Diodes ZXCL SERIES - Manual (Page 10)

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
the device for normal operation (P
max
), is a
function of the package junction to ambient
thermal resistance (
θ
ja
), maximum junction
temperature (Tj
max
), and ambient temperature
(T
amb
), according to the expression:
P
max
= (Tj
max
– T
amb
) /
θ
ja
The maximum output current (I
max
) at a given
value of Input voltage (V
IN
) and output voltage
(V
OUT
) is then given by
I
max
= P
max
/ (V
IN
- V
OUT
)
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 limit,(I
OLIM
). Power
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
ЊC to 85ЊC temperature
range, whereas a capacitor with a Y5V dielectric
loses 80% of its capacitance at -40
ЊC and 75% at
85
ЊC.
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 20 40 60 80 100
0
100
200
300
400
500
SOT23
SC70
Derating Curve
Max
P
ower
D
issipatio
n
(mW
)
Temperature (°C)