Product Specification

PE99153

Page 11 of 15

Document No. DOC-29414-2 │ www.psemi.com

©2010-2013 Peregrine Semiconductor Corp. All rights reserved.

Efficiency Estimation and Improvement

The efficiency of a switch mode power supply can be

estimated by identifying and estimating all sources of loss

in the power supply system. These loss terms include

switching losses, resistive losses, losses incurred on chip

and losses associated with external passive components.

External passive losses occur primarily in the output

inductor, the output capacitor and the input capacitor.

Internal losses at high current are dominated by the high

and low side switch resistance. At low current, internal

losses are dominated by quiescent bias current and

switching related losses.

The PE99153 Design Guide provides a simple tool for

estimating loss. Losses are parameterized across input

voltage, output voltage and switching frequency to

provide accurate estimates of the performance of the part

under a variety of conditions.

The following sections give the mathematical expressions

of six main loss terms calculated in the design guide

spreadsheet.

Input Capacitor

The loss in the input capacitor can be calculated by using

the estimate of the RMS capacitor current calculated in

the input capacitor selection section. Given that:

Power lost in the input capacitor can be calculated as:

2

Output Capacitor

The RMS current through the output capacitor in steady

state was calculated in the output capacitor selection

then calculated as:

capacitors absorbing the ripple current.

Inductor

The inductor RMS current is given by:

Power lost in the DC resistance of the inductor is then

given as:

High Side Switch Loss

During the time the HSS is on, it is supporting the load

current plus the inductor ripple current. RMS current

through the HSS, when it is on, is given by:

and is included because power is only dissipated in the

HSS when it is on.

Low Side Switch Loss

During the time the LSS is on, it is supporting the load

current plus the inductor ripple current. RMS current

through the LSS, when it is on, is given by:

ratio of the LSS and is included because power is only

dissipated in the LSS when it is on.

Other Internal Loss

A complete list of internal losses in the PE99153 regulator

is estimated and available in the PE99153 design guide

spreadsheet available online. The internal losses are

parameterized across input voltage, output voltage and

switching frequency to provide accurate estimates of the

performance under a variety of conditions.

Setting the Current Limit

When the RSEL pin is grounded, the PE99153 uses an

internal current limiting resistor that will limit the output

current to a value of ILIMXINT listed in Table 2 of the

datasheet. See Figure 10 for a visual description of the

various current limits. The part can be programmed to

use an alternate current limit by tying the RSEL pin to

various output current limits through the selection of a

resistor connecting the RSET pin to ground.

Current Threshold

IL

I

Max Current Limit:

ILIMXEXT or ILIMXINT

Operating Max: Imax

Absolute Max: Io

Time

Figure 10. PE99153 Current Limit