LT3476 datasheet(21/36 Pages) AD

0.45A • fSW

⎛

⎝⎜

⎞

⎠⎟

(13)

Table 4 provides some recommended inductor vendors.

Table 4. Inductor Manufacturers

VENDOR

WEB

Wurth Elektronik

www.we-online.com

Coilcraft

www.coilcraft.com

Cooper

www.cooperet.com

Output Capacitor Selection

In addition to smoothing the output voltage, the output

capacitor in combination with the small signal forward

resistance of the LEDs provides an output pole for the

frequency compensation.

The LED forward resistance (RLED) is determined from

the LED data sheet, and is roughly 10Ω for the case of

a typical 150mA LED. Forward resistance is highest at

low currents, and lowest at the maximum drive current.

The total forward resistance of a series strand of LED is

n • RLED, where n is the number of LEDs in the strand.

For the LT3966, we choose COUT to keep the minimum

frequency of the output pole > 2kHz, Equation 14.

COUT ≤

12500 •n • RLED

(14)

In most cases, a 2.2µF output capacitor is a suitable choice.

Schottky Rectifier Diode

The power Schottky diode conducts the switching current

during the power switch off time. Select a diode rated for

at least 1.5 • ILED to provide operating margin. The reverse

breakdown voltage should be at least 20% greater than

the maximum output voltage expected in circuit. Keep in

mind that in the case of disconnected LEDs, the output

voltage will be driven to the limit defined by the FB divider.

FAULTS AND FAULT HANDLING

Status Bits and ALERT

The LT3966 has independent fault handling for each

LED driver channel. Four types of faults are detected:

LED Overcurrent, Shorted LED, Open LED, and Output

Overvoltage. Additional information on the detection con-

ditions is provided in the LED current sense amplifier, and

FB amplifier sections.

As described, each type of fault is indicated in a channel’s

status bit and can optionally be indicated on the open-

drain ALERT pin through the setting of fault enable bits in

the same register. When a fault’s enable bit is set, the fault

status will be latched and the ALERT pin will be asserted

if a fault is detected. Write a 0 to the status bit to clear, or

write a 0 to the status bit enable bit to clear and disable

the fault. If a fault’s EN mask bit is not set, reading the

status bit will always give the status of the fault at that

time, but the status will not be latched nor indicated on

the ALERT pin.

In standalone mode, the mask bits are ignored and the

logical OR of all faults is indicated on the ALERT pin. This

indicator is not latched, and will only be asserted as long as

a fault is present. The ALERT pin will return to high imped-

ance when no faults are detected in standalone mode.

Hiccup and Latchoff Mode

The LED overcurrent and shorted LED conditions result in

an internal fault response that is dependent on the state

of the channels’ latchoff (LATOFF) bit. In the case of one

of these faults, switching is terminated, the channel’s TG

pin is pulled high to disconnect the output, and the device

waits for a 7168-cycle cooldown period.

At this point if the channel’s LATOFF bit is set, the device

will stay in this non-switching rest state until reset by tog-

gling the EN pin or system power, toggling the channel’s

OFF bit, or by clearing the LATOFF bit itself.

If the channel’s LATOFF bit is not set, the device will

attempt a new soft-start cycle after completion of the

cooldown period. Sustained faults will result in continuing

cooldown and retry attempts, often referred to as “hic-

cup” mode (see Figure 17 and Figure 18).

LT3476 Datasheet(PDF) 21 Page - Analog Devices

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