SC4509
POWER MANAGEMENT
Block Diagram
PRELIMINARY
SW
Internal Schottky
FB
VIN
V
REF
1.25V
199mV
VOUT
PWM
R
S
Q
OVP
∑
S
EN/SS
Ramp
Generator
1.2MHz
Oscillator
GND
Theory of Operation
The SC4509 is a constant frequency PWM step-up regu-
lator IC designed to operate with an input voltage range
from 2.7V to 10V.
A maximum output voltage of 20V allows for up to
fi
ve LEDs
to be connected in series, providing even illumination by
passing the same output current through each LED.
Designed for a low 199mV, the feedback voltage has a
direct effect on the converter efficiency. Because the volt-
age drop across the feedback resistor does not contribute
to the output power (LED brightness), the lower the feed-
back voltage, the higher the efficiency. A 1.2MHz oscillator
frequency saves board space by reducing external compo-
nent sizes, making the SC4509 ideal for battery-operated
products.
The SC4509 can be configured as a voltage regulated
boost converter or as a current-regulating boost converter
for driving white LEDs. The device is designed to be stable
without additional external components in both cases.
©
2007 Semtech Corp.
Over-Voltage Protection (OVP)
To prevent the output voltage from exceeding the maxi-
mum switch voltage rating (30V), an over-voltage protec-
tion circuit is integrated. As soon as the output voltage
exceeds the OVP threshold, the converter stops switching
and the output voltage starts to drop. When the output
voltage drops back below the OVP threshold, the convert-
er resumes operation. This sequence will repeat for as
long as is necessary.
Enable and PWM Dimming
The SC4509 provides the EN/SS pin to serve multiple
functions. Its main function is as an enable pin (active
high). It also can be used to control LED brightness
when a PWM signal (up to 10kHz) of varying duty cycle is
applied. A third function acts as a soft-start control. Soft
start can be implemented by ramping the voltage applied
to the pin at startup or by slowly increasing the PMW duty
cycle.
6
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