LT3755 datasheet(13/30 Pages) LINEAR

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LT3922

3922f

For more information www.linear.com/LT3922

operaTion

The LT3922 is a step-up LED driver that utilizes a 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 providing power

to the top switch driver. The switches connect an external

inductor at 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

frequencymodulation(SSFM),whichreducesradiatedand

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

oscillatorwhichstabilizestheinductorcurrent.Anetworkof

passive components 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 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

resistorbetweenthesepinsshouldbeplacedinserieswith

the string of LEDs such that the voltage across it provides

the feedback to regulate the LED current. The current

regulation amplifier then compares the actual LED current

to the desired LED current and adjusts VC as necessary.

The voltage regulation amplifier overrides the current

regulation 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 VOUTandthestringofLEDs.Anexternalpulse

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 LT3922 preserves the

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

recovery for the next pulse.

applicaTions inForMaTion

The following is a guide to selecting the external com-

ponents and configuring the LT3922 according to the

requirements of an application.

Programming LED Current with the CTRL Pin

The primary function of the LT3922 is to regulate the cur-

rent in a string of LEDs. This current should pass through

a series current sense resistor. The voltage across this

resistor is sensed by the current regulation amplifier

through the ISP and ISN pins and regulated to a level pro-

grammed by the CTRL pin. The maximum resistor voltage

that can be programmed is 100mV which corresponds to

1A through the LED string when a 100mΩ current sense

resistor is used.

To allow for this maximum current, the CTRL pin may

be connected directly to the VREF pin which provides

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