TOP221-227
TOP S w it c h Family Functional Description
TOPSwitch is a self biased and protected linear control current-
to-duty cycle converter with an open drain output. High
efficiencyisachievedthrough theuseofCMOSandintegration
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.
Auto-restart
I
B
D
MAX
Slope = PWM Gain
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
D
MIN
I
2.0
6.0
CD1
I
(mA)
C
PI-2040-050197
Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.
below. Refer to Figure 2 for a block diagram and to Figure 6 for
timing and voltage waveforms of the TOPSwitch integrated
circuit.
I
C
Charging C
T
5.7 V
4.7 V
V
C
0
Off
V
IN
0
DRAIN
Switching
(a)
I
CD2
Discharging C
I
I
T
C
CD1
Discharging C
Charging C
T
T
5.7 V
4.7 V
V
C
8 Cycles
95%
0
5%
Off
Off
Off
V
IN
0
DRAIN
Switching
Switching
(b)
C is the total external capacitance
T
connected to the CONTROL pin
PI-1956-092496
Figure 5. Start-up Waveforms for (a) Normal Operation and (b) Auto-restart.
C
3
12/97
POWERINT [ Power Integrations ]
