NCP1381 datasheet(8/26 Pages) ONSEMI

Part #	NCP1381

Description	Low?뭆tandby High Performance PWM Controller

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1381

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8

Figure 5. A Typical Startup Sequence Followed by a Faulty Condition

Startup Resistor Calculation

For the sake of the example, we will go through the

calculation of the startup element. Suppose that we have the

following information:

VCCON = 15 V.

VCCOFF = 10 V.

ICC2 = 4 mA, given by the selected MOSFET Qg.

Startup duration below 2 s at minimum input voltage.

Input voltage from 85 VAC to 265 VAC.

Standby power below 500 mW.

1. From a startup

DV of 15 − 10 = 5 V and a 4 mA

total consumption, we can obtain the necessary

VCC capacitor to keep enough voltage, assuming

the feedback loop is closed within 10ms: CVCC =

4 m x 10 m / 5 = 8

mF or 22 mF for the normalized

value if we account for the natural dispersion.

2. If we want a startup below 2 s, then the charging

current flowing inside the VCC capacitor must be

above: Icharge > 15 x 22 u / 2 > 165 mA. If we add

the 15

mA of ICC1, the total startup current shall be

above 180

mA.

3. The minimum input voltage is 85 x 1.414 = 120 V.

Then, Rstartup should be below (120 − 15) / 180 m

< 580 k

W.

4. From this value, we can calculate the dissipated

power at high line: Pstartup = (265 x 1.414)2 / 580 k

= 242 mW.

In latched mode, an internal zener diode is activated and

clamps VCC to around 6 V. When VCC goes below 4 V, this

zener is relaxed and the circuit can startup again.

Please note that in fault mode the VCC comparator has the

priority and stops the pulses anytime VCC falls below its

minimum operating level VCCOFF.

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