RT8482GS Richtek USA Inc, RT8482GS Datasheet - Page 10

• 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
Applications Information
The RT8482 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. The LED current can be programmed by an
external resistor. The input voltage range of the RT8482
can be up to 36V and the output voltage can be up to 48V.
The RT8482 provides analog and PWM dimming to achieve
LED current control.
GBIAS Regulator and Bypass Capacitor
The GBIAS pin requires a capacitor for stable operation
and to store the charge for the large GATE switching
currents. Choose a 10V rated low ESR, X7R or X5R
ceramic capacitor for best performance. A 1μF capacitor
will be adequate for most applications.
Place the capacitor close to the IC to minimize the trace
length to the GBIAS pin and also to the IC ground. An
internal current limit on the GBIAS output protects the
RT8482 from excessive on chip power dissipation.
The GBIAS pin has its own Under Voltage Lockout (UVLO)
set to 4.3V (typical) to protect the external FETs from
excessive power dissipation caused by not being fully
enhanced. If the input voltage, VIN, does not exceed 8V,
then the GBIAS pin should be connected to the input
supply. Be aware if GBIAS supply is used to drive extra
circuits in addition to the RT8482. Typically, the extra
GBIAS load should be limited to less than 10mA.
Loop Compensation
The RT8482 uses an internal error amplifier via the
compensation pin (VC) to optimize the loop response for
specific application. The external inductor, output capacitor,
and compensation resistor and capacitor determine the
loop stability. The inductor and output capacitor are chosen
based on performance, size and cost. The compensation
resistor and capacitor at VC are selected to optimize
control loop response and stability. For typical LED
applications, a 3.3nF compensation capacitor at VC is
adequate. A series resistor should always be used to
increase the slew rate on the VC pin to maintain tighter
regulation of LED current during fast transients on the input
supply to the converter. An external resistor in series with
a capacitor is connected from the VC pin to GND to provide
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10
a pole and a zero for proper loop compensation. The typical
compensation for the RT8482 is 10kΩ and 3.3nF.
Soft-Start
The soft-start of the RT8482 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 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 soft-start interval is set by the soft-start capacitor
selection according to the equation :
A typical value for the soft-start capacitor is 0.1μF. The
soft-start capacitor is discharged when EN/UVLO falls
below its threshold, during an over-temperature event or
during a GBIAS under-voltage event.
LED Current Setting
The LED current is programmed by placing an appropriate
valued current sense resistor between the ISP and ISN
pins. Typically, sensing of the current should be done at
the top of the LED string. The ACTL pin should be tied to
a voltage higher than 1.2V to get the full-scale 190mV
(typical) threshold across the sense resistor. The ACTL
pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
voltage sense threshold. When the ACTL pin voltage is
less than 1.2V, the LED current is :
where,
R
When the voltage of ACTL is higher than 1.2V, the LED
current is regulated to :
The ACTL pin can also be used in conjunction with a
thermistor to provide over-temperature protection for the
I
I
t
LED(MAX)
LED
SS
SENSE
=
=
C
(V
is the resistor between ISP and ISN.
SS
ACTL
=
×
R
190mV
R
2.4V
6 A
SENSE
μ
SENSE
−
0.2) 0.19
×
DS8482-00 August 2010