MAX16801A Datasheet, PDF (7/15 Pages) Maxim Integrated Products – Off-Line and DC-DC PWM Controllers for High-Brightness LED Drivers

Off-Line and DC-DC PWM Controllers for
High-Brightness LED Drivers
When in the bootstrapped mode with a transformer
(Figure 5), the circuit is protected against most output
short-circuit faults when the tertiary voltage drops
below +10V, causing the UVLO to turn off the gate
drive of the external MOSFET. This re-initiates a startup
sequence with soft-start.
When the LED current needs to be tightly regulated, an
internal error amplifier with 1% accurate reference can
be used (Figure 9). This additional feedback minimizes
the impact of passive circuit component variations and
tolerances, and can be implemented with a minimum
number of additional external components.
A wide dimming range can be implemented using a
low-frequency PWM dimming signal fed directly to the
DIM/FB pin.
LED driver circuits designed with the MAX16801 use a
high-value startup resistor R1 that charges a reservoir
capacitor C1 (Figure 5 or Figure 9). During this initial
period, while the voltage is less than the internal boot-
strap UVLO threshold, the device typically consumes
only 45µA of quiescent current. This low startup current
and the large bootstrap UVLO hysteresis help minimize
the power dissipation across R1, even at the high end
of the universal AC input voltage.
An internal shutdown circuit protects the device when-
ever the junction temperature exceeds +130°C (typ).
Dimming
Linear dimming can be implemented by creating a
summing node at CS, as shown in Figures 6 and 7.
Low-frequency PWM (chopped-current) dimming is
possible by applying an inverted-logic PWM signal to
the DIM/FB pin of the IC (Figure 8). This might be a pre-
ferred way of dimming in situations where it is critical to
retain the light spectrum unchanged. It is accom-
plished by keeping constant the amplitude of the
chopped LED current.
MAX16801/MAX16802 Biasing
Implement bootstrapping from the transformer when it
is present (Figure 5). Biasing can also be realized
directly from the LEDs in non-isolated topologies
(Figure 1).
Bias the MAX16802 directly from the input voltage of
10.8VDC to 24VDC. The MAX16802 can also be used
AC
BRIDGE
IN
RECTIFIER
C1
R1
VCC
R5
NDRV
R2
Q1
IN
GND
MAX16801B
COMP
CS
C2
C3
DIM/FB
UVLO/EN
R3
R6
R4
L1
C4
D3
TOTAL LED VOLTAGE:
11V TO 23V
Figure 1. Biasing the IC using LEDs in Nonisolated Flyback Driver
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