10
Applications Information
SC4540
General Description
The SC4540 contains an 800kHz xed-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 SC4540
receives information from the internal LED current regula-
tor and drives the output to the proper voltage with no
user intervention.
The current owing through the LED string is indepen-
dently controlled by an internal current regulator, unlike
the ballasting resistor methodology that many LED current
regulators use. The internal current regulator can be shut
o 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 1A. 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 pro le 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
SC4540 provides dimming between 100Hz and 50kHz.
The SC4540 is compatible with a wide range of devices by
using dimming technology that avoids 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
e ciency and color purity. The SC4540 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 e ect 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, 10% for 32kHz
PWM frequency and 1% for 200Hz PWM frequency. Refer
to the Normalized I
OUT
versus Duty Cycle in the Typical
Characteristics section for PWM performance across duty
cycle for di erent 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 SC4540 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 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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