AN8028 Просмотр технического описания (PDF) - Panasonic Corporation

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AN8028

Self-excited RCC pseudo-resonance type AC-DC switching power supply control IC

Panasonic Corporation 

Voltage Regulators

AN8028

s Application Notes (continued)

[1] Operation descriptions (continued)

8. Timer latch (continued)

VO

Power supply

output voltage

0

IDS

Power MOSFET

current

0

Power supply stop

Time

Power supply stop

Time

VOVP

TIM/OVP

terminal voltage

0

Power supply stop

OVP

VTH = 7.3 V (typ.)

Figure 10. Timer latch basic operation

Time

9. Output block

In order to drive the power MOSFET which is a capacitive load at high speed, this IC is adopting the totem pole

(push-pull) type output circuit which performs the sink and source of the current with the NPN transistor as

shown in figure 11.

The maximum sink/source current is ±0.1 A (DC) and the current at peak is ±1.0 A (peak). The circuit is

provided with the sink capability even if the supply voltage VCC is under the stop voltage so that it turns off the

power MOSFET without fail.

The peak current capability is mainly required

and a particularly too large current is not required

constantly.

Because the power MOSFET becomes a ca-

pacitive load for the output, a large peak current

is required for driving it at a high speed. How-

ever, after the charge and discharge, a particularly

Schottky barrier

diode

large current is not required to keep such condi-

tion. In the case of this IC, the peak current capa-

bility of ±1 A is ensured by taking a capacitance

value of the power MOSFET used into account.

Figure 11

The parasitic LC of the power MOSFET may produce ringing to decrease the output pin under the GND

potential. When the voltage decrease of the output pin becomes larger than the voltage drop of diode and its

voltage becomes negative, the parasitic diode consisting of the substrate and collector of the output NPN turns

on. This phenomenon can cause the malfunction of device. In such a case, the Schottky barrier diode should be

connected between the output and GND.

11

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