Advanced Monolithic Systems, Inc.
www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723
AMS5105
APPLICATION NOTES
Definition of Terms
Dropout Voltage
: The input-output voltage differential at which
the circuit stops to regulate against further reduction in input
voltage. Measured when the output voltage has dropped 100mV
from the nominal voltage obtained at 1V input, dropout voltage is
dependent upon load current and junction temperature.
Input Voltage
: The DC voltage applied to the input terminal with
respect to ground.
Input-Output Differential
: The voltage difference between the
unregulated input voltage and the regulated output voltage for
which the regulator will regulate.
Line Regulation
: The change in output voltage for a change in
the input voltage. The line regulation is measured under conditions
of low dissipation or by using low duty cycle pulse testing such
that the average chip temperature is not significantly affected.
Load Regulation
: The change in output voltage for a change in
load current at constant chip temperature.
Long term stability
: Output voltage stability under accelerated
life-test conditions after 1000 hours with maximum rated voltage
and junction temperature.
Quiescent Current
: That part of the positive input current that
does not contribute to the positive load current. The regulator
ground lead current.
Temperature Stability of V
O
: The percentage change in output
voltage for a thermal variation from room temperature to either
temperature extreme.
External capacitor
The AMS5105 series requires an output capacitor of 2.2
μ
F or
greater to ensure device stability, as shown in Figures 1 through 4.
Without the capacitor the device may oscillate. Most types of
tantalum or electrolytic capacitor can be used in the applications.
The value of this capacitor can be increased without limits. For
higher loads, the value of the capacitor should be increased,
especially when the output voltage is set for 2.5V or less. The
AMS5105 lowest fixed output voltage value is 1.5V.
Programming the Output Voltage
The AMS5105 may be programmed for any output voltage
between its 1.25V reference and its 6V maximum rating, allowing
for dropout and power dissipation. As shown in Figure 3, an
external pair of resistors is required.
The equation for the output voltage is:
V
OUT
= V
REF
(1 + R
1
/ R
2
)
where V
REF
is the nominal 1.25 reference voltage.
Typical application circuit (adjustable output)
TYPICAL APPLICATION
+V
IN
GND
ENABLE
2
3
1
5
+
2.2 F
C
IN
+
+V
IN
V
OUT
AMS5105-X
2.2 F
C
OUT
+V
IN
V
OUT
Figure 2
GND
V
OUT
2
3
1
+V
IN
V
OUT
AMS5105-X
+
2.2 F
C
IN
+
2.2 F
C
OUT
Figure 1
R
1
GND
FB
4
V
OUT
V
REF
R
2
ENABLE
2
3
1
5
+V
IN
+
AMS5105
2.2 F
C
IN
+
2.2 F
C
OUT
+V
IN
V
OUT
1.25V
Figure 3