LT1129/LT1129-3.3/LT1129-5
PI FU CTIO S
Adjust Pin:
For the LT1129 (adjustable version) the adjust
pin is the input to the error amplifier. This pin is internally
clamped to 6V and – 0.6V (one V
BE
). This pin has a bias
current of 150nA which flows into the pin. See Bias Current
curve in the Typical Performance Characteristics. The
adjust pin reference voltage is equal to 3.75V referenced
to ground.
R
P
IN
1µF
LT1129
SHDN
GND
SENSE
10µF
OUT
APPLICATIO S I FOR ATIO
The LT1129 is a micropower low dropout regulator with
shutdown, capable of supplying 700mA of output current
at a dropout voltage of 0.4V. The device operates with very
low quiescent current (50µA). In shutdown the quiescent
current drops to only 16µA. In addition to the low quies-
cent current the LT1129 incorporates several protection
features which make it ideal for use in battery-powered
systems. The device is protected against reverse input
voltages. In battery backup applications where the output
can be held up by a backup battery when the input is pulled
to ground, the LT1129 acts like it has a diode in series with
its output and prevents reverse current flow.
Adjustable Operation
The adjustable version of the LT1129 has an output
voltage range of 3.75V to 30V. The output voltage is set by
the ratio of two external resistors as shown in Figure 2. The
device servos the output voltage to maintain the voltage at
the adjust pin at 3.75V. The current in R1 is then equal to
3.75V/R1. The current in R2 is equal to the sum of the
current in R1 and the adjust pin bias current. The adjust pin
bias current, 150nA at 25°C, flows through R2 into the
adjust pin. The output voltage can be calculated according
to the formula in Figure 2. The value of R1 should be less
than 400k to minimize errors in the output voltage caused
by the adjust pin bias current. Note that in shutdown the
output is turned off and the divider current will be zero.
Curves of Adjust Pin Voltage vs Temperature and Adjust
Pin Bias Current vs Temperature appear in the Typical
U
W
U
U
U
U
U
+
LOAD
R
P
LT1129 • F01
Figure 1. Kelvin Sense Connection
Performance Characteristics. The reference voltage at the
adjust pin has a positive temperature coefficient of ap-
proximately 15ppm/°C. The adjust pin bias current has a
negative temperature coefficient. These effects are small
and will tend to cancel each other.
The adjustable device is specified with the adjust pin tied
to the output pin. This sets the output voltage to 3.75V.
Specifications for output voltages greater than 3.75V will
be proportional to the ratio of the desired output voltage to
3.75V (V
OUT
/3.75V). For example: load regulation for an
output current change of 1mA to 700mA is – 6mV typical
at V
OUT
= 3.75V. At V
OUT
= 12V, load regulation would be:
⎛
12V
⎞
⎜
⎟
•
(
– 6mV
)
=
(
–19mV
)
⎝
3.75V
⎠
IN
LT1129
SHDN
GND
OUT
R2
ADJ
R1
V
OUT
+
LT1129 • F02
V
OUT
= 3.75V 1 + R2 + I
ADJ
• R2
R1
V
ADJ
= 3.75V
I
ADJ
= 150nA at 25°C
OUTPUT RANGE = 3.75V to 30V
(
)
(
)
Figure 2. Adjustable Operation
112935fc
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
