14

SCD5031 Rev B

Pre

lim

in

ary

Recalling that the EA output is level shifted and divided before being applied to the comparator input, the peak current

Clamping the EA output to the soft pin also makes implementing a softstart ciruit easy. Rsoft and Csoft are connected as

in Figure 9 to provide the SOFT pin an asymptotically rising voltage. Because of the internal clamp on the EA output,

the PWM duty cycle will increase only as fast as the chosen time constant will allow. In this way, excessive duty cycle

and surge currents into the output capacitors are avoided. A transistor may be optionally connected across the softstart

capacitor to force zero duty cycle on command. This is a particularly convenient method for implementing an externally

controlled turn-on delay.

The discussion so far assumes the user operates the chip in the current mode: switch current is sensed and compared to

the error between the output voltage and a precision reference. Alternatively, the user may wish to implement voltage

mode control in which the control loop is dependent only on the output voltage. The PWM chip readily supports this

configuration with the following modification:

FIGURE 10 – Circuit for implementing voltage mode control.

stage. The operation is now identical to the current mode application: when the sawtooth voltage exceeds the amplified

difference between the output and a voltage reference, the comparator fires and latches off the output until the start of the

next cycle.

SELECTED APPLICATION EXAMPLES

The flexibility and performance of the chip makes it suitable for an enormous range of power converter applications –

step-up, step-down, DC-DC, AC-DC, isolated/non-isolated, and many more. This section will cover two of the more

popular power converter applications for which this chip is particularly well suited although many more can be

envisioned.

5V Input, 3.3V Isolated Output (Single Ended Forward Converter)

The isolated step down DC/DC converter is a staple of many satellite and aerospace systems. A common bus distributes

raw primary power to various system loads which must then convert the primary to one or more low voltage secondary

outputs. These outputs are filtered, regulated, and ground isolated from the primary side to keep EMI and undesired

subsystem interaction at a minimum. Figure 9 is one example of a circuit that very efficiently performs this conversion.

The values here were chosen to work for a 5V input and 3.3V output but the circuit topology is general enough to

support an infinite variety of applications. For example, output voltages can be adjusted by changing values of just a few

components. A wider input voltage range can be supported by varying the transformer’s turns ratios and by proper

selection of M1. Thus, a very wide range of power converter applications can be satisfied by simple variations of the

circuit.

At the start of each switching cycle, the PWM output goes high and turns on M1. Energy is coupled across T1’s turns

ratios to the secondary side where it is caught, rectified, and filtered to produce a clean DC voltage. A sampling network

on the output side feeds back a portion of the output across the isolation barrier into the error amplifier negative input.

I

pk

V

soft

1.4

–

3

R

s

×

-------------------------

=

1.4V

V

soft

4.4V

≤≤

Out

Isense

Vref

2N2222

Cset

Cset

M1

Switch

Current