1
C6
C5
C4
Figure 7. Typical Application Circuit for a SEPIC LED Driver
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