LT1963A Series
W U U
APPLICATIO S I FOR ATIO
U
The adjustable device is tested and specified with the ADJ
pin tied to the OUT pin for an output voltage of 1.21V.
Specifications for output voltages greater than 1.21V will
be proportional to the ratio of the desired output voltage to
1.21V: VOUT/1.21V. For example, load regulation for an
output current change of 1mA to 1.5A is –3mV typical at
VOUT = 1.21V. At VOUT = 5V, load regulation is:
The LT1963A series are 1.5A low dropout regulators op-
timized for fast transient response. The devices are ca-
pable of supplying 1.5A at a dropout voltage of 350mV.
The low operating quiescent current (1mA) drops to less
than 1mA in shutdown. In addition to the low quiescent
current, the LT1963A regulators incorporate several pro-
tection features which make them ideal for use in battery-
powered systems. The devices are protected against both
reverseinputandreverseoutputvoltages.Inbatterybackup
applications where the output can be held up by a backup
battery when the input is pulled to ground, the LT1963A-X
actslikeithasadiodeinserieswithitsoutputandprevents
reverse current flow. Additionally, in dual supply applica-
tions where the regulator load is returned to a negative
supply, the output can be pulled below ground by as much
as 20V and still allow the device to start and operate.
(5V/1.21V)(–3mV) = –12.4mV
Output Capacitance and Transient Response
The LT1963A regulators are designed to be stable with a
wide range of output capacitors. The ESR of the output
capacitor affects stability, most notably with small capaci-
tors. A minimum output capacitor of 10mF with an ESR of
3W orlessisrecommendedtopreventoscillations. Larger
values of output capacitance can decrease the peak devia-
tions and provide improved transient response for larger
loadcurrentchanges.Bypasscapacitors,usedtodecouple
individual components powered by the LT1963A, will
increase the effective output capacitor value.
Adjustable Operation
The adjustable version of the LT1963A has an output
voltage range of 1.21V to 20V. The output voltage is set by
theratiooftwoexternalresistorsasshowninFigure2.The
deviceservostheoutputtomaintainthevoltageatthe ADJ
pin at 1.21V referenced to ground. The current in R1 is
then equal to 1.21V/R1 and the current in R2 is the current
in R1 plus the ADJ pin bias current. The ADJ pin bias
current, 3mA at 25∞C, flows through R2 into the ADJ pin.
The output voltage can be calculated using the formula in
Figure 2. The value of R1 should be less than 4.17k to
minimize errors in the output voltage caused by the ADJ
pinbiascurrent.Notethatinshutdowntheoutputisturned
off and the divider current will be zero.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior over
temperature and applied voltage. The most common
dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit strong voltage and tem-
perature coefficients as shown in Figures 3 and 4. When
used with a 5V regulator, a 10mF Y5V capacitor can exhibit
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
0
IN
OUT
LT1963A
ADJ
V
OUT
X5R
+
V
R2
R1
IN
–20
–40
GND
1963 F02
–60
Y5V
R2
ˆ
˜
¯
R1
Ê
Ë
–80
VOUT = 1.21V 1+
+ I
R2
(
ADJ)(
)
Á
VADJ = 1.21V
IADJ = 3mA AT 25∞C
OUTPUT RANGE = 1.21V TO 20V
–100
0
8
12 14
2
4
6
10
16
DC BIAS VOLTAGE (V)
1963 F03
Figure 2. Adjustable Operation
Figure 3. Ceramic Capacitor DC Bias Characteristics
1963af
11
Linear [ Linear ]
