LP3905 Datasheet, PDF (13/15 Pages) National Semiconductor (TI) – Power Management Unit For Low Power Handheld Applications

Application Information (Continued)
The ceramic capacitor’s capacitance can vary with tempera-
ture. The capacitor type X7R, which operates over a tem-
perature range of -55˚C to +125˚C, will only vary the capaci-
tance to within ±15%. The capacitor type X5R has a similar
tolerance over a reduced temperature range of -55˚C to
+85˚C.
Tantalum capacitors are less desirable than ceramic for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
1µF to 4.7µF range.
Another important consideration is that tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a tantalum
capacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly ) than a ceramic capacitor with the same ESR
value. It should also be noted that the ESR of a typical
tantalum will increase about 2:1 as the temperature goes
from 25˚C down to -40˚C, so some guard band must be
allowed.
LDO Input Capacitor
An input capacitor is required for stability. The input capacitor
should be at least equal to or greater than the output capaci-
tor. It is recommended that a 1µF capacitor be connected
between V_IN2 input pin and ground (this capacitance value
may be increased without limit).
This capacitor must be located a distance of not more than
1cm from the input pin and returned to a clean analogue
ground. Any good quality ceramic, tantalum, or film capacitor
may be used at the input.
Important: Tantalum capacitors can suffer catastrophic fail-
ures due to surge current when connected to a low-
impedance source of power (like a battery or a very large
capacitor). If a tantalum capacitor is used at the input, it must
be guaranteed by the manufacturer to have a surge current
rating sufficient for the application. There are no require-
ments for the ESR (Equivalent Series Resistance) on the
input capacitor, but tolerance and temperature coefficient
must be considered when selecting the capacitor to ensure
the capacitance will remain 1.0µF ±30% over the entire
operating voltage and temperature range.
LDO Output Capacitor
The LP3905 LDOs are designed specifically to work with
very small ceramic output capacitors. A ceramic capacitor
(dielectric types X5R or X7R) in the 0.47µF to 10µF range,
and with ESR between 5mΩ to 500mΩ, is suitable in the
application circuit. For this device the output capacitor
should be connected between the LDO1 and LDO2 pins and
a good ground connection and should be mounted within
1cm of the device.
The output capacitor must meet the requirement for the
minimum value of capacitance and also have an ESR value
that is within the range 5mΩ to 500mΩ for stability.
No-Load Stability
The LP3905 LDOs will remain stable and in regulation with
no external load.
Enable Control
A 1MΩ pulldown resistor ties the EN_1/2 input to ground, this
ensures that the device will remain off when the enable pin is
left open circuit. To ensure proper operation, the signal
source used to drive the EN_1/2 input must be able to swing
above and below the specified turn-on/off voltage thresholds
listed in the Electrical Characteristics section under V_IL and
V_IH. EN1 can be used to turn ON Buck1 and LDO1/2. In this
case Buck1 will be turned on first. Once Buck1 is powered
up, after a typical 150µs delay LDO1/2 will be turned on
concurrently.
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