Kingbor Technology Co.,Ltd
KB4312
TEL:(86)0755-26508846 FAX:(86)0755-26509052
Soft-Start
Soft-start allows a gradual increase of the internal current-limit level for the step-up converter during
power-up to reduce input surge currents. As the internal current source charges the internal soft-start
capacitor, the peak N-MOS current is limited by the voltage on the capacitor. In another story, when
toggle or a logic-level transition on EN pin from low to high, soft-start function must work to enable
constant current charging internal capacitor. When soft-start process has finished or appeared falling
edge of PWM signal on EN pin , soft-start capacitor must be discharged to ground level.
Cycle-by-Cycle Over-Current Protection
The KB4312 provides cycle-by-cycle over-current protection. Current limit is accomplished using a
separate dedicated comparator. The cycle-by-cycle current limit abbreviates the on-time of the N-MOS
in event that the current of flowing N-MOS is greater than the current limit value. The current-limit
feature protection against a hard short or over-current fault at the output.
Over-Voltage Protection
If VOVP is above 17V or LEDs are disconnected from the circuit, the FB pin is similar to pull down to
ground with a sense resistor. This will cause N-MOS to switch with a maximum duty cycle and come
out output over-voltage. This may cause the LX pin voltage to exceed its maximum voltage rating to
damage built-in N-MOS. In the state, the OVLO protection circuitry stops the internal N-MOS .
Power dissipation consideration
The KB4312 maximum power dissipation depends on the thermal resistance of the IC package and
circuit board, the temperature difference between the die junction and ambient air, and the rate of any
airflow. The power dissipation in the device depends on the operating conditions of the regulator.
The step-up converter dissipates power across the internal N-MOS as the controller ramps up the
inductor current. In continuous condition, the power dissipated internally can be approximated by :
IO uV
Vin
Vin u D
12 fOSC u L
1
P
[(
O )2 ꢁ
(
)2 ]u RDS (ON ) u D
boost
where
IO : It is the load current.
fOSC : It is a switching frequency.
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KINGBOR [ KINGBOR TECHNOLOGY CO ]
