LT1963 Series
W U U
U
APPLICATIO S I FOR ATIO
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
beproportional to theratioofthe desiredoutput 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 LT1963 series are 1.5A low dropout regulators opti-
mized for fast transient response. The devices are capable
of supplying 1.5A at a dropout voltage of 350mV. The low
operating quiescent current (1mA) drops to less than 1µA
in shutdown. In addition to the low quiescent current, the
LT1963regulatorsincorporateseveralprotectionfeatures
which make them ideal for use in battery-powered sys-
tems.Thedevicesareprotectedagainstbothreverseinput
and reverse output voltages. In battery backup applica-
tions where the output can be held up by a backup battery
when the input is pulled to ground, the LT1963-X acts like
it has a diode in series with its output and prevents reverse
current flow. Additionally, in dual supply applications
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 LT1963 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 10µF with an ESR in
the range of 50mΩ to 3Ω is recommended to prevent
oscillations. Larger values of output capacitance can de-
creasethepeakdeviationsandprovideimprovedtransient
response for larger load current changes. Bypass capaci-
tors, used to decouple individual components powered by
the LT1963, will increase the effective output capacitor
value.
Adjustable Operation
The adjustable version of the LT1963 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, 3µA 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. In some applications, the use of ceramic
capacitors with an ESR below 50mΩ can cause oscilla-
tions.PleaseconsultourApplicationsEngineeringdepart-
ment for help with any issues concerning the use of
ceramic output capacitors. Ceramic capacitors are manu-
factured with a variety of dielectrics, each with different
behavior over temperature and applied voltage. The most
commondielectricsusedareZ5U, Y5V, X5RandX7R. The
Z5U and Y5V dielectrics are good for providing high
capacitancesinasmallpackage,butexhibitstrongvoltage
and temperature coefficients as shown in Figures 3 and 4.
When used with a 5V regulator, a 10µF Y5V capacitor can
exhibit an effective value as low as 1µF to 2µF over the
operatingtemperaturerange.TheX5RandX7Rdielectrics
result in more stable characteristics and are more suitable
for use as the output capacitor. The X7R type has better
stability across temperature, while the X5R is less expen-
sive and is available in higher values.
IN
OUT
ADJ
V
OUT
+
V
LT1963
GND
R2
R1
IN
1963 F02
R2
R1
VOUT = 1.21V 1+
ADJ = 1.21V
ADJ = 3µA AT 25°C
OUTPUT RANGE = 1.21V TO 20V
+ I
R2
(
ADJ)(
)
V
I
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
1963fa
Figure 2. Adjustable Operation
10
Linear Systems [ Linear Systems ]
