15

SCD5031 Rev B

Pre

lim

in

ar

y

This feedback can be accomplished in a number of different ways: pulse transformers, optocouplers, or capacitive

coupling are a few methods. The compensation network may need modification depending on the feedback method

chosen. The additional winding and rectifier on T1 are used to reset the transformer core after the PWM latches off M1

to prevent staircase saturation of the core.

Note the chip is powered directly from the main power bus (via a zener and current limit resistor) without the need for

additional bootstrap transformer windings. This is one of the main advantages this PWM chip provides over other

products. This scheme could not be implemented with other chips which draw significantly more current. On the other

hand, supplying bias to our PWM chip is about as simple as it gets.

5V to 1.8V Synchronous Buck Converter

A second application is a secondary side, non-isolated synchronous buck converter. The circuit takes a high voltage (5V

in this case) and steps down to a lower voltage (5V to 1.8V in this example, although as pointed out above, these values

are completely adjustable with proper component selection). The distinguishing feature of this implementation is the

synchronous rectification scheme used to replace the standard Schottky rectifiers for more efficient power conversion of

low voltage outputs.

FIGURE 11 – Buck Converter

is the desired output voltage. The LC filter then simply smoothes this pulse train to produce a clean DC output. The

control loop regulates against operating point perturbations (temperature, line, load) by adjusting M1's duty cycle. The

circuit is operated in the voltage mode since switch current is not referenced to circuit ground. Alternatively, a current

the output path to properly drive the P-channel MOSFET. For low current applications (less than -50mA output current),

it may be possible to use the PWM's output drive stage as the switching elements and eliminate M1 and D1 altogether.

Rcomp

1V/1.8V/2.5V/3.3V

M1

Ccomp

0.1µF

COMP

50%

Out A

DRVP

SOFT

D1