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ZXCL1250H5

ULTRA SMALL MICROPOWER SC70 LOW DROPOUT REGULATORS

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APPLICATIONS INFORMATION (Cont)

Capacitor Selection and Regulator Stability

Power Dissipation

The maximum allowable power dissipation of the

device for normal operation (Pmax), is a function of the

package junction to ambient thermal resistance (θja),

maximum junction temperature (Tjmax), and ambient

temperature (Tamb), according to the expression:

Pmax = (Tjmax – Tamb) / θja

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 1µF 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 0.5Ω are recommended for best results.

The maximum output current (Imax) at a given value of

Input voltage (VIN) and output voltage (VOUT) is then

given by

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

recommended to filter supply noise at the device input

and will improve ripple rejection.

Imax = Pmax / (VIN - VOUT )

The value of θja 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 shutdown state, Tjmax

should be assumed to be 125°C and Imax less than the

over-current limit,(IOLIM). Power derating for the SC70

and SOT23-5 packages is shown in the following

graph.

500

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.

400

300

200

100

0

-40 -20

SOT23

SC70

0

20 40 60 80

Temperature (°C)

Derating Curve

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.

100

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.

ISSUE 5 - NOVEMBER 2001

9

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