TOP242 Power Integrations Inc., TOP242 Datasheet - Page 6

• Manufacturer Part Number: TOP242
• Description: Family Extended Power/ Design Flexible/Integrated Off-line Switcher
• Manufacturer: Power Integrations Inc.

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
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
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
Figure 8. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-restart (4) Power Down.
6
TOP242-249
V
V
V
Note: S0 through S7 are the output states of the auto-restart counter
LINE
C
DRAIN
V
0 V
0 V
0 V
0 V
OUT
E
7/01
C
is the supply voltage for the control circuitry
V
UV
1
August 8, 2000
S7
S0
2
S1
S2
S6
S7
C
reaches
3
S0
S1
S2
in excess of the consumption of the chip is shunted to SOURCE
through resistor R
through R
provide closed loop regulation. The shunt regulator has a finite
low output impedance Z
when used in a primary feedback configuration. The dynamic
impedance Z
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
typically 4.8 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-8 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-8 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 internal
capacitance between two voltage levels to create a sawtooth
S6
S7
E
2
controls the duty cycle of the power MOSFET to
C
of the CONTROL pin together with the external
S0
E
as shown in Figure 2. This current flowing
C
that sets the gain of the error amplifier
S1
S2
4
S6
S7
C
within a window of
S7
4.8 V
PI-2545-082299
5.8 V