SC5010 datasheet(15/36 Pages) SEMTECH

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SC5010

General Description

The SC5010 contains a high frequency, current-mode,

internally compensated boost controller and eight con-

stant current sinks for driving LED strings. The LED current

for all strings is programmed by an external resistor and

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

14 LEDs for each string, with current driven up to 30mA.

The unique control loop of the SC5010 allows fast tran-

sient response in dealing with line and load disturbances.

The SC5010 operating with an external power MOSFET

regulates the boost converter output voltage based on

instantaneous requirement of the eight 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 100Hz

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

when all LED strings are off.

Start-Up

When the EN pin is pulled up high (>2.1V), 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 UVLO pin

voltage is greater than 1.23V, the SC5010 goes into a

startup 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

Application Circuit on page 21) as shown by the following

equations:

IN_TurnOn [V] = 1.23 X (R1 + R2) / R1

IN_Hysteresis [V] = 10

-5

X R

2 [Ω]

In the next phase, the SC5010 checks each IO pin to deter-

mine if the respective LED string is enabled. Each IO pin is

pulled up with a 100μA current source. If any IO pin is con-

nected to GND, it will be detected as an unused string,

and will be turned off. This unused string checking proce-

dure takes 1ms (typical). After this the SC5010 enters into

a soft-start sequence.

The soft-start function helps to prevent excess inrush

current through the input rail during startup. In the

SC5010, 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 SC5010 is controlled by an internal timing

circuit which is used during start-up and it’s based on the

boost converter switching frequency. For example, with

switching frequency at 1MHz, it is typical 8ms and it

becomes typical 4ms when the switching frequency is

2MHz.

If PWMI voltage goes low while the SC5010 is in soft start

operation, the SC5010 switches to standby mode. Under

such mode, the external power MOSFET and the LED

current sources will be turned off immediately. The inter-

nal soft-start timer is turned off and the soft-start value is

saved. When the PWMI voltage goes high again, the soft-

start resumes from the previously saved value.

Each LED current source (IO1 to IO8) 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 typical 0.6V.

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

1μA(max).

In the scenario when the EN pin voltage is high, but either

or VCC voltage falls below their respective UVLO

threshold, the SC5010 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 150°C is reached,

the boost converter and all IO current sources are turned

off. FLT pin is forced low in this situation. As temperature

falls below the TSD trip point by 10°C, the SC5010 will

restart following the startup 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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