Rev. G 08/16

3

TOP221-227

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TOPSwitch-II Family Functional Description

TOPSwitch is a self biased and protected linear control cur-

rent-to-duty cycle converter with an open drain output. High

efficiency is achieved through the use of CMOS and integra-

tion 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 CON-

TROL 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 inte-

grated circuit.

PI-2040-050197

DMAX

DMIN

IC (mA)

2.0

6.0

Slope = PWM Gain

IB

ICD1

Auto-restart

Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.

DRAIN

0

VIN

VC

0

4.7 V

5.7 V

DRAIN

0

VIN

VC

0

4.7 V

5.7 V

connected to the CONTROL pin

Figure 5. Start-up Waveforms for (a) Normal Operation and (b) Auto-restart.