TOP242-250
CONTROL (C) Pin Operation
voltage of 5.8 V, current in excess of the consumption of the
chip is shunted to SOURCE through resistor RE as shown in
Figure2. ThiscurrentflowingthroughRE controlsthedutycycle
of the power MOSFET to provide closed loop regulation. The
shunt regulator has a finite low output impedance ZC that sets
the gain of the error amplifier when used in a primary feedback
configuration. The dynamic impedance ZC of the CONTROL
pin together with the external CONTROL pin capacitance sets
the dominant pole for the control loop.
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
feedbacksignalfromthesupplycurrent. CONTROLpinvoltage
VC is the supply voltage for the control circuitry including the
MOSFET gate driver. An external bypass capacitor closely
connectedbetweentheCONTROLandSOURCEpinsisrequired
tosupplytheinstantaneousgatedrivecurrent. Thetotalamount
of capacitance connected to this pin also sets the auto-restart
timing as well as control loop compensation.
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 VC 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
outputMOSFETonlywhenthedivide-by-eightcounterreaches
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.
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
voltagecurrentsourceconnectedinternallybetweentheDRAIN
and CONTROL pins. When the CONTROL pin voltage VC
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
Oscillator and Switching Frequency
The internal oscillator linearly charges and discharges an
VUV
VLINE
0 V
S0
S0
S7
S1
S2
S6
S7 S0
S1
S2
S6
S7
S1 S2
S6
S7
S7
5.8 V
4.8 V
VC
0 V
VDRAIN
0 V
VOUT
0 V
1
2
3
2
4
Note: S0 through S7 are the output states of the auto-restart counter
PI-2545-082299
Figure 8. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-Restart (4) Power Down.
M
6
12/04
POWERINT [ Power Integrations ]
