RT8482GS Richtek USA Inc, RT8482GS Datasheet - Page 12

• Manufacturer Part Number: RT8482GS
• Description: IC LED DVR BUCK/BOOST 2A 8SOP
• Manufacturer: Richtek USA Inc

Specifications of RT8482GS

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 1028-1005-2

RT8482
switch current selected with the R
inductor value based on operating frequency, input and
output voltage to provide a current mode ramp of
approximately 20mV magnitude on the 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-MOSFET switch is typically chosen for drain
voltage, V
switch on-resistance, R
because switching losses dominate power loss. The
GBIAS regulator on the RT8482 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen such that the
product of Qg at 5V and switching frequency does not
exceed the GBIAS current limit.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for the RT8482
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are also important
parameters for the Schottky diode selection. Choose a
suitable Schottky diode with reverse voltage rating greater
than the 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 temperature, from
the output during the PWM low interval. Therefore, a
Schottky diode with sufficiently low leakage current is
suggested.
www.richtek.com
12
L
L
Buck
Boost
=
=
R
R
DS
SW
SW
, rating and low gate charge. Consideration of
0.02 V
×
0.02 V
×
V
V
OUT
IN
×
×
×
OUT
×
(
IN
V
(
V
OUT
×
IN
×
f
DS(ON),
SW
f
−
SW
−
V
V
OUT
IN
is usually secondary
SW
)
)
resistor. Choose an
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most of the RT8482 applications, a 10μF ceramic capacitor
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.
The maximum power dissipation can be calculated by
following formula :
P
Where T
temperature, T
the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT8482, The maximum junction temperature is 125°C.
The junction to ambient thermal resistance θ
dependent. For WQFN-16L 3x3 packages, the thermal
resistance θ
four layers thermal test board. For SOP-16 packages,
the thermal resistance θ
JEDEC 51-7 four layers thermal test board. The maximum
power dissipation at T
following formula :
C
D(MAX)
OUT
=
= (T
V
J(MAX)
IN
J(MAX)
I
JA
×
OUT
V
A
is 68°C/W on the standard JEDEC 51-7
RIPPLE
is the ambient temperature and the θ
is the maximum operation junction
×
− T
V
OUT
OUT
A
) / θ
×
, is approximately given by the
A
f
SW
JA
JA
= 25°C can be calculated by
is 85°C/W on the standard
DS8482-00 August 2010
JA
is layout
JA
is