LT1020 Datasheet, PDF (8/16 Pages) Linear Technology – Micropower Regulator and Comparator

LT1020
APPLICATI S I FOR ATIO
other precautions are necessary and the reference is
stable with nominal stray capacitances.
REF
4
OUTPUT
20Ω
OR
10µF
REF
4
20Ω
OUTPUT
10µF
1020 F01
Figure 1. Bypassing Reference
Overload Protection
The main regulator in the LT1020 is current limited at
approximately 350mA. The current limit is stable with both
input voltage and temperature. A current limit pin, when
strapped to ground, decreases the output current. This
allows the output current to be set to a lower value than
250mA. The output current available with the current limit
pin strapped to ground is not well controlled so if precise
current limiting is desired it should be provided externally
as is shown in some of the application circuits.
If the device is overloaded for long periods of time, thermal
shutdown turns the output off. In thermal shutdown, there
may be some oscillations which can disturb external
circuitry. A diode connected between the reference and
feedback terminal provides hysteresis under thermal shut-
down, so that the device turns on and off with about a 5
second period and there are no higher frequency oscilla-
tions. This is shown in Figure 2. This diode is recom-
mended for most applications. Thermal shutdown tem-
perature is set at approximately 145°C.
2
V_OUT
LT1020
REF
FB ¹¹
4
*
0.001µF
* DIODE ADDS FEEDBACK
10µF
1020 F02
Figure 2. Minimizing Oscillation in Thermal Shutdown
Like most other IC regulators, a minimum load is required
on the output of the LT1020 to maintain regulation. For
most standard regulators this is normally specified at
5mA. Of course, for a micropower regulator this would be
a tremendously large current. The output current must be
large enough to absorb all the leakage current of the pass
transistor at the maximum operating temperature. It also
affects the transient response; low output currents have
long recovery times from load transients. At high operat-
ing temperatures the minimum load current increases and
having too low of a load current may cause the output to
go unregulated. Devices are tested for minimum load
current at high temperature. The output voltage setting
resistors to the feedback terminal can usually be used to
provide the minimum load current.
Frequency Compensation
The LT1020 is frequency compensated by a dominant pole
on the output. An output capacitor of 10µF is usually large
enough to provide good stability. Increasing the output
capacitor above 10µF further improves stability. In order
to insure stability, a feedback capacitor is needed between
the output pin and the feedback pin. This is because stray
capacitance can form another pole with the large value of
feedback resistors used with the LT1020. Also, a feedback
capacitor minimizes noise pickup and improves ripple
rejection.
With the large dynamic operating range of the output
current, 10000:1, frequency response changes widely.
Low AC impedance capacitors are needed to insure stabil-
ity. While solid tantalum are best, aluminum electrolytics
can be used but larger capacitor values may be needed.
The CURRENT LIMIT pin allows one of the internal nodes
to be rolled off with a 0.05µF capacitor to ground. With this
capacitor, lower values of regulator output capacitance
can be used (down to 1µF) for low (<20mA) output
currents. Values of capacitance greater than 0.05µF de-
grade the transient response, so are not recommended.
If the CURRENT LIMIT pin is connected to GND, the
current limit is decreased and only a 1µF output capacitor
is needed.
When bypassing the reference, a 20Ω resistor must be
connected in series with the capacitor.
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