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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
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.
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
I IN
V OUT I OUT
V IN
the following relationship.
I LED = 200/R SENSE
where R SENSE value is in Ohms.
I LED value is in mA.
I IN - Input current;
I OUT – Output current;
V OUT – Boost output voltage;
V IN – Input voltage;
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
η – Efficiency of t he boost converter.
10
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