RT8482GS Richtek USA Inc, RT8482GS Datasheet - Page 11

• 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

LED load, or with a resistive voltage divider to V
output power and switching current when V
presence of a time varying differential voltage signal (ripple)
across ISP and ISN at the switching frequency is expected.
The amplitude of this signal is increased by high LED load
current, low switching frequency and/or a smaller value
output filter capacitor. The compensation capacitor on the
VC pin filters the signal so the average difference between
ISP and ISN is regulated on the user-programmed value.
Dimming Control
For LED applications where a wide dimming range is
required, two competing methods are available: analog
dimming and PWM dimming. The easiest method is to
simply vary the DC current through the LED by analog
dimming.
However, the better dimming method is PWM dimming,
which switches the LED on and off by different duty cycle
to control the average LED current. The PWM dimming
offers several advantages over analog dimming and is much
preferred by LED manufacturers. One advantage is the
chromatic ity of the LEDs which remains unchanged in
this scheme since the LED current is either zero or at
programmed current. Another advantage of PWM dimming
over analog dimming is that a wider dimming range is
possible.
The RT8482 features both analog and digital dimming
control. Analog dimming is linearly controlled by an
external voltage (0.2V < V
output clamping amplifier and a resistor. PWM dimming
signal fed at the DCTL pin can be easily low-pass filtered
to an analog dimming signal with one external capacitor
from the ACTL pin to GND for noise-free PWM dimming. A
very high contrast ratio true digital PWM dimming can be
achieved by driving the ACTL pin with a PWM signal from
100Hz to10kHz.
Output Over Voltage Setting
The RT8482 is equipped with an Over Voltage Protection
(OVP) function. When the voltage at the OVP pin exceeds
a threshold of approximately 1.18V, the power switch is
turned off. The power switch can be turned on again once
the voltage at the OVP pin drops below 1.18V. For Boost
DS8482-00 August 2010
ACTL
< 1.2V). With an on-chip
IN
IN
is low. The
to reduce
and Buck-Boost applications, the output voltage can be
set by the following equation :
where,
R1 and R2 are the voltage divider resistors from V
GND with the divider center node connected to the OVP
pin.
ISW Sense Resistor Selection
The resistor, R
N-MOSFET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the 110mV (typical) current limit threshold on
the ISW pin of the RT8482. For real applications, select a
resistor that gives a switch current at least 30% greater
than the required LED current.
For Buck application, select a resistor according to :
For Buck-Boost application, select a resistor according
to :
For Boost application, select a resistor according to :
The placement of R
the N-MOSFET and GND of the RT8482. The ISW pin
input to the RT8482 should be a Kelvin connection to the
positive terminal of R
Over Temperature Protection
The RT8482 has an Over Temperature Protection (OTP)
function to prevent excessive power dissipation from
overheating the device. 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 RT8482 should have a
saturation current rating appropriate to the maximum
R
R
R
V
OUT, OVP
SW, Buck
SW, Buck-Boost
SW, Boost
=
= ⎜
= ⎜
1.18
⎛
⎝
⎛
⎝
0.08V
I
V
V
SW
OUT
OUT
= ⎜
IN
×
, between the source of the external
⎛
⎝
⎛
⎜
⎝
×
(V
1
SW
0.08V
SW
⎞
⎟
⎠
×
+
IN
I
R2
OUT
R1
should be close to the source of
.
V
+
IN
⎞
⎟
⎠
V
×
OUT
⎞
⎟
⎠
0.08V
) I
×
OUT
RT8482
⎞
⎟
⎠
www.richtek.com
OUT
11
to