SC4541 datasheet(10/19 Pages) SEMTECH

10 / 19 page

SC4541

Applications Information

Operation

The SC4541 is a fixed frequency step-up current-mode

switching regulator with an integrated Schottky diode

and power transistor (see the Block Diagram for more

information).

When EN is low, the device is in shutdown mode and

draws less than 1μA of current. The internal circuitry is

enabled and turns on when EN is logic high. The voltage

drop across OUT and FB is reflected to ground and fed to

the negative input of the error amplifier and a 200mV

reference signal is fed to the positive input of the error

amplifier. The output of the error amplifier is one input to

the PWM comparator. The device is designed to be stable

without additional external compensation components.

Switch current is sensed with an integrated sense resistor

and is summed with the slope-compensating ramp which is

then fed as the modulating ramp input to the PWM com-

parator. The latch is reset and Q1 is turned off when the

modulating ramp intersects the error amplifier output. The

latch will also be reset if the current in Q1 exceeds the

current limit threshold, typically 350mA. The latch is set and

Q1 turns on at the beginning of every oscillator clock cycle.

The SC4541 has a built-in Schottky diode. When input

voltage is applied an inrush current flows through the

inductor and the Schottky diode and charges up the

output capacitor connected to OUT. The Schottky diode

inside the SC4541 can sustain a maximum current of 1A.

Programming the LED Current

The circuit will maintain the necessary output voltage to

ensure 200mV across OUT and FB pins. Refer to the Typical

Application Circuit on page 1. The LED current is pro-

grammed by connecting a resistor (R

SENSE) between the

OUT pin and FB pin. The LED current, in mA, is set by using

the following relationship.

LED = 200/RSENSE

where R

SENSE value is in Ohms.

LED value is in mA.

The feedback voltage has a direct effect on the converter

efficiency. Because the voltage drop across the feedback

resistor does not contribute to the output power (LED

brightness), the lower the feedback voltage, the higher

the efficiency. The internal oscillator sets a high switching

frequency of 2.0MHz which further reduces the total solu-

tion size by minimizing external L/C filter size.

Over-Voltage Protection (OVP)

An integrated over-voltage protection circuit prevents the

OUT voltage from exceeding the maximum switch voltage

rating (28V). When EN is high, a resistor divider from OUT

to GND is switched in to monitor OUT for over-voltage

protection. If OUT rises higher than the OVP threshold,

typically 27V, the OVP comparator will trip and reset the

latch to turn Q1 off. When OUT drops back below the OVP

threshold, Q1 will turn back on in the next oscillator cycle.

This sequence will repeat for as long as the over-voltage

condition persists.

Enable and PWM Dimming

The EN pin could be toggled for direct PWM dimming. In

a typical application, a microcontroller sets a register

which varies the pulse width on a GPIO which is tied to EN

pin. Direct PWM dimming can be used to control LED

brightness when a PWM signal (up to 1kHz) with wide

duty ratio is applied.

Inductor Selection

The inductance value affects the converter’s steady state

operation, transient response, and its loop stability.

Attention must be given to inductor parameters; includ-

ing inductance, DC resistance, and saturation current. The

inductance and operating conditions determine the

inductor ripple current. The converter can operate in con-

tinuous conduction mode (CCM) or discontinuous con-

duction mode (DCM). The inductor DC current or input

current can be calculated using the following equation.

OUT

IN - Input current;

OUT – Output current;

OUT – Boost output voltage;

IN – Input voltage;

η–Efficiencyofthe boost converter.

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