TOP245GN- 参数 Datasheet PDF下载

TOP242-250

CONTROL (C) Pin Operation

The CONTROL pin is a low impedance node that is capable

of receiving a combined supply and feedback current. During

normal operation, a shunt regulator is used to separate the

feedback signal from the supply current. CONTROL pin voltage

V

C

is the supply voltage for the control circuitry including the

MOSFET gate driver. An external bypass capacitor closely

connected between the CONTROL and SOURCE pins is required

to supply the instantaneous gate drive current. The total amount

of capacitance connected to this pin also sets the auto-restart

timing as well as control loop compensation.

When rectified DC high voltage is applied to the DRAIN

pin during start-up, the MOSFET is initially off, and the

CONTROL pin capacitor is charged through a switched high

voltage current source connected internally between the DRAIN

and CONTROL pins. When the CONTROL pin voltage V

C

reaches approximately 5.8 V, the control circuitry is activated

and the soft-start begins. The soft-start circuit gradually

increases the duty cycle of the MOSFET from zero to the

maximum value over approximately 10 ms. If no external

feedback/supply current is fed into the CONTROL pin by the

end of the soft-start, the high voltage current source is turned

off and the CONTROL pin will start discharging in response

to the supply current drawn by the control circuitry. If the

power supply is designed properly, and no fault condition

such as open loop or shorted output exists, the feedback loop

will close, providing external CONTROL pin current, before

the CONTROL pin voltage has had a chance to discharge to

the lower threshold voltage of approximately 4.8 V (internal

supply under-voltage lockout threshold). When the externally

fed current charges the CONTROL pin to the shunt regulator

voltage of 5.8 V, current in excess of the consumption of the

chip is shunted to SOURCE through resistor R

E

as shown in

Figure 2. This current flowing through R

E

controls the duty cycle

of the power MOSFET to provide closed loop regulation. The

shunt regulator has a finite low output impedance Z

C

that sets

the gain of the error amplifier when used in a primary feedback

configuration. The dynamic impedance Z

C

of the CONTROL

pin together with the external CONTROL pin capacitance sets

the dominant pole for the control loop.

When a fault condition such as an open loop or shorted output

prevents the flow of an external current into the CONTROL

pin, the capacitor on the CONTROL pin discharges towards

4.8 V. At 4.8 V, auto-restart is activated which turns the output

MOSFET off and puts the control circuitry in a low current

standby mode. The high-voltage current source turns on and

charges the external capacitance again. A hysteretic internal

supply under-voltage comparator keeps V

C

within a window

of typically 4.8 V to 5.8 V by turning the high-voltage current

source on and off as shown in Figure 8. The auto-restart

circuit has a divide-by-eight counter which prevents the output

MOSFET from turning on again until eight discharge/charge

cycles have elapsed. This is accomplished by enabling the

output MOSFET only when the divide-by-eight counter reaches

full count (S7). The counter effectively limits

TOPSwitch-GX

power dissipation by reducing the auto-restart duty cycle

to typically 4%. Auto-restart mode continues until output

voltage regulation is again achieved through closure of the

feedback loop.

Oscillator and Switching Frequency

The internal oscillator linearly charges and discharges an

~

~

~

~

V

UV

V

LINE

0V

S7

S0

S1

S2

S6

S7

S0

S1

S2

S6

S7

S0

S1

S2

S6

S7

S7

~

~

~

~

~

~

~

~

V

C

0V

5.8 V

4.8 V

~

~

~

~

~

~

~

~

V

DRAIN

0V

~

~

V

OUT

0V

~

~

~

~

~

~

1

2

3

2

4

PI-2545-082299

Note: S0 through S7 are the output states of the auto-restart counter

Figure 8. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-Restart (4) Power Down.

6

M

12/04