D

7/96

TOP200-4/14

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

or bias voltage for the 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

auto-restart 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

C

current is supplied 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