SMM151/152

Preliminary Datasheet

Summit Microelectronics, Inc

2131 2.1 8/15/2008

14

Maximizing Accuracy

Maximum margining accuracy is obtained by placing a

resistor between the SMM151/152 TRIM output and the

TRIM input of the converter. From the manufacturer’s

data sheet obtain the value of the internal voltage

reference and equivalent TRIM input series resistance.

Figure 7 below displays the internal trimming circuit for

a typical isolated DC-DC converter. In this example, the

converter uses positive trimming, i.e., an increase in

voltage at the TRIM pin causes an increase in output

voltage.

DC-DC

Converter

TRIM

VREF

V-

V+

-S

+S

L

O

A

D

VREF

R

TRIM

SMM151/152

TRIM Pin

R1

R2

Figure 7 - Simplified TRIM circuit of an isolated DC-DC converter connects to SMM151/152 TRIM output

()

()

()

× -0.3

2×

(

-0.3)

=

×

-0.3

1-

-0.3

TRIM

VREF k

R

VREF

R

k VREF

VREF







Where:

Nom

Low

V

V

k

=

0.3 = SMM151/152 TRIM output saturation voltage

VLow = Margin Low target voltage

VNom = Nominal (non-trimmed output voltage)

VREF = Converter internal reference voltage

The next example applies to most non-isolated DC-DC

converters, LDO’s and in-system designed converters

using monolithic PWM controllers. Figure 8 is a

simplified schematic showing the resistor divider

network used to close the loop from the output to the

circuit’s feedback node. These type circuits employ

negative trimming, meaning any decrease in voltage

into the feedback node cause an increase in output

voltage.

(

)

()

1×

-0.3

=

×-1

TRIM

R

VREF

R

VNom k

Nom

High

V

V

k

=

0.3 = SMM151/152 TRIM output saturation voltage

VHIGH = Margin Low target voltage

VNom = Nominal (non-trimmed output voltage)

VREF = Converter internal reference voltage

APPLICATIONS INFORMATION (CONTINUED)