LTM8042 datasheet(12/22 Pages) AD

12 / 22 page

LT3935

Rev. 0

For more information www.analog.com

OPERATION

The LT3935 is a step-down 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 charge pump for providing

power to the top switch driver. The switches connect an

external inductor at the SW pin alternately to the input

supply and then to ground. 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 top 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 overridden

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

SPRD pin can also be used to command spread spectrum

frequencymodulation(SSFM),whichreducesradiatedand

conducted electromagnetic interference (EMI).

Thetopswitchisturnedoffbythepeakcurrentcomparator

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 capacitor at the VC

pin is necessary to stabilize 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-to-digital detector and the control

buffer convert either a DC voltage or digital pulses at the

CTRL pin into the input for the current regulation ampli-

fier. The other input to this amplifier comes from the ISP

and ISN pin voltages. An external current sense resistor

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

programmed LED current and adjusts VC as necessary.

The voltage regulation amplifier overrides the current

regulation amplifier, when the FB pin voltage approaches

an internal 1V 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 FB voltage is also monitored to detect fault conditions

like open and short-circuits, which are then reported by

pulling the FAULT pin low. The response to a fault can be

selected either to try hiccup restarts or to latch-off by the

choice of an external resistor connected to the SS pin.

Refer to Applications Information for a detailed explana-

tion of fault responses.

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