LP3891 Datasheet, PDF (8/12 Pages) National Semiconductor (TI) – 0.8A Fast-Response Ultra Low Dropout Linear

Application Hints
V_BIAS RESTRICTIONS FOR PROPER START-UP
To prevent misoperation, ensure that V_BIAS is below 50mV
before start-up is initiated. This scenario can occur in sys-
tems with a backup battery using reverse-biased "blocking"
diodes which may allow enough leakage current to flow into
the V_BIAS node to raise it’s voltage slightly above ground
when the main power is removed. Using low leakage diodes
or a resistive pull down can prevent the voltage at V_BIAS from
rising above 50mV. Large bulk capacitors connected to
V_BIAS may also cause a start-up problem if they do not
discharge fully before re-start is initiated (but only if V_BIAS is
allowed to fall below 1V). A resistor connected across the
capacitor will allow it to discharge more quickly. It should be
noted that the probability of a "false start" caused by incor-
rect logic states is extremely low.
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 capaci-
tor must be within the "stable" range as shown in the graph
below over the full operating temperature range for stable
operation.
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 capaci-
tors 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.
SHUTDOWN OPERATION
Pulling down the shutdown (S/D) pin will turn-off the regula-
tor. Pin S/D must be actively terminated through a pull-up
resistor (10 kΩ to 100 kΩ) for a proper operation. If this pin
is driven from a source that actively pulls high and low (such
as a CMOS rail to rail comparator), the pull-up resistor is not
required. This pin must be tied to Vin if not used.
20069531
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.
POWER DISSIPATION/HEATSINKING
A heatsink may be required depending on the maximum
power dissipation and maximum ambient temperature of the
application. Under all possible conditions, the junction tem-
perature must be within the range specified under operating
conditions. The total power dissipation of the device is given
by:
P_D = (V_IN−V_OUT)I_OUT+ (V_IN)I_GND
where I_GND is the operating ground current of the device.
The maximum allowable temperature rise (T_Rmax) depends
on the maximum ambient temperature (T_Amax) of the appli-
cation, and the maximum allowable junction temperature
(T_Jmax):
T_Rmax = T_Jmax− T_Amax
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ_JA, can be calculated using the formula:
θ_JA = T_Rmax / P_D
These parts are available in TO-220 and TO-263 packages.
The thermal resistance depends on amount of copper area
or heat sink, and on air flow. If the maximum allowable value
of θ_JA calculated above is ≥ 60 ˚C/W for TO-220 package
and ≥ 60 ˚C/W for TO-263 package no heatsink is needed
since the package can dissipate enough heat to satisfy these
requirements. If the value for allowable θ_JA falls below these
limits, a heat sink is required.
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