top221 Power Integrations, Inc., top221 Datasheet - Page 3

• Manufacturer Part Number: top221
• Description: Three-terminal Off-line Pwm Switch
• Manufacturer: Power Integrations, Inc.

TOPSwitch-II Family Functional Description
TOPSwitch is a self biased and protected linear control current-
to-duty cycle converter with an open drain output. High
efficiency is achieved through the use of CMOS and integration
of the maximum number of functions possible. CMOS process
significantly reduces bias currents as compared to bipolar or
discrete solutions. Integration eliminates external power
resistors used for current sensing and/or supplying initial start-
up bias current.
During normal operation, the duty cycle of the internal output
MOSFET decreases linearly with increasing CONTROL pin
current as shown in Figure 4. To implement all the required
control, bias, and protection functions, the DRAIN and
CONTROL pins each perform several functions as described
below. Refer to Figure 2 for a block diagram and to Figure 6 for
timing and voltage waveforms of the TOPSwitch integrated
circuit.
Figure 5. Start-up Waveforms for (a) Normal Operation and (b) Auto-restart.
DRAIN
DRAIN
V C
V C
5.7 V
4.7 V
5.7 V
4.7 V
V IN
V IN
0
0
0
0
Charging C T
Charging C T
C
T
connected to the CONTROL pin
Off
Off
I C
I C
is the total external capacitance
Switching
(a)
(b)
Discharging C T
Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.
Off
I CD1
95%
8 Cycles
D MAX
D MIN
Switching
Auto-restart
I CD1
Switching
5%
2.0
Discharging C T
I B
I CD2
Off
I C (mA)
Slope = PWM Gain
TOP221-227
6.0
PI-1956-092496
7/01
PI-2040-050197
D
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