C

12/97

TOP221-227

3

TOPSwitch 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.

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

8 Cycles

95%

5%

Off

Switching

Switching

Off

IC

Charging CT

ICD1

Discharging CT

ICD2

Discharging CT

IC

Charging CT

Off

PI-1956-092496

DRAIN

0

VIN

VC

0

4.7 V

5.7 V

Off

Switching

(b)

(a)

connected to the CONTROL pin

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