RT4531
b. PWM Brightness Dimming
it achieves LED brightness dimming. The relationship
between the PWM duty cycle and IFBx current is given by
Besides programmable built-in 1-wire backlight LED
current control, the RT4531 features a built-in PWM
dimming current control by PWM pin, offering a linear
current dimming by external clock source. In order to
guarantee the PWM dimming resolution, recommending
PWM dimming frequency have to be operated at range of
20kHz to 100kHz. The PWM mode ramp time is 64μs
each step.
Equation 3.
I
I Duty %
FB
(3)
FBX
Where :
IFBX is the current of each current sink, IFB is the full-
scale LED current, Duty is the duty cycle information
detected from the PWM signals.
When the PWM pin is constantly high, the dual channel
current is regulated to full-scale according to Equation 1.
The PWM pin allows PWM signals to reduce this
regulation current according to the PWM duty cycle;
therefore,
L1
4.7µH
D1
V
IN
2.5V to 5.5V
C1
1µF
C3
1µF / 50V
R1
10
SW
VIN
C2
1µF
RT4531
2P10S
WLEDs
EN
PWM
IFB1
IFB2
ISET
GND AGND
R2
63.4k
Figure 2. RT4531 PWM Dimming Typical Application
OCP Protection
Open LED Protection
The RT4531 features a 1.5A OCP. The driver provides
cycle-by-cycle current limit function to control the current
on power switch. The boost switch turns off when the
inductor current reaches this current threshold and it
remains off until the beginning of the next switching cycle.
This protects the RT4531 and external component under
overload conditions.
Open LEDprotection circuitry prevents IC damage as the
result of LED disconnection. The RT4531 monitors the
voltage at the LX pin during each switching cycle. The
circuitry turns off the switch and shuts down the IC as
soon as the LX voltage exceeds the VOVP threshold. The
device remains in shutdown mode until it is enabled by
toggling the EN pin logic. The RT4531 is designed to work
with inductor values for 4.7μH (TaiyoNR4018T4R7M) and
Cout values for 1μF/50V (Murata 0603/X5R)that the OVP
is 37.5V typ. Generally, the VF of the backlight LED is
about 3.1 to 3.3V, the RT4531 can support up to 10 LEDs
and it would not be trigger OVP.
OTP Protection
The OTP function will disable boost and current source
when the junction temperature exceeds 150°C. Then
restart to soft-start when the junction temperature is
smaller than 135°C.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS4531-01 July 2016
www.richtek.com
11
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