AN8021L 查看數據表（PDF） - Panasonic Corporation

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AN8021L

AC-DC switching power supply control IC

Panasonic Corporation 

AN8021L, AN8021SB

Voltage Regulators

s Application Notes (continued)

[2] Operation descriptions (continued)

7. Output Block

The AN8021L and AN8021SB employ the out-

put circuit using the totem pole (push-pull) method,

by which sink/source of current is performed with

the NPN transistor as shown in figure 9, in order to

drive the power MOSFET at high speed.

The maximum sink/source current is ±0.1 A

(DC) and ±1.0 A (peak). Even when the supply

Schottky

barrier diode

voltage VCC is under the stop voltage, the sink

function works to ensure that the power MOSFET

be turned off.

Figure 9

For the current capability, the peak current is major concern, and the particularly large current is not required

normally: the power MOSFET which works as load on the output is capacitive load. Therefore, in order to drive

it at high speed, the large peak current is required. However, after charge/discharge, particularly large current is

not required to keep that condition.

For the AN8021L or AN8021SB, capacitance value of the power MOSFET used is taking into account, and

the capability of peak value ±1 A is ensured.

The parasitic LC of the power MOSFET may produce ringing which makes the output pin go under the GND

potential. When the 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 may cause the malfunction of the device. In such a case, the Schottky barrier diode should be

connected between the output and GND.

[3] Design reference data

1. Setting the output frequency

The output is controlling the triangular oscillation with PWM control: Triangular oscillation frequency = output

frequency

CT (C6) = capacitor terminal for triangular oscillation

RT (C7) = resistor terminal for triangular oscillation

VOSC − H−

[Reference calculation formula]

T1 = T2 =

C6 · V

2IRT (charge/discharge current)

VOSC − L−

V

Since the IRT is given by rough calculation of 2.5 V/RT , and

V becomes approximately 3 V, the output frequency is obtained

in the following equation:

T1

T2

1

fOUT = T1 + T2 =

IRT

C6 · V

=

5

6 · C6 · R7

However, it may deviate a little from the design value due to delay of the internal circuit.

(Reference value)

fOUT = approximately 200 kHz

at CT (C6) = 220 pF and RT (R7) = 19 kΩ

14

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