NCP1653, NCP1653A

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15

Figure 40. Self−biasing Scheme in Constant Output

Voltage Mode

V

C1

C2

out

When the NCP1653 circuit is required to be startup

independently from the second−stage converter, it is

recommended to use a circuit in Figure 41. When there is

maximum). It is good for saving the current to charge the

current the startup current rises to as high as 1.5 mA in the

be implemented: a PNP bipolar transistor derives the

so that the startup current keeps low and an initial voltage

Figure 41 are just for reference.

Figure 41. Recommended Startup Biasing Scheme

180k

NCP1653

100uF

560k

Input

Output

1.5M

180k

180k

BC556

The device typically starts to operate when the supply

excessive supply voltage. After startup, the operating range

is between 8.7 V and 18 V.

Thermal Shutdown

An internal thermal circuitry disables the circuit gate

drive and then keeps the power switch off when the junction

temperature exceeds 150

_C. The output stage is then

enabled once the temperature drops below typically 120

_C

(i.e., 30

_C hysteresis). The thermal shutdown is provided

to prevent possible device failures that could result from an

accidental overheating.

Output Drive

The output stage of the device is designed for direct drive

of power MOSFET. It is capable of up to

±1.5 A peak drive

current and has a typical rise and fall time of 88 and

61.5 ns with a 2.2 nF load.