LT1963A-1.5 LINER [Linear Technology], LT1963A-1.5 Datasheet - Page 11

• Manufacturer Part Number: LT1963A-1.5
• Description: 1.5A, Low Noise, Fast Transient Response LDO Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
reverse input and reverse output voltages. In battery backup
applications where the output can be held up by a backup
battery when the input is pulled to ground, the LT1963A-X
acts like it has a diode in series with its output and prevents
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.
Adjustable Operation
The adjustable version of the LT1963A has an output
voltage range of 1.21V to 20V. The output voltage is set by
the ratio of two external resistors as shown in Figure 2. The
device servos the output to maintain the voltage at the 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
pin bias current. Note that in shutdown the output is turned
off and the divider current will be zero.
V
IN
Figure 2. Adjustable Operation
I
V
V
OUTPUT RANGE = 1.21V TO 20V
ADJ
OUT
ADJ
IN
=
U
=
=
LT1963A
3
1 21
1 21 1
m AT 25 C
.
GND
.
A
V
V
Ê
Á
Ë
U
OUT
ADJ
+
∞
R
R
2
1
ˆ
˜ +
¯
(
R2
R1
I
ADJ
W
)( )
+
R
2
1963 F02
V
OUT
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: V
output current change of 1mA to 1.5A is – 3mV typical at
V
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 or less is recommended to prevent oscillations. Larger
values of output capacitance can decrease the peak devia-
tions and provide improved transient response for larger
load current changes. Bypass capacitors, used to decouple
individual components powered by the LT1963A, will
increase the effective output capacitor value.
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
OUT
(5V/1.21V)(–3mV) = – 12.4mV
= 1.21V. At V
Figure 3. Ceramic Capacitor DC Bias Characteristics
OUT
–100
–20
–40
–60
–80
20
0
/1.21V. For example, load regulation for an
0
2
OUT
4
DC BIAS VOLTAGE (V)
= 5V, load regulation is:
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
6
8
Y5V
X5R
LT1963A Series
10
12
14
1963 F03
16
11
1963af