RT4503/A
Application Information
Soft-Start
PWM Dimming
The RT4503/A includes a soft-start function to avoid high
inrush current during start-up. The soft-start function is
achieved by clamping the output voltage of the error
amplifier with another voltage source that is increased
slowly from zero to near VIN.
When the EN pin is constantly high, the FB voltage is
regulated to 200mV typically. The RT4503 allows a PWM
signal from EN pin to reduce the regulation voltage; and
achieve LED brightness dimming. The relationship
between the duty cycle and FB voltage is given by
VFB = Duty VREF
LED Current
Where :
The loop control of the Boost converter keeps VFB equal
to a reference voltage, VREF. Therefore, when RSET is
connected between the FB pin andGND, the LEDcurrent
will be determined by the current through RSET, which is
Duty = duty cycle of the PWM signal
VREF = internal reference voltage (200mV typ.)
The RT4503 chops up the internal 200mV reference voltage
via the duty cycle of the PWM signal. Therefore, although
a PWM signal is used for brightness dimming, only the
LEDDC current is modulated, which is often referred as
analog dimming. This eliminates the audible noise which
often occurs when the LED current is pulsed in replica of
the frequency and duty cycle of PWM control. For optimum
performance, use the PWM dimming frequency in the
range of 1kHz to 100kHz.
equal to VFB / RSET
.
Current Limit
The current flowing through the inductor during a charging
period is detected by a current sensing circuit. If the value
exceeds the current limit, the N-MOSFET will be turned
off. The inductor will then be forced to leave charging stage
and enter discharging stage. Therefore, the inductor current
will not increase to reach current limit.
Shutdown Delay
When the EN voltage is in logic low for 20ms during PWM
dimming for RT4503 and 2ms during pulse dimming for
RT4503A, the system will enter shutdown.
1kHz < f
< 100kHz
T
> 20ms
PWM
SHDN
Period1
Period2
EN
FB
Duty1
Duty2
Duty1 x 200mV
Duty2 x 200mV
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS4503/A-03 June 2015
www.richtek.com
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