SC440AEVB datasheet(12/21 Pages) SEMTECH

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SC440A

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If PWM pin voltage goes low while SC440A is at normal

operation, SC440A will run in standby mode. The internal

switcher and the LED current sources will be immediately

turned off. The SS pin will not be affected by the PWM

signal and remains at its final value.

Main Power Stage Operation

SC440A is an 800kHz fixed-frequency, peak current-

mode boost switching regulator with an integrated 2.5A

(typical), power transistor. Referring to the Block Diagram,

the clock from the oscillation section resets the latch and

turns on the power transistor. Switch current is sensed

with an integrated sense resistor. The sensed current is

summed with the slope-compensating ramp and fed

into the modulating ramp input of the PWM comparator.

The latch is set and the power transistor conduction is

terminated when the modulating ramp intersects the

error amplifier output (COMP).

The current-mode switching regulator is a dual-loop

feedback control system. In the inner current loop, the EA

output (COMP) controls the peak inductor current. In the

outer loop, the error amplifier regulates the output voltage

to keep the LED current in set point. The double reactive

poles of the output LC filter are reduced to a single real

pole by the inner current loop, allowing the simple loop

compensation network to accommodate a wide range of

input and output voltages.

Over-Current Protection

If the switch current exceeds 2.5A (the typical current-

limit trip point), the current-limit comparator I

LIM

will set

the latch and immediately turn off the internal power

switch.

Due to separate pulse-width modulating and

current limiting paths, the OCP trip point is not affected

by slope compensation (i.e., trip point is not affected by

switching duty cycle).

Over-Voltage Protection (OVP)

SC440A includes an external programming over-voltage

protection circuit to prevent damage to the IC and output

capacitor in the event of an open-circuit condition. The

output voltage of the boost converter is detected at the

OVPIN pin. If the voltage at the OVPIN pin exceeds 1.5V,

the boost converter will shut off and a 1mA pull down

current source will be applied to the VOUT pin to quickly

Applications Information (continued)

discharge the over-voltage capacitor. This additional level

of protection prevents a condition where the output

capacitor and Schottky diode must endure high voltage

for an extended period of time, which can pose a reliability

risk for the system. The total resistance of the divider for

the OVP protection should be more than 200kΩ.

The output over-voltage trip point can be programmed

by R2 and R4 resistor divider (see the schematic on page

17). The relationship can be described as follows:

OVPIN_TH

OVP_trip

Where OVPIN_TH is 1.535V typical.

An OVP event causes a fault which disables the boost

converter and enables the strong pulldown and the FFLAG

pin will also go high. Meanwhile, the soft-start capacitor

is discharged. When the soft-start capacitor voltage falls

below 0.5V, SC440A enters a soft-start process.

The OVP detection circuitry provides a disconnect feature

during the shutdown state to prevent any leakage from

the output. The external OVP resistor divider should be

connected between VOUT and OVPRTN while its central

tap is connected to OVPIN. If this disconnect function

is not desired, just connect the end of the OVP resistor

divider directly to GND. The OVPIN pin is sensitive to noise,

a proper decoupling cap (1nF ~ 10nF) is required.

LED Short-Circuit Protection

If one or more LEDs are detected as shortened, that

corresponding string will be latched off if SCP_EN is

floating. The voltages on all internal LED current sources

(IO pins) are monitored to see if any exceeds 2.3V. (The IO

voltage on abnormal LED string will rise earlier than other

floating LED strings). If any IO pin voltage exceeds 2.3V,

that IO current source will be latched off and the FFLAG

will go high. The latch can be reset if VIN falls below UVLO

or recycle EN signal. Other normal LED strings remain at

their normal operation. The protection will be disabled if

SCP_EN is tied to GND. If all IO pin voltages reach 0.8V

then the internal main switch will be off until any of the IO

voltages is lower than 0.7V.

Unused Strings

The SC440A may be operated with less than 6 strings. In

this case, all unused strings should be tied to VIN and

leave the SCP_EN pin floating.

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