Product Specification

PE99151

Page 9 of 15

Document No. DOC-29314-2 │ www.psemi.com

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

DESIGN GUIDE

Setting the Output Voltage

The PE99151 can be configured to output a DC voltage

from +1.0V to +3.6V. The user can set the output voltage by

feedback resistors divide down the output voltage to be

compared to a +1.000V reference voltage. The error

amplifier uses this comparison to determine the amount of

current to send to the load.

To set the output voltage, a resistor divider must selected

that will produce at +1.000V DC voltage at the EAINM pin

Figure 8. Output Voltage Selection

The PE99151 reference design uses a value of 10 kΩ for

Example:

* For a desired output voltage of 1V, Rfb1 can be replaced with a 0 Ω resistor and

Output Inductor Selection

The output Inductor serves as the main energy storage

element in a switching regulator. It is perhaps the most

critical component influencing the performance of the buck

regulator. It impacts many aspects of the power supply

system performance, including power supply bandwidth,

output voltage ripple and ripple spectrum, and switching,

conduction, and core losses. Additionally, specific aspects

of the buck regulator itself place requirements on the

range of allowable Inductor values. These aspects

include the internal current detector sensitivity, the slope

compensation ramp dynamic range, and the current

limitations of the part. The selection of the Inductor is

also a function of the specifics of the application

including input voltage, output voltage, load current

range, switching frequency, PCB area, efficiency targets,

power supply bandwidth, and ripple requirements, to

name a few.

Many performance requirements and other component

selections place restrictions on the Inductor selection.

However, since the Inductor selection plays a central

role in the performance of the power supply, its selection

needs to be made early in the design process.

Therefore, as a starting point, the Inductor needs to be

initially selected based on a few rough calculations and

selection can be refined iteratively as more system

requirements are introduced.

∆

IL is defined to be the Inductor peak-to-peak current

ripple. The ripple current is the change in the Inductor

current during each switching cycle. For the PE99151, the

mode compensation dynamic range.

Given the output voltage, switching frequency, input

Inductance can be calculated as:

and to GND, the inductor peak to peak current ripple

(triangular current waveform magnitude) is expressed as:

∆

Example:

∆

IL = 0.5A

L = (+2.5/(1 MHz x 0.5) * [(1 – (+2.5/+5.0))] = 2.5 µH

The Inductor self resonant frequency (SRF) should be

selected to be at least 10x higher than the switching

stability, reduce output ripple and improve efficiency.

Vout

Rfb1

Error Amp

Rfb2

+

_

1.000V

EAINM

EAINP

VREF