ZXCL250 ZXCL260 ZXCL280
ZXCL300 ZXCL330 ZXCL400
APPLICATIONS INFORMATION (Cont)
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 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
ESRlessthan0.5Ω arerecommendedforbestresults.
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:
P
max = (Tjmax – Tamb) / θja
An input capacitor of 1µF (ceramic or tantalum) is
recommended to filter supply noise at the device input
and will improve ripple rejection.
The maximum output current (Imax) at a given value of
Input voltage (VIN) and output voltage (VOUT) is then
given by
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.
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.
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.
500
400
Power Supply Rejection and Load
Transient Response
SOT23
300
Line and Load transient response graphs are shown in
the typical characteristics.
200
SC70
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
0
-40
-20
0
20
40
60
80
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.
Temperature (°C)
Derating Curve
ISSUE 5 - NOVEMBER 2001
9
