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/MAX16821B/MAX16821C.
MAX16821A/MAX16821B/MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
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N.C.
N.C.
I.C. OUTV RT/SYNC EN MODE CLKOUT SGND
