LT1020_15 Datasheet, PDF (7/16 Pages) Linear Technology – Micropower Regulator and Comparator

BLOCK DIAGRA
REF OUT 4
10
CURRENT
LIMIT
3 V_IN
LT1020
PNP OUT 5
NPN OUT 6
NON- ₇
INVERTING
INVERTING 8
2 ^VOUT
13
DROPOUT
DETECTOR
REF
2.5V
11 FB
9 GND
1020 BD
APPLICATIO S I FOR ATIO
The LT1020 is especially suited for micropower system
applications. For example, the comparator section of the
LT1020 may be used as a battery checker to provide an
indication of low battery. The dropout detector can shut
down the system when the battery voltage becomes too
low to regulate. Another type of system application for the
LT1020 would be to generate the equivalent of split
supplies from a single power input. The regulator section
provides regulated output voltage and the reference, which
can both source and sink current, is then an artificial
system ground providing a split supply for the system.
For many applications the comparator can be frequency
compensated to operate as an amplifier. Compensation
values for various gains are given in the data sheet. The
comparator gain is purposely low to make it easier to
frequency compensate as an amplifier. Two outputs are
available on the comparator, the NPN output is capable of
sinking 10mA and can drive loads connected to voltages
in excess of the positive power supply. This is useful for
driving switches or linear regulators from a higher input
voltage. The PNP output, which is capable of sourcing
100µA can drive loads below ground. It can be used to
make negative regulators with the addition of an external
pass transistor. Both outputs can be tied together to
provide an output that swings from rail-to-rail for com-
parator or amplifier applications. Although it is not speci-
fied, the gain for the PNP output is about 500 to 1000.
If the PNP output is being used, to maximize the gain a 1µA
to 5µA load should be placed upon the NPN output
collector. This is easily done by connecting a resistor
between the NPN collector and the reference output.
(Providing this operating current to the NPN side in-
creases the internal emitter base voltages and maximizes
the gain of the PNP stage.) Without this loading on the
NPN collector, at temperatures in excess of 75°C, the gain
of the PNP collector can decrease by a factor of 2 or 3.
Reference
Internal to the LT1020 is a 2.5V trimmed class B output
reference. The reference was designed to be able to source
or sink current so it could be used in supply splitting
applications as well as a general purpose reference for
external circuitry. The design of the reference allows it to
source typically 4mA or 5mA and sink 2mA. The available
source and sink current decreases as temperature in-
creases. It is sometimes desirable to decrease the AC
output impedance by placing an output capacitor on them.
The reference in the LT1020 becomes unstable with large
capacitive loads placed directly on it. When using an
output capacitor, about 20Ω should be used to isolate the
capacitor from the reference pin. This 20Ω resistor can be
placed directly in series with the capacitor or alternatively
the reference line can have 20Ω placed in series with it and
then a capacitor to ground. This is shown in Figure 1. Other
sn1020 1020fcs
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