A

4/99

TOP412/414

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

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 internal output MOSFET duty

cycle linearly decreases 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 Figure 6 for timing and voltage

waveforms of the TOPSwitch integrated circuit.

Control Voltage Supply

CONTROL pin voltage V

controller and driver circuitry. An external bypass capacitor

closely connected between the CONTROL and SOURCE pins

is required to supply the gate drive current. The total amount of

capacitance connected to this pin (C

timing as well as control loop compensation. V

either of two modes of operation. Hysteretic regulation is used

for initial start-up and overload operation. Shunt regulation is

used to separate the duty cycle error signal from the control

circuit supply current. During start-up, V

from a high-voltage switched current source connected internally

between the DRAIN and CONTROL pins. The current source

provides sufficient current to supply the control circuitry as

well as charge the total external capacitance (C

PI-1691-112895

DMAX

DMIN

ICD1

IC (mA)

2.5

6.5

45

Slope = PWM Gain

-16%/mA

IB

Auto-restart

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

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

Auto-restart.

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-1124A-060694

DRAIN

0

VIN

VC

0

4.7 V

5.7 V

Off

Switching

(b)

(a)

connected to the CONTROL pin