參數(shù)資料
型號(hào)： LP2981A-125DBVR
廠商： Texas Instruments, Inc.
英文描述： 100-MA ULTRA DROPOUT REGULATOR WITH SHUTDOWN
中文描述： 100馬超差穩(wěn)壓器具有關(guān)斷
文件頁(yè)數(shù)： 13/20頁(yè)
文件大?。?/td> 346K
代理商： LP2981A-125DBVR
SLVS521B JULY 2004 REVISED OCTOBER 2004
13
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
output capacitor (C
out
) (continued)
It is critical that both the minimum capacitance and ESR requirement be met over the entire operating
temperature range.
Depending on the type of capacitor used, both of these parameters can vary significantly
with temperature (see capacitor characteristics section).
capacitor characteristics
ceramics
Due to their very low ESR values, ceramic capacitors are not suitable for use as the output capacitor. For
instance, a typical 2.2-
μ
F ceramic capacitor has an ESR in the range of 10 m
to 20 m
and, thus, easily can
fall out of minimum ESR requirements under certain operating conditions.
If a ceramic capacitor is used at the output, a 1-
resistor should be placed in series with the capacitor to raise
the ESR seen by the regulator.
tantalum
Solid tantalum capacitors are optimal choices for the LP2981, but they still must meet the minimum ESR
requirement. Note that the ESR of a tantalum capacitor increases as temperature drops, as much as double
from 25
°
C to 40
°
C. Thus, ESR margins must be maintained over the temperature range to prevent regulator
instability. For operation at very low temperatures, paralleling a tantalum capacitor with a ceramic one keeps
the combined ESR from increasing near the upper limit of the ESR curve.
aluminum
Aluminum capacitors can be used, but use with the LP2981 is impractical due to their large physical dimensions.
They also must meet the ESR requirements over the full temperature range. In this regard, aluminium capacitors
are at a big disadvantage due to their sharp ESR increase as temperature drops. For example, over a
temperature drop from 20
°
C to 40
°
C, the ESR of an aluminum electrolytic capacitor can increase by a factor
of 50. In addition, some of the electrolytes used in these capacitors can freeze at 25
°
C, making the capacitor
nonoperational.
ON/OFF operation
The LP2981 allows for a shutdown mode via the ON/OFF pin. If the shutdown feature is not used, ON/OFF
should be connected to the input to ensure that the regulator is on at all times. To drive ON/OFF:
A LOW (
≤
0.3 V) turns the regulator OFF; a HIGH (
≥
1.6 V) turns it ON.
Use either a totem-pole output or an open-collector output with a pullup resistor tied to V
IN
(or another
logic supply). The HIGH signal can exceed V
IN
, but must not exceed the absolute maximum ratings of
20 V for the ON/OFF pin.
Apply a signal with a slew rate of
≥
40 mV/
μ
s. A slow slew rate can cause the shutdown function to
operate incorrectly.
ADJ version
From Figure 3, V
OUT
is set according to the following equation:
R
ADJ
R
1
V
OUT
= V
REF
1
where V
REF
= 1.23 V (nominal). Under no-load conditions, a minimum “bias” current is required through the R
ADJ
and R
1
divider to maintain regulator stability. Therefore, the maximum value for R
1
should be
39.5 k
.
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