PS22921

Plessey Semiconductors Ltd, Tamerton Road, Roborough, Plymouth, UK PL6 7BQ

Data Sheet 292203 issue 1

Tel: +44 (0)1752 693000

Fax: +44 (0)1752 693200

email: sales@plesseysemi.com; October 12, 2010

6

APPLICATION HINT

Input capacitor may be required when the device is not near

the source power supply or when supplied by a battery.

This capacitor will reduce the circuit’s sensitivity when

powered from complex source impedance and significantly

enhance the output transient response. The input bypass

should be mounted with the short possible track directly

across the regulator’s input and ground terminals. A 10µF

ceramic capacitor should be adequate for most applications.

The output capacitor provides not only stability to the

regulator, but also, enhances the load transient response. A

minimum capacitance of 10µF is required. When selecting

a ceramic capacitor, only X5R and X7R dielectric types

should be used. Other types such as Z5U and Y5F have

such severe loss of capacitance due to effects of

temperature variation and applied voltage, they may

provide as little as 20% of rated capacitance in many

typical applications.

Always consult capacitor manufacturer’s data curves before

selecting a capacitor. High-quality ceramic capacitors can

be obtained from Taiyo-Yuden, AVX, and Murata. Higher

values of the output capacitance can be used to enhance

loop stability and transient response.

Output Voltage

The adjustable output voltage option allows the user to

select an output voltage by using an external resistor

divider. PS22921 uses a 0.5V reference voltage at the

positive terminal of the error amplifier. To set the output

voltage a programming resistor from the feedback pin (FB)

to ground must be selected.

A 10k

resistor is a good

selection for a programming resistor R2. A higher value

may result in an excessively sensitive feedback node while

a lower value will draw more current and degrade the light

load efficiency. The equation for selecting the voltage

specific resistor is:

FB

O

V

R

R

V

*

2

1

1













+

=

For the fixed output devices, R1 and R2 are included within

the device.

ByPass Capacitor

Connecting a capacitor between the BYP pin and ground

can significantly reduce output noise. Values can range

from 0pF to 100nF, depending on the sensitivity to output

noise in the application. The start up speed of the PS22291

is inversely proportional to the size of the bypass capacitor.

Applications requiring a slow ramp-up of output voltage

should consider larger values of bypass capacitance.

Likewise, if rapid turn on is necessary, consider omitting

Enable/Shutdown

The enable (EN) pin is active high and is compatible with

regulator on. If not used, EN can be connected to the input

as close as possible to the largest capacitance on the input

to prevent voltage droops on that line from triggering the

enable circuit.

Thermal consideration

The PS22291 is designed to provide 500mA of continuous

current in a very small package. Maximum power

dissipation can be calculated based on the output current

and the voltage drop across the part. To determine the

maximum power dissipation of the package, use the

junction-to-ambient thermal resistance of the device and the

following basic equation:













−

=

JA

A

J

D

T

T

P

θ

(max)

(max)

layout dependent.

The actual power dissipation of the

regulator circuit can be determined using the equation:

Substituting PD(max) for PD and solving for the operating

conditions that are critical to the application will give the

maximum operating conditions for the regulator.