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OB3350CP View Datasheet(PDF) - On-Bright Electronics

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OB3350CP
On-Bright
On-Bright Electronics  On-Bright
OB3350CP Datasheet PDF : 10 Pages
1 2 3 4 5 6 7 8 9 10
OB3350
Cost Effective LED Controller
the slope which representing inductor current ramp
up until the comp signal is reached, then ramp
down. If the duty > 50%, any distribution won’t be
converged from duty to duty and thus
sub-harmonic oscillation happens. For the
equation below:
I (n 1)
Sf
Sn
Se
Se
I (n)
To avoid sub-harmonic oscillation happening, just
make sure (Sf-Se) / (Sn+Se) smaller than 1.
For boost applications, the slopes Se, Sf and Sn
can be calculated with the formulas below and a
typical value of VSL is 50mV.
Se VSL f s
S f Rsen Vout Vin / L
S n Rsen Vin / L
When Se increases then the factor which can
determine whether the sub-harmonic oscillation
will occur decreases. When the duty cycle is
greater than 50% and the inductance becomes
less, the factor increases. And for more flexibility
slope compensation amplitude can be increased
by adding one external resistor, RSL, in the CS
path. Figure 4 shows the setup. The externally
generated slope compensation is then added to
the internal slope compensation of the OB3350.
When using external slope compensation, the
formula for Se becomes:
Se (VSL I s RSL ) f s
A typical value of Is is 40uA.
Dimming Control
The LED brightness is controlled by the PWM
signal at PWM pin which has different duty cycle.
OB3350 can accept an external PWM signal to
PWM pin in the range of 100Hz to 1 KHz with a
swing voltage of 0V to a level greater than 2.5V.
OVP Protection
OB3350 shuts down the output if the output
voltage is higher than the predetermined threshold
level. In case of LED failing, the current of LED
and the resulted voltage drop on sensing resistor
is nearly zero; the converter will run at maximum
duty cycle that boosts the output voltage to a very
high level. To prevent this happen, the output
voltage is monitored through Pin OVP and
converter is shut down if the output voltage
exceeds the predetermined level. This protection
is latched and OB3350 can be reset from VIN or
PWM.
VOUT SHORT Protection
VOUT short condition is monitored by the voltage
at pins OVP. During normal operation, when the
voltage at OVP pin drops below a threshold of
approximately 0.2V, the VOUT-short protection
function is activated and the GATE is turned off.
This state is latched and can be reset from VIN or
PWM.
Thermal Shutdown
The OB3350 includes a thermal protection circuit.
When the junction temperature exceeds 160°C
(typical), the controller and current sources shut
down and do not restart until the junction
temperature drops below 140°C (typical).
Figure5 Adding external slope compensation to OB3350
It’s good design practice to only add as much
slope compensation as needed to avoid
sub-harmonic oscillation. Additional slope
compensation minimized the influence of the
sensed current in the control loop. With very large
slope compensation the control loop
characteristics are similar to a voltage mode
regulator which compares the error voltage to a
saw tooth waveform rather than the inductor
current.
©On-Bright Electronics
Confidential
-7-
Preliminary datasheet
OB_DOC_DS_335000
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