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Applications Information

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. This keeps the output capacitor

charged and ready for instant activation of the LED

string.

The 800kHz switching speed provides high output power

while allowing the use of a low profile inductor, maximiz-

ing efficiency for space constrained and cost-sensitive

applications. The converter and output capacitor are pro-

tected 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.

The SC4538 is compatible with a wide range of devices,

allowing 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 accommodate

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 activation of the internal current regulator

causes nonlinearity between the I

BL

current and the duty

cycle of the PWM frequency seen by the EN pin. As the

PWM signal frequency increases, the total on time per

cycle of the PWM signal decreases. Since the start up delay

time remains constant, the effect of the delay becomes

more noticeable, causing the average I

BL

to be less predict-

able at lower duty cycles. Recommended minimum duty

cycles are 20% for 50kHz PWM frequency, 15% for 32kHz

PWM frequency and 2% for 200Hz PWM frequency. Refer

to the I

BL

vs. Duty Cycle in the Typical Characteristics

section for PWM performance across duty cycle for differ-

ent PWM frequencies.

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 V

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)

A built-in over-voltage protection circuit prevents 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 inter-

nally. 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 capacitor and Schottky diode

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