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1.5MHz, 30A High-Efficiency, LED Driver
with Rapid LED Current Pulsing
The oscillator also generates a 2V P-P voltage-ramp sig-
nal for the PWM comparator and a 180° out-of-phase
clock signal for CLKOUT to drive a second LED regula-
tor out-of-phase.
Synchronization
The MAX16818 can be easily synchronized by connect-
ing an external clock to RT/SYNC. If an external clock is
present, then the internal oscillator is disabled and the
external clock is used to run the device. If the external
clock is removed, the absence of clock for 32μs is
detected and the circuit starts switching from the inter-
nal oscillator. Pulling RT/SYNC to ground for at least
50μs disables the converter. Use an open-collector
transistor to synchronize the MAX16818 with the exter-
nal system clock.
Control Loop
The MAX16818 uses an average-current-mode control
scheme to regulate the output current (Figure 7). The
main control loop consists of an inner current loop for
controlling the inductor current and an outer current
loop for regulating the LED current. The inner current
loop absorbs the inductor pole reducing the order of the
outer current loop to that of a single-pole system. The
current loop consists of a current-sense resistor (R S ), a
current-sense amplifier (CA), a current-error amplifier
(CEA), an oscillator providing the carrier ramp, and a
PWM comparator (CPWM) (Figure 7). The precision CA
amplifies the sense voltage across R S by a factor of
34.5. The inverting input to the CEA senses the CA out-
put. The CEA output is the difference between the volt-
age-error amplifier output (EAOUT) and the amplified
voltage from the CA. The RC compensation network
connected to CLP provides external frequency compen-
sation for the CEA. The start of every clock cycle
enables the high-side drivers and initiates a PWM on-
cycle. Comparator CPWM compares the output voltage
from the CEA with a 0V to 2V ramp from the oscillator.
The PWM on-cycle terminates when the ramp voltage
exceeds the error voltage. Compensation for the outer
LED current loop varies based upon the topology.
The MAX16818 outer LED current control loop consists
of the differential amplifier (DIFF AMP), reference volt-
age, and VEA. The unity-gain differential amplifier pro-
vides true differential remote sensing of the voltage
across the LED current set resistor, R LS . The differential
amplifier output connects to the inverting input (EAN) of
the VEA. The DIFF AMP is bypassed and the inverting
input is available to the pin for direct feedback. The
noninverting input of the VEA is internally connected to
an internal precision reference voltage, set to 0.6V. The
VEA controls the inner current loop (Figure 6). A feed-
back network compensates the outer loop using the
EAOUT and EAIN pins.
C CF
C CFF
R CF
EAOUT
SENSE+
600mV
CSN
CA
CSP
CLP
MAX16818
V IN
I L
SENSE-
DIFF
AMP
VEA
CEA
CPWM
DRIVE
LED
STRING
EAN
DIFF
Figure 7. MAX16818 Control Loop
V REF + V CM = 1.2V
C OUT
R S
R LS
16
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