TOP250 datasheet(6/54 Pages) POWERINT

6 / 54 page

Rev. R 10/20

TOP242-250

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66 kHz can be chosen by connecting this pin to CONTROL

pin instead. Leaving this pin open is not recommended.

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

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

undervoltage 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

undervoltage 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%.

PI-2545-082299

5.8 V

4.8 V

0 V

VLINE

VDRAIN

VOUT

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

0 V

VUV

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

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