RT8452GS Richtek USA Inc, RT8452GS Datasheet - Page 13

• Manufacturer Part Number: RT8452GS
• Description: IC PWM CTRLR LED DVR 16SOP
• Manufacturer: Richtek USA Inc

Specifications of RT8452GS

• Internal Driver: No
• Type - Primary: Backlight, General Purpose
• Mounting Type: Surface Mount
• Topology: PWM, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Frequency: 280kHz ~ 420kHz
• Voltage - Supply: 4.5 V ~ 36 V
• Package / Case: 16-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -

the N-MOSFET and GND of the RT8452. The ISW pin
input to RT8452 should be a Kelvin connection to the
positive terminal of R
Over Temperature Protection
The RT8452 has over temperature protection (OTP)
function to prevent the excessive power dissipation from
overheating. The OTP function will shut down switching
operation when the die junction temperature exceeds
150°C. The chip will automatically start to switch again
when the die junction temperature cools off.
Inductor Selection
The inductor used with the RT8452 should have a
saturation current rating appropriate to the maximum
switch current selected with the RSW resistor. Choose
an inductor value based on operating frequency, input and
output voltage to provide a current mode ramp on ISW pin
during the switch on-time of approximately 20mV
magnitude. The following equations are useful to estimate
the inductor value.
For Buck application :
For Boost application
For Buck-Boost application
Power MOSFET Selection
For applications operating at high input or output voltages,
the power N-MOS FET switch is typically chosen for drain
voltage VDS rating and low gate charge. Consideration of
switch on-resistance, R
because switching losses dominate power loss. The
GBIAS regulator on the RT8452 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen so that the
product of
exceed the GBIAS current limit.
Schottky Diode Selection
DS8452-01
L
L
Buck
Boost
=
=
R
R
SW
SW
Qg
September 2010
0.02 V
×
0.02 V
×
at 5V and switching frequency does not
V
V
OUT
IN
×
×
×
SW
OUT
×
(
IN
V
(
.
V
OUT
×
IN
×
f
DS(ON),
SW
f
−
SW
−
V
V
OUT
IN
is usually secondary
)
)
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for the RT8452
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are the important
parameters for the Schottky diode selection. Choose a
suitable Schottky diode whose reverse voltage rating is
greater than maximum output voltage. The diode’s
average current rating must exceed the average output
current. The diode conducts current only when the power
switch is turned off (typically less than 50% duty cycle).
If using the PWM feature for dimming, it is important to
consider diode leakage, which increases with the
temperature, from the output during the PWM low interval.
Therefore, choose the Schottky diode with sufficiently low
leakage current.
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most the RT8452 applications, a
is sufficient. A value higher or lower may be used
depending on the noise level from the input supply and
the input current to the converter.
In Boost application, the output capacitor is typically a
ceramic capacitor and is selected based on the output
voltage ripple requirements. The minimum value of the
output capacitor C
following equation :
For LED applications, the equivalent resistance of the LED
is typically low and the output filter capacitor should be
sized to attenuate the current ripple. Use of X7R type
ceramic capacitors is recommended. Lower operating
frequencies will require proportionately higher capacitor
values.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum operation junction temperature. The maximum
power dissipation depends on the thermal resistance of
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
C
OUT
=
V
IN
I
×
OUT
V
RIPPLE
×
V
OUT
OUT
×
is approximately given by the
f
SW
10μF
ceramic capacitor
RT8452
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