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OB3350 View Datasheet(PDF) - On-Bright Electronics

Part Name
Description
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OB3350
On-Bright
On-Bright Electronics  On-Bright
OB3350 Datasheet PDF : 10 Pages
1 2 3 4 5 6 7 8 9 10
OB3350
Cost Effective LED Controller
Function Description
General Operation
The OB3350 is a LED driver designed for boost
DC-DC converters in a constant frequency mode.
The controller implements a peak current mode
control scheme and an internal transconductance
amplifier to accurately control the output current
over a wide input and load conditions.
Low frequency PWM dimming input that can
accept an external control signal with a duty ratio
of 1%-100%. OB3350 offers comprehensive
protection features to protect the system in various
fault conditions such as output over voltage
protection, boost diode disconnection protection
and output short circuit protection. The
cycle-by-cycle current limit function limits the
maximum current flowing through the external
MOSFET. The over temperature protection
ensures that the system will not run into condition
of thermal runaway and blow up.
Startup
OB3350 is enabled by applying a voltage of
greater than approximately 2.5V to PWM pin. An
on-chip internal 150k ohm pull down resistor is
inserted between PWM pin and ground. When the
first rising edge is applied to PWM input, also the
pin of PWM is logic high, the OB3350 will power up
immediately, and remains power up until the PWM
input has been hold low for at least 50ms(typ), at
this time the chip will enter sleep mode. At sleep
mode, the power of theOB3350 will be lower than
400uA (typ).
Once OB3350 is enabled the internal 5V regulator
will be activated to source a current less than 4mA
to the whole chip.
When PWM is active high, the OB3350 checks the
topology connection first. The chip monitors the
OVP pin to see if the Schottky diode (boost diode)
is connected or the boost output is short to GND, if
the voltage at OVP pin is lower than 100mV, the
output will be disabled. The OB3350 will also
check other faults (UVLO, OCP and OTP), if there
is no fault, then the boost converter will boost up
the output with the internal soft start.
switching operation when the voltage at pin VIN
increases to a voltage above 7.5V.
LED Current Regulation
The LED current is sensed by current sense
resistors connected between pin FB and GND.
The sensed error signal is amplified and compared
to the CS pin sensing current signal plus the slope
compensation to determine the 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 load changes. The
slope-compensation signal is added to the
current-sense signal to improve the stability at high
duty cycles.
The average LED current is approximated by the
following equation
300[mV ]
I LED [mA] RFB [ ]
At light loading, the OB3350 automatically skips
some pulses to improve efficiency and prevent
overcharging the output capacitor. In this
pulse-skipping mode, the inductor current ramps
up for a minimum on-time (typical 300ns), then
deliver the stored energy to the output. The switch
remains off until another pulse is needed to boost
the output voltage.
Slope Compensation
The OB3350 uses a current mode control scheme.
The main advantages of current mode control are
inherent cycle-by-cycle current limit for the switch
and simpler control loop characteristics. However,
current mode control has an inherent instability for
duty cycles greater than 50%, which is also called
sub-harmonic oscillation. The OB3350 has a
build-in slope compensation to avoid sub-harmonic
oscillation.
UVLO
An under-voltage lockout protection feature with a
hysteresis of about 300mv is provided for VDDH.
When the voltage at this pin exceeds a threshold
of approximately 7.5V (typ), the IC starts the
normal operation. If the voltage at this pin drops
below a threshold of approximately 7(typ) V, the IC
stops switching operation. The IC resumes
Figure4 Sub-harmonic oscillation at duty>50%
and compensation slope to avoid sub-harmonic
oscillation
If the control signal (which applied to the negative
input of the PWM comparator) has no slope and
©On-Bright Electronics
Confidential
-6-
Preliminary datasheet
OB_DOC_DS_335000
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