LP3891ES-1.2 Datasheet, PDF (8/19 Pages) Texas Instruments – 1.5% Output Accuracy (25°C), 60 nA Typical Quiescent Current in Shutdown

LP3891
SNVS235D – SEPTEMBER 2003 – REVISED APRIL 2013
Application Hints
www.ti.com
EXTERNAL CAPACITORS
To assure regulator stability, input and output capacitors are required as shown in the Typical Application Circuit.
OUTPUT CAPACITOR
At least 10µF of output capacitance is required for stability (the amount of capacitance can be increased without
limit). The output capacitor must be located less than 1cm from the output pin of the IC and returned to a clean
analog ground. The ESR (equivalent series resistance) of the output capacitor must be within the "stable" range
as shown in the graph below over the full operating temperature range for stable operation.
10
1.0
0.1
COUT > 10 PF
STABLE REGION
.01
.001
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
Figure 18. Minimum ESR vs Output Load Current
Tantalum capacitors are recommended for the output as their ESR is ideally suited to the part's requirements
and the ESR is very stable over temperature. Aluminum electrolytics are not recommended because their ESR
increases very rapidly at temperatures below 10°C. Aluminum caps can only be used in applications where lower
temperature operation is not required.
A second problem with Al caps is that many have ESR's which are only specified at low frequencies. The typical
loop bandwidth of a linear regulator is a few hundred kHz to several MHz. If an Al cap is used for the output cap,
it must be one whose ESR is specified at a frequency of 100 kHz or more.
Because the ESR of ceramic capacitors is only a few milli Ohms, they are not suitable for use as output
capacitors on LP389X devices. The regulator output can tolerate ceramic capacitance totaling up to 15% of the
amount of Tantalum capacitance connected from the output to ground.
INPUT CAPACITOR
The input capacitor must be at least 10 µF, but can be increased without limit. It's purpose is to provide a low
source impedance for the regulator input. Ceramic capacitors work best for this, but Tantalums are also very
good. There is no ESR limitation on the input capacitor (the lower, the better). Aluminum electrolytics can be
used, but their ESR increase very quickly at cold temperatures. They are not recommended for any application
where temperatures go below about 10°C.
BIAS CAPACITOR
The 0.1µF capacitor on the bias line can be any good quality capacitor (ceramic is recommended).
BIAS VOLTAGE
The bias voltage is an external voltage rail required to get gate drive for the N-FET pass transistor. Bias voltage
must be in the range of 4.5 - 6V to assure proper operation of the part.
UNDER VOLTAGE LOCKOUT
The bias voltage is monitored by a circuit which prevents the regulator output from turning on if the bias voltage
is below approximately 4V.
8
Submit Documentation Feedback
Product Folder Links: LP3891
Copyright © 2003–2013, Texas Instruments Incorporated