ML4863

6

SYNCHRONOUS RECTIFIER CONTROL

The control circuitry for the synchronous rectifier does not

influence the operation of the main controller. The

synchronous rectifier is turned on during the minimum off

time, or whenever the SENSE pin is less than –18mV.

During transitions where the primary switch is turned on

before the voltage on the SENSE pin goes above –18mV,

the gate of the synchronous rectifier is discharged softly to

avoid accidently triggering the current-mode comparator

with the gate discharge spike on the sense resistor.

The part will also operate with a Schottky diode in place

of the synchronous rectifier, but the conversion efficiency

will suffer.

CURRENT LIMIT AND MODES OF OPERATION

The normal operating range and current limit point are

determined by the current programming comparator. They

are dependent on the value of the synchronous rectifier

R

V

VV

mV

I

V

VI

SENSE

IN MIN

OUT

IN

OUT MAX

IN MIN

IN MAX

OUT MAX

=

+

´+

´´









´

05

05

05

05

05

150

20

where

h = converter efficiency.

LV

R

P

IN MAX

SENSE

=×

×

×

−

()

25 10

6

05

(2)

Three operational modes are defined by the voltage at the

SENSE pin at the end of the off-time: discontinuous mode,

continuous mode, and current limit. The following values

can be used to determine the current levels of each mode:

operational mode into the following equation will

determine the maximum current levels for each

operational mode:

I

V

VV

V

R

tV

L

OUT

IN

OUT

IN

SENSE

SENSE

ON

IN

P

=

+

×+

×

×







2

η

(3)