LT1965
APPLICATIONS INFORMATION
The LT1965 is a 1.1A low dropout regulator with shut-
down. The device is capable of supplying 1.1A at a typical
dropout voltage of 290mV. The low operating quiescent
current (500μA) drops to less than 1μA in shutdown. In
addition to its low quiescent current, the LT1965 regulator
incorporates several protection features that make it ideal
for use in battery-powered systems. The device protects
itself against both reverse input and reverse output volt-
ages. In battery backup applications, if a backup battery
holds up the output when the input is pulled to ground,
the LT1965 performs like it has a diode in series with its
output, preventing reverse current flow. Also, in dual sup-
ply applications where the regulator load is returned to a
negative supply, the output can be pulled below ground
by as much as 20V. The LT1965 still starts and operates
normally in this situation.
The adjustable device is tested and specified with the ADJ
pin tied to the OUT pin for an output voltage of 1.20V.
Specifications for output voltages greater than 1.20V are
proportional to the ratio of the desired output voltage to
1.20V: V /1.20V. For example, load regulation for an
OUT
outputcurrentchangeof1mAto1.1Aistypically–4.25mV
at V
= 1.20V. At V
= 5V, load regulation is:
OUT
OUT
5V
1.20V
• –4.25mV = –17.71mV
Output Capacitance
The LT1965’s design is stable with a wide range of out-
put capacitors. The ESR of the output capacitor affects
stability, most notably with small capacitors. A minimum
output capacitor of 10μF with an ESR of 3Ω or less is
recommended to prevent oscillations. The LT1965 is a
low quiescent current device and output load transient
responseisafunctionofoutputcapacitance.Largervalues
of output capacitance decrease the peak deviations and
provide improved transient response for larger current
changes.
Adjustable Operation
The LT1965 has an output voltage range of 1.20V to 20V.
Figure 1 illustrates that the ratio of two external resistors
sets the output voltage. The device servos the output to
maintaintheADJpinvoltageat1.20Vreferencedtoground.
R1’s current equals 1.20V/R1. R2’s current equals R1’s
current plus the ADJ pin bias current. The ADJ pin bias
current, 1.3μA at 25°C, flows through R2 into the ADJ pin.
Use the formula in Figure 1 to calculate output voltage.
Linear Technology recommends that R1’s value be less
than 12.1k to minimize output voltage errors due to the
ADJ pin bias current. In shutdown, the output turns off
and the divider current is zero. For curves depicting ADJ
Pin Voltage vs Temperature and ADJ Pin Bias Current vs
Temperature, see the Typical Performance Characteristics
section.
Ceramic capacitors require extra consideration. Manufac-
turersmakeceramiccapacitorswithavarietyofdielectrics,
eachwithdifferentbehavioracrosstemperatureandapplied
voltage. The most common dielectrics used are specified
with EIA temperature characteristic codes of Z5U, Y5V,
X5R and X7R. The Z5U and Y5V dielectrics provide high
C-V products in a small package at low cost, but exhibit
strong voltage and temperature coefficients as shown in
Figures 2 and 3. When used with a 5V regulator, a 16V
10μF Y5V capacitor can exhibit an effective value as low
as 1μF to 2μF for the DC bias applied and over the operat-
ing temperature range. The X5R and X7R dielectrics yield
much more stable characteristics and are more suitable
for use as the output capacitor. The X7R type works over
a wider temperature range and has better temperature
stability whereas X5R is less expensive and is available in
highervalues.CarestillmustbeexercisedwhenusingX5R
and X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
R2
⎠
R1
⎛
⎝
⎞
IN
OUT
LT1965
V
VOUT = 1.20V 1+
+IADJ •R2
⎟
OUT
⎜
+
V
R2
R1
IN
V
ADJ = 1.20V
ADJ = 1.3µA AT 25ºC
OUTPUT RANGE = 1.20V TO 19.5V
ADJ
I
GND
1965 F01
Figure 1. Adjustable Operation
Z5U capacitors, but can still be significant enough to drop
1965f
8
Linear [ Linear ]
