LT3922 datasheet(14/28 Pages) AD

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LT8386

Rev. 0

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OPERATION

The LT8386 is a Silent Switcher step-up LED driver that

utilizes fixed-frequency peak current control to accurately

regulate the current through a string of LEDs. It includes

two power switches, their drivers, and a diode for provid-

ing power to the top switch driver. The switches connect

the external inductor terminal connected to the SW pin

alternately to the ground and then to the output (VOUT).

The inductor current rises and falls accordingly and the

peak current can be regulated by adjusting the duty ratio

of the power switches through the combined effect of the

other circuit blocks.

The synchronous controller ensures the power switches

do not conduct at the same time, and a programmable

oscillator turns on the bottom switch at the beginning of

each switching cycle. The frequency of this oscillator is set

by an external resistor at the RT pin and can be overrid-

den by external pulses at the SYNC/SPRD pin. The SYNC/

SPRD pin can also be used to command spread spectrum

frequency modulation (SSFM), which reduces radiated

and conducted electromagnetic interference (EMI).

The bottom switch is turned off by the peak current com-

parator which waits during the on-time for the increasing

inductor current to exceed the target set by the voltage

at the VC pin. This target is modified by a signal from the

oscillator which stabilizes the inductor current. A network

of passive components at the VC pin is necessary to sta-

bilize this regulation loop.

The target for the inductor current is derived from the

desired LED current programmed by the voltage at the

CTRL pin. The analog-or-digital detector and the control

buffer convert either a DC voltage or duty cycle of pulses

at the CTRL pin into the input for the current regulation

amplifier. The other input to this amplifier comes from the

ISP and ISN pin voltages. An external current sense resis-

tor between these pins should be placed in series with the

string of LEDs such that the voltage across it provides the

feedback to regulate the LED current. The current regula-

tion amplifier then compares the actual LED current to the

desired LED current and adjusts VC as necessary.

The voltage regulation amplifier overrides the current reg-

ulation amplifier when the FB pin voltage is higher than

an internal 1.2V reference. An external resistor network

from the LED string to the FB pin provides an indication

of the LED string voltage and allows the voltage amplifier

to prevent overvoltage of the LED string.

The ISP, ISN, and FB pin voltages are also monitored to

detect fault conditions like open and short circuits, which

are then reported by pulling FAULT pin low. The response

to a fault can be selected either to try hiccup restarts or to

latchoff by the choice of an external resistor connected to

the SS pin. Refer to the Applications Information section

for a detailed explanation of fault responses.

Finally, pulse-width modulation (PWM) of the LED current

is achieved by turning on and off an external PMOS switch

between the VOUT and the string of LEDs. An external

pulse at the PWM pin controls the state of the PWM driver

or a DC voltage at the PWM pin dictates the duty ratio of

an internal PWM pulse, whose frequency is programmed

with an external resistor at the RP pin. After each pulse,

when the PMOS switch opens, the LT8386 controls the

voltages of the capacitors at VC and VOUT to ensure a rapid

recovery for the next pulse.

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