Datasheet

LP3999
www.ti.com SNVS207E –JUNE 2003–REVISED MAY 2013
NO-LOAD STABILITY
The LP3999 will remain stable and in regulation with no external load. This is an important consideration in some
circuits, for example CMOS RAM keep-alive applications.
CAPACITOR CHARACTERISTICS
The LP3999 is designed to work with ceramic capacitors on the output to take advantage of the benefits they
offer. For capacitance values in the range of 1 µF to 4.7 µF, ceramic capacitors are the smallest, least expensive
and have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a
typical 1 µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR requirement for
stability for the LP3999.
The temperature performance of ceramic capacitors varies by type. Most large value ceramic capacitors (≥ 2.2
µF) are manufactured with Z5U or Y5V temperature characteristics, which results in the capacitance dropping by
more than 50% as the temperature goes from 25°C to 85°C.
A better choice for temperature coefficient in a ceramic capacitor is X7R. This type of capacitor is the most stable
and holds the capacitance within ±15% over the temperature range. 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 1 µF to 4.7 µF 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.
NOISE BYPASS CAPACITOR
A bypass capacitor should be connected between the C
BYPASS
pin and ground to significantly reduce the noise at
the regulator output. This device pin connects directly to a high impedance node within the bandgap reference
circuitry. Any significant loading on this node will cause a change on the regulated output voltage. For this
reason, DC leakage current through this pin must be kept as low as possible for best output voltage accuracy.
The use of a 0.01µF bypass capacitor is strongly recommended to prevent overshoot on the output during start-
up.
The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High quality ceramic
capacitors with NPO or COG dielectric typically have very low leakage. Polypropolene and polycarbonate film
capacitors are available in small surface-mount packages and typically have extremely low leakage current.
Unlike many other LDO’s, the addition of a noise reduction capacitor does not effect the transient response of the
device.
ENABLE OPERATION
The LP3999 may be switched ON or OFF by a logic input at the ENABLE pin, V
EN
. A high voltage at this pin will
turn the device on. When the enable pin is low, the regulator output is off and the device typically consumes 3
nA. If the application does not require the shutdown feature, the V
EN
pin should be tied to V
IN
to keep the
regulator output permanently on. To ensure proper operation, the signal source used to drive the V
EN
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
.
FAST TURN ON
Fast turn-on is ensured by control circuitry within the reference block allowing a very fast ramp of the output
voltage to reach the target voltage. There is no active turn-off on this device. Refer to LP3995 for a similar device
with active turn-off.
DSBGA MOUNTING
The DSBGA package requires specific mounting techniques which are detailed in TI's AN-1112 Application
Report (SNVA009).
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