10

SC4538

Applications Information

General Description

The SC4538 contains an 800kHz fixed-frequency current-

mode boost converter and an independent LED current

regulator. The LED current set point is chosen using an

external resistor, while the PWM controller operates inde-

pendently to keep the current in regulation. The SC4538

receives information from the internal LED current regula-

tor and drives the output to the proper voltage with no

user intervention.

The current flowing through the LED string is indepen-

dently controlled by an internal current regulator, unlike

the ballasting resistor scheme that many LED current reg-

ulators use. The internal current regulator can be shut off

entirely without leaking current from a charged output

capacitor or causing false-lighting with low LED count and

high V

IN

. The backlight current (I

BL

) is programmed using

an external resistor.

The path from the EN pin to the output control is a high

bandwidth control loop. This feature allows the PWM

dimming frequency to range between 100Hz and 50kHz.

In shutdown mode, leakage through the current regulator

output is less than 1μA, keeping the output capacitor

charged and ready for instant activation of the LED

strand.

The 800kHz switching speed provides high output power

while allowing the use of a 1.0mm-high inductor, maxi-

mizing efficiency for space constrained and cost-sensitive

applications. In addition, the converter and output capaci-

tor are protected from open-LED conditions by

over-voltage protection.

PWM Dimming

The enable pin can be toggled to allow PWM dimming. In

a typical application, a microcontroller sets a register or

counter that varies the pulse width on a GPIO pin. The

SC4538 provides dimming between 100Hz and 50kHz,

allowing for compatibility with a wide range of devices,

including dimming strategies that avoid the audio band

by using high frequency PWM dimming. A wide range of

illumination can be generated while keeping the instanta-

neous LED current at its peak value for luminescent

efficiency and color purity. The SC4538 can acomodate

any PWM duty cycle between 0 and 100%. A low duty

cycle PWM signal used for a few milliseconds provides the

additional advantage of reduced in-rush at start up.

The start-up delay time between the enable signal going

high and the internal current regulator activating causes a

small offset dependent upon the PWM frequency seen by

the EN pin. As the PWM signal frequency goes higher, the

effect of the delay becomes noticeable.

Zero Duty Cycle Mode

Zero duty cycle mode is activated when the voltage on

the BL pin exceeds 1.3V. In this mode, the COMP pin

voltage is pulled low, suspending all switching. This allows

the V

OUT

and V

BL

voltages to fall. The COMP voltage is held

low until the V

BL

falls below 1V, allowing COMP to return to

its normal operating voltage and switching to resume.

Protection Features

The SC4538 provides several protection features to safe-

guard the device from catastrophic failures. These features

include:

Over-voltage protection (OVP)

Soft-start

Thermal shutdown

Current Limit

Over-Voltage Protection (OVP)

The SC4538 includes a built-in 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 OUT pin

and divided internally. If the voltage at the OUT pin

exceeds the OVP limit, the boost converter is shut down,

and a strong pull down is applied to the OUT pin to quickly

discharge the output capacitor. This additional level of

protection prevents a condition where the output capaci-

tor and Schottky diode must endure high voltage for an

extended period of time.

Soft-Start

The soft-start mode reduces in-rush current by utilizing

the external compensation network. As the error amplifier

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