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SC4539
Applications Information
General Description
The SC4539 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 SC4539
receives information from the internal LED current regu-
lator 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
regulators 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 pro-
seconds provides the additional advantage of reducing
in-rush current 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 predictable 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 different PWM frequencies.
grammed using an external resistor.
Zero Duty Cycle Mode
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 regula-
tor output is less than 1 μA. This keeps the output
capacitor charged and ready for instant activation of the
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.
LED string.
Protection Features
The 800kHz switching speed provides high output power
while allowing the use of a low profile inductor, maximiz-
ing effi ciency for space constrained and cost-sensitive
The SC4539 provides several protection features to safe-
guard the device from catastrophic failures. These features
include:
applications. The converter and output capacitor are
protected from open-LED conditions by over-voltage
protection.
PWM Dimming
The enable pin can be toggled to allow PWM dimming. In
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Over-Voltage Protection (OVP)
Soft-start
Thermal Shutdown
Current Limit
a typical application, a microcontroller sets a register or
counter that varies the pulse width on a GPIO pin. The
device is compatible with a wide range of applications by
allowing dimming strategies that avoid the audio band
by using a frequency between 100Hz and 50kHz for PWM
dimming. Various intensity levels can be generated while
keeping the instantaneous LED current at its peak value
for luminescent effi ciency and color purity. The SC4539
can accommodate any PWM duty cycle between 0 and
100%. A low duty cycle PWM signal used for a few milli-
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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