SC5014AEVB datasheet(12/27 Pages) SEMTECH

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SC5014A

General Description

The SC504A contains a high frequency, current-mode,

internally compensated boost controller with 2 constant

current sinks for driving LED strings. The LED current for

both strings is programmed by an external resistor. The

boost converter operates to maintain minimal required

output voltage for regulating the LED current to the pro-

grammed value. A typical backlight application uses 3 to

4 LEDs per each string, with current driven up to 240mA.

The unique control loop of the SC504A allows fast tran-

sient response in dealing with line and load disturbances.

The SC504A, operating with an external power MOSFET,

regulates the boost converter output voltage based on

the instantaneous requirement of the 2 string current

sources. This provides power to the entire lighting subsys-

tem with increased efficiency and reduced component

count. It supports PWM dimming frequencies from 00Hz

to 30kHz and the supply current is reduced to 2mA typical

when both LED strings are off.

Start-Up

When the EN pin is pulled up high (>2.V), the device is

enabled and the UVLO and VCC pin voltages are checked.

The VCC voltage has fixed under-voltage rising and falling

trip points. If the VCC pin is higher than 4.2V and the UVLO

pin voltage is greater than .23V, the SC504A goes into a

start-up sequence. The UVLO pin voltage can be used to

program the input power source voltage VIN turn-on

threshold and its hysteresis (refer to the detailed applica-

tion circuit on page 20, Figure 2) as shown by the follow-

ing equations:

IN_TurnOn [V] = .23 X (R + R2) / R

IN_Hysteresis [V] = 0

-5

X R

2 [Ω]

In the next phase, the SC504A checks each IO pin to

determine if the respective LED string is enabled. Each IO

pin is pulled up with a 00µA current source. If any IO pin

is connected to ground, it will be detected as an unused

string, and will be turned off. This unused string checking

procedure typically takes ms. After this, the SC504A

enters into a soft-start sequence.

The soft-start function helps to prevent excess inrush

current through the input rail during start-up. In the

SC504A, the soft-start is implemented by slowly ramping

up the reference voltage fed to the error amplifier. This

closed loop start-up method allows the output voltage

to ramp up without any overshoot. The duration of the

soft-start in the SC504A is controlled by an internal

timing circuit, which is used during start-up and is based

on the boost converter switching frequency. For example,

with switching frequency at MHz, it is 8ms typical and

becomes 4ms typical when the switching frequency is

2MHz.

If the PWM voltage goes low while the SC504A is in soft-

start operation, the SC504A switches to standby mode,

where the external power MOSFET and the LED current

sources will be turned off immediately. The internal soft-

start timer is turned off and the soft-start value is saved.

When the PWM voltage goes high again, the soft-start

resumes from the previously saved value.

Each LED current source tries to regulate the LED current

to its set point. The control loop will regulate the output

voltage such that all the IO pin voltages are at least 0.9V

typical.

Shutdown

When the EN pin is pulled down below 0.8V, the device

enters into shutdown mode. In this mode, all the internal

circuitry is turned off and the supply current is less than

µA (max).

In the scenario where the EN pin voltage is high, but VCC

voltage falls below the respective UVLO threshold, the

SC504A goes into a suspend mode. In this mode, all the

internal circuitry except the reference and the oscillator

are turned off.

Thermal Shutdown (TSD)

If the thermal shutdown temperature of typical 50°C is

reached, the boost converter and all IO current sources

are turned off. The FLT pin is forced low in this condition.

When the temperature falls below the TSD trip point by

0°C, the SC504A will restart following the start-up

sequence as described before. The FLT pin is latched and

will stay low, it is reset by cycling the EN, VCC or UVLO.

Applications Information

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