MAX16821A/MAX16821B/MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
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Detailed Description
The MAX16821A/MAX16821B/MAX16821C are high-per-
formance average current-mode PWM controllers for
high-power and high-brightness LEDs (HBLEDs). The
average current-mode control technique offers inherently
stable operation, reduces component derating and size
by accurately controlling the inductor current. The
devices achieve high efficiency at high currents (up to
30A) with a minimum number of external components. A
logic input (MODE) allows the LED driver to switch
between buck and boost modes of operation.
The MAX16821A/MAX16821B/MAX16821C feature a
CLKOUT output 180° out-of-phase with respect to either
the high-side or low-side driver, depending on MODE’s
logic level. CLKOUT provides the drive for a second
out-of-phase LED driver for applications requiring
reduced input capacitor ripple current while operating
another LED driver.
The MAX16821A/MAX16821B/MAX16821C consist of
an inner average current regulation loop controlled by
an outer loop. The combined action of the inner current
loop and outer voltage loop corrects the LED current
errors by adjusting the inductor current resulting in a
tightly regulated LED current. The differential amplifier
(SENSE+ and SENSE- inputs) senses the LED current
using a resistor in series with the LEDs and produces
an amplified version of the sense voltage at DIFF. The
resulting amplified sensed voltage is compared against
an internal 0.6V reference at the error amplifier input.
Input Voltage
The MAX16821A/MAX16821B/MAX16821C operate
with a 4.75V to 5.5V input supply range when the inter-
nal LDO is disabled (V
CC
connected to IN) or a 7V to
28V input supply range when the internal LDO is
enabled. For a 7V to 28V input voltage range, the inter-
nal LDO provides a regulated 5V output with 60mA of
sourcing capability. Bypass V
CC
to SGND with 4.7µF
and 0.1µF low-ESR ceramic capacitors.
The MAX16821A/MAX16821B/MAX16821C’s V
DD
input
provides supply voltage for the low-side and the high-
side MOSFET drivers. Connect V
DD
to V
CC
using an
R-C filter to isolate the analog circuits from the MOSFET
drivers. The internal LDO powers up the MAX16821A/
MAX16821B/MAX16821C. For applications utilizing a
5V input voltage, disable the internal LDO by connect-
ing IN and V
CC
together. The 5V power source must be
in the 4.75V to 5.5V range of for proper operation of the
MAX16821A/MAX16821B/MAX16821C.
Undervoltage Lockout (UVLO)
The MAX16821A/MAX16821B/MAX16821C include
UVLO and a 2048 clock-cycle power-on-reset circuit.
The UVLO rising threshold is set to 4.3V with 200mV
hysteresis. Hysteresis at UVLO eliminates chattering
during startup. Most of the internal circuitry, including
the oscillator, turns on when the input voltage reaches
4V. The MAX16821A/MAX16821B/MAX16821C draw up
to 3.5mA of quiescent current before the input voltage
reaches the UVLO threshold.
Soft-Start
The MAX16821A/MAX16821B/MAX16821C include an
internal soft-start for a glitch-free rise of the output volt-
age. After 2048 power-on-reset clock cycles, a 0.6V
reference voltage connected to the positive input of the
internal error amplifier ramps up to its final value after
1024 clock cycles. Soft-start reduces inrush current
and stress on system components. During soft-start,
the LED current will ramp monotonically towards its
final value.
Internal Oscillator
The internal oscillator generates a clock with the fre-
quency inversely proportional to the value of R
T
(see
the
Typical Operating Circuit
). The oscillator frequency
is adjustable from 125kHz to 1.5MHz range using a sin-
gle resistor connected from RT/SYNC to SGND. The
frequency accuracy avoids the overdesign, size, and
cost of passive filter components like inductors and
capacitors. Use the following equation to calculate the
oscillator frequency:
For 120kΩ≤R
T
≤ 500kΩ:
For 40kΩ≤R
T
≤ 120kΩ:
The oscillator also generates a 2V
P-P
ramp signal for
the PWM comparator and a 180° out-of-phase clock
signal at CLKOUT to drive a second out-of-phase LED
current regulator.
.
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