ZXCL SERIES

Issue 8 - October 2007

10

www.zetex.com

© Zetex Semiconductors plc 2007

Applications information (Cont)

Power dissipation

The maximum allowable power dissipation of

function of the package junction to ambient

thermal resistance (

θ

maximum junction

The value of qja is strongly dependent upon the

type of PC board used. Using the SC70 package

it will range from approximately 280°C/W for a

multi-layer board to around 450°C/W for a single

sided board. It will range from 180°C/W to

300°C/W for the SOT23-5 package. To avoid

entering the thermal shutdo wn state, Tjmax

should be assumed to be 125°C and Imax less

than

the

over-current

derating for the SC70 and SOT23-5 packages is

shown in the following graph.

Capacitor selection and regulator stability

The device is designed to operate with all types

of output capacitor, including tantalum and low

ESR ceramic. For stability over the full operating

range from no load to maximum load, an output

capacitor with a minimum value of 1

μF is

recommended, although this can be increased

without

limit

to

improve

load

transient

performance. Higher values of output capacitor

will also reduce output noise. Capacitors with

ESR less than 0.5V are recommended for best

results.

The dielectric of the

ceramic capacitance is an

important consideration for the ZXCL Series

operation over temperature. Zetex recommends

minimum dielectric specification of X7R for the

input and output capacitors. For example a

ceramic capacitor with X7R dielectric will lose 20%

of its capacitance over a -40 C to 85 C temperature

range, whereas a capacitor with a Y5V dielectric

loses 80% of its capacitance at -40 C and 75% at

85 C.

An input capacitor of 1 F (ceramic or tantalum) is

recommended to filter supply noise at the device

input and will improve ripple rejection.

The input and output capacitors should be

positioned close to the device, and a ground plane

board layout should be used to minimise the

effects of parasitic track resistance.

Dropout voltage

The output pass transistor is a large PMOS device,

which acts like a resistor when the regulator enters

the dropout region. The dropout voltage is

therefore proportional to output current as shown

in the typical characteristics.

Ground current

The use of a PMOS device ensures a low value of

ground current under all conditions including

dropout, start-up and maximum load.

Power

supply

rejection

and

load

transient

response

Line and Load transient response graphs are

shown in the typical characteristics.

These show both the DC and dynamic shift in the

output voltage with step changes of input voltage

and load current, and how this is affected by the

output capacitor.

If improved transient response is required, then an

output capacitor with lower ESR value should be

used.

Larger

capacitors

will

reduce

over/

undershoot, but will increase the settling time.

Best results are obtained using a ground plane

layout to minimise board parasitics.

-40

-20

0

20406080

100

0

100

200

300

400

500

SOT23

SC70

Derating Curve

Temperature (°C)