LT3496IFE(RevE) Ver la hoja de datos (PDF) - Linear Technology

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LT3496IFE
(Rev.:RevE)

Triple Output LED Driver

Linear Technology 

LT3496

APPLICATIONS INFORMATION

dimming waveforms and the start-up time should be

checked across all operating conditions.

Open-LED Protection

The LT3496 has open-LED protection for all the three

converters. As shown in Figure 1, the OVP1 pin receives

the output voltage (the voltage across the output capacitor)

feedback signal from an external resistor divider. OVP1

voltage is compared with a 1V internal voltage reference by

comparator A6. In the event the LED string is disconnected

or fails open, converter 1 output voltage will increase, caus-

ing OVP1 voltage to increase. When OVP1 voltage exceeds

1V, the power switch Q1 will turn off, and cause the output

voltage to decrease. Eventually, OVP1 will be regulated to

1V and the output voltage will be limited. In the event one

of the converters has an open-LED protection, the other

converters will continue functioning properly.

Switching Frequency and Soft-Start

The LT3496 switching frequency is controlled by FADJ

pin voltage. Setting FADJ voltage to be less than 1V will

reduce switching frequency.

If FADJ voltage is higher than 1V, the default switch-

ing frequency is 2.1MHz. In general, a lower switching

frequency should be used where either very high or very

low switch duty cycle is required or higher efficiency is

desired. Selection of a higher switching frequency will

allow use of low value external components and yield a

smaller solution size and profile.

Connecting FADJ pin to a lowpass filter (R5 and C4 in

Figure 1) from the REF pin provides a soft-start function.

During start-up, FADJ voltage increases slowly from 0V

to the setting voltage. As a result, the switching frequency

increases slowly to the setting frequency. This function

limits the inrush current during start-up.

Undervoltage Lockout

The LT3496 has an undervoltage lockout circuit that

shuts down all the three converters when the input volt-

age drops below 2.4V. This prevents the converter from

switching in an erratic mode when powered from a low

supply voltage.

Input Capacitor Selection

For proper operation, it is necessary to place a bypass

capacitor to GND close to the VIN pin of the LT3496. A

1μF or greater capacitor with low ESR should be used. A

ceramic capacitor is usually the best choice.

In the buck mode configuration, the capacitor at PVIN has

large pulsed currents due to the current returned though

the Schottky diode when the switch is off. For the best

reliability, this capacitor should have low ESR and ESL

and have an adequate ripple current rating. The RMS

input current is:

( ) IIN(RMS) = ILED • 1– D • D

where D is the switch duty cycle. A 1μF ceramic type ca-

pacitor placed close to the Schottky diode and the ground

plane is usually sufficient for each channel.

Output Capacitor Selection

The selection of output filter capacitor depends on the load

and converter configuration, i.e., step-up or step-down.

For LED applications, the equivalent resistance of the LED

is typically low, and the output filter capacitor should be

large enough to attenuate the current ripple.

To achieve the same LED ripple current, the required filter

capacitor value is larger in the boost and buck-boost mode

applications than that in the buck mode applications. For the

LED buck mode applications, a 0.22μF ceramic capacitor

is usually sufficient for each channel. For the LED boost

and buck-boost applications, a 1μF ceramic capacitor is

usually sufficient for each channel. If higher LED current

ripple can be tolerated, a lower output capacitance can be

selected to reduce the capacitor’s cost and size.

Use only ceramic capacitors with X7R or X5R dielectric,

as they are good for temperature and DC bias stability of

the capacitor value. All ceramic capacitors exhibit loss of

capacitance value with increasing DC voltage bias, so it

may be necessary to choose a higher value capacitor to get

the required capacitance at the operation voltage. Always

check that the voltage rating of the capacitor is sufficient.

Table 1 shows some recommended capacitor vendors.

3496fe

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