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OB3362FHPA View Datasheet(PDF) - Unspecified

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OB3362FHPA Datasheet PDF : 13 Pages
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OB3362F
4 Channels High Performance LED Driver
Function Description
load changes. The slope-compensation signal is
added to the current-sense signal to improve the
General Operation
The OB3362F LED driver is designed for LCD
backlight system applications. It incorporates a
fixed frequency, current mode boost converter, a
10V regulator specially to power the gate driver,
dimming control circuits and 4 channels current
sources which regulate the current of LED array
made up by 4 LED strings. When enabled, the
converter boosts the output voltage to provide
sufficient headroom for the current sources to
regulate their respective string current. OB3362F
features programmable operation frequency
(100KHz to 600KHz) by setting the resistor on RI
pin which allows the trade-off between the
stability at high duty cycles.
The OB3362F has a gate driver which can provide
10V high level voltage (when VCC=10V), this can
greatly extend the select range of external power
MOSFET as well as increase the overall system
efficiency.
At light loads, the OB3362F automatically skips
T some pulses to improve efficiency and also
V prevent overcharging the output capacitor. In the
I pulse-skipping mode, the inductor current ramps
up for a minimum on-time (typical 140ns), then
deliver the stored energy to the output. The switch
o remains off until another pulse is needed to boost
t the output voltage.
external components and the efficiency.
Internal burst and external PWM dimming modes
l are provided to offer the flexible dimming control.
OB3362F offers comprehensive protection
a features to protect the system in various fault
i conditions. The open/short LED string protection
ensures that one or more LED strings fail do not
t cause overall LED array out of function. The
n cycle-by-cycle current limit function limits the
maximum current flowing through the external
e power MOSFET. The over temperature protection
d ensures that the system will not run into condition
of thermal runaway and blow up.
i The principle of operation can be understood by
f referring to the block diagram.
n Internal 10V Regulator VCC and UVLO
o The OB3362F includes an internal switching mode
C regulator which is specially used to power the gate
LED Current Sources
Maintaining uniform LED brightness and dimming
capability are critical for LCD backlighting
applications. The OB3362F is equipped with 4
matched current sources. These specialized
current sources are accurate to within 3%. All LED
full-scale currents are identical by setting through
the resistor at ISET pin (18mA < ILED < 200mA).
I LED
=
3000 ×
1.2V
RSET
The minimum voltage drop across each current
source is approximately 350mV. The low voltage
drop helps to reduce the dissipation while
maintaining sufficient compliance to control the
LED current within the required tolerances.
If any one channel out of the total 4 channels is not
used, the relative LEDX pin should be connected
to a voltage that higher than 5.5V to disable this
driver. When the VIN is great than 10V, the
regulator generates a 10V supply. If VIN smaller
t than 10V, the output of regulator can follow VIN
voltage until VIN drop below the UVLO voltage
h threshold. The OB3362F also has an internal 5V
g regulator to power the rest analog and logic
circuitry.
i The OB3362F have Under Voltage Lockout
r (UVLO) function. The chip is disabled when the
Vin is below the UVLO threshold (typically 5.2V)
B and the hysteresis of UVLO is nearly 200mV.
-Fixed-Frequency Boost converter
nA fixed-frequency, current mode, boost converter
Ois built in and it automatically chooses the lowest
channel, the LEDX voltage must lower than 40V to
prevent IC be damaged.
All LEDX pins in use are measured and the lowest
voltage is extracted and fed into the step-up
converter’s error amplifier to regulate the step-up
converter’s output voltage.
Frequency Setting
The internal oscillator frequency is set by the
resistor on at RI pin as shown in the equation.
High frequency operation optimizes the regulator
for small component size at the expense of
efficiency due to increased switching losses. While
low frequency operation offers the better overall
efficiency, but requires larger components and
active LEDX voltage to regulate the output voltage. more PCB area.
The resulting error signal is amplified and
compared to the internal sensing current signal
plus the slope compensation to determine the
FOSC
=
67850
RRI (kΩ)
on-time of the switching MOSFET. The error
amplifier sources or sinks the current to the COMP
pin to adjust the required inductor current as the
©On-Bright Electronics
Confidential
-7-
Preliminary Datasheet
OB_DOC_DS_3362F00
 

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