Datasheet

O
1 2
V
R R 1
1.25V
é ù
= -
ê ú
ë û
V
O
=
V
REF
R
1
R
2
+
1
OFF
THERMAL
PROTECTION
1.25V
BANDGAP
FAST
START-UP
CIRCUIT
CURRENT
LIMIT
FAULT
COMPARATORS
R1
R2
V
IN
V
O
SET
CC
SHDN
FAULT
GND
ERROR
AMP
+
-
LP3982
www.ti.com SNVS185D –FEBRUARY 2002–REVISED APRIL 2013
APPLICATION INFORMATION
General Information
The LP3982 is package, pin and performance compatible with Maxim's MAX8860 excluding reverse battery
protection and Dual Mode function (fixed and adjustable combined).
Figure 16 shows the functional block diagram for the LP3982. A 1.25V bandgap reference, an error amplifier and
a PMOS pass transistor perform voltage regulation while being supported by shutdown, fault, and the usual
Temperature and current protection circuitry
The regulator's topology is the classic type with negative feedback from the output to one of the inputs of the
error amplifier. Feedback resistors R
1
and R
2
are either internal or external to the IC, depending on whether it is
the fixed voltage version or the adjustable version. The negative feedback and high open loop gain of the error
amplifier cause the two inputs of the error amplifier to be virtually equal in voltage. If the output voltage changes
due to load changes, the error amplifier provides the appropriate drive to the pass transistor to maintain the error
amplifier's inputs as virtually equal. In short, the error amplifier keeps the output voltage constant in order to keep
its inputs equal.
Figure 16. Functional Block Diagram for the LP3982
Output Voltage Setting (Adj Version Only)
The output voltage is set according to the amount of negative feedback (Note that the pass transistor inverts the
feedback signal.) Figure 17 simplifies the topology of the LP3982. This type of regulator can be represented as
an op amp configured as non-inverting amplifier and a fixed DC Voltage (V
REF
) for its input signal. The special
characteristic of this op amp is its extra-large output transistor that only sources current. In terms of its non-
inverting configuration, the output voltage equals V
REF
times the closed loop gain:
(1)
Utilize the following equation for adjusting the output to a particular voltage:
(2)
Choose R
2
= 100k to optimize accuracy, power supply rejection, noise and power consumption.
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