MAX16818_09 Datasheet, PDF(16/25 Page) Maxim Integrated Products – 1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

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MAX16818_09 Datasheet, PDF (16/25 Pages) Maxim Integrated Products – 1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

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1.5MHz, 30A High-Efficiency, LED Driver

with Rapid LED Current Pulsing

The oscillator also generates a 2VP-P voltage-ramp sig-

nal for the PWM comparator and a 180Â° 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 32Âµs is

detected and the circuit starts switching from the inter-

nal oscillator. Pulling RT/SYNC to ground for at least

50Âµs 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 (RS), 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 RS 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, RLS. 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.

CCF

RCF

EAOUT

SENSE+

DIFF

AMP

SENSE-

EAN

600mV

DIFF

CSN

CSP

CEA

VEA

VREF + VCM = 1.2V

CCFF

CLP

MAX16818

CPWM

DRIVE

VIN

COUT

LED

STRING

RLS

Figure 7. MAX16818 Control Loop

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