RT8453BGSP Richtek USA Inc, RT8453BGSP Datasheet - Page 8

• Manufacturer Part Number: RT8453BGSP
• Description: IC LED DVR BUCK 40V 1.5A 8SOP
• Manufacturer: Richtek USA Inc

Specifications of RT8453BGSP

• Constant Current: Yes
• Internal Driver: Yes
• Type - Primary: Backlight, General Purpose
• Mounting Type: Surface Mount
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Frequency: 400kHz ~ 600kHz
• Voltage - Supply: 4.5 V ~ 40 V
• Package / Case: 8-SOIC (0.154", 3.90mm Width) Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -
• Other names: 1028-1004-2

RT8453B
Applications Information
The RT8453B is specifically designed to be operated in
buck converter applications. This device uses a fixed
frequency, current mode control scheme to provide
excellent line and load regulation. The control loop has a
current sense amplifier which senses the voltage between
the VCC and ISN pins and provides an output voltage at
the VC pin. A PWM comparator then turns off the internal
power switch when the sensed power switch current
exceeds the compensated VC pin voltage. The power
switch will not be reset by the oscillator clock in each
cycle. If the comparator does not turn off the switch in a
cycle, the power switch will be on for more than a full
switching period until the comparator is tripped. In this
manner, the programmed voltage across the sense resistor
is regulated by the control loop.
The current through the sense resistor is set by the
programmed voltage and the sense resistance. The voltage
across the sense resistor can be programmed by the
analog or digital signal at the ACTL pin.
The protection schemes in the RT8453B include over
temperature and switch current limit to prevent abnormal
situations.
Frequency Compensation
The RT8453B has an external compensation pin (VC),
allowing the loop response to be optimized for specific
applications. An external resistor in series with a capacitor
is connected from the VC pin to GND to provide a pole
and a zero for proper loop compensation. The typical value
for the RT8453B is 10k and 3.3nF.
Soft Start
The soft-start of the RT8453B can be achieved by
connecting a capacitor from the SS pin to GND.
The built in soft-start circuit reduces the start-up current
spike and output voltage overshoot. The soft-start time is
determined by the external capacitor which is charged by
an internal 6μA constant charging current. The SS pin
directly limits the rate of voltage rise on the VC pin, which
in turn limits the peak switch current.
The value of the soft-start capacitor is user-defined to
satisfy the designer's request.
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LED Current Setting
The LED current can be calculated by the following
equation :
where V
(190mV typ. if ACTL dimming is not applied) and the R2
is the resister between VCC and ISN.
Current Limit Protection
The RT8453B can limit the peak switch current with its
internal over current protection feature. In normal operation,
the power switch is turned off when the switch current
hits the loop-set value. The over current protection function
will turn off the power switch independent of the loop control
when the peak switch current reaches around 1.5A.
Over Temperature Protection
The RT8453B has an Over Temperature Protection (OTP)
function to prevent over heating caused by excessive power
dissipation. The OTP function will shut down switching
operation when the die junction temperature exceeds 150°C
. Once temperature falls below 150°C, the chip will
automatically resume operation again.
Inductor Selection
Choose an inductor that can handle the necessary peak
current without saturating and ensure that the inductor has
a low DCR (copper-wire resistance) to minimize I
losses. A 4.7μH to 22μH inductor will meet the demand of
most of the RT8453B applications.
Inductor manufacturers specify the maximum current rating
as the current where the inductance falls to certain
percentage of its nominal value, typically 65%.
In buck application where the transition between
discontinuous and continuous modes occurs, the value of
the required output inductor (L) can be approximated by
the following equation :
I
L
LED(MAX)
=
⎡
⎢
⎣
f
× Δ
V
CC
OUT
I
L(MAX)
=
− V
V
CC
ISN
R2
⎤ ⎡
⎥ ⎢
⎦ ⎣
−
×
is the voltage between VCC and ISN
V
ISN
1
−
V
IN(MAX)
V
OUT
DS8453B-00 August 2010
⎤
⎥
⎦
2
R power