LP2985-N
SNVS018V –MARCH 2000–REVISED JUNE 2013
www.ti.com
Noise Bypass Capacitor
Connecting a 10 nF capacitor to the Bypass pin significantly reduces noise on the regulator output. It should be
noted that the capacitor is connected directly to a high-impedance circuit in the bandgap reference.
Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause a
change in the regulated output voltage. For this reason, DC leakage current through the noise bypass capacitor
must never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy.
The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic
capacitors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
CAPACITOR CHARACTERISTICS
The LP2985-N was designed to work with ceramic capacitors on the output to take advantage of the benefits
they offer: for capacitance values in the 2.2 µF to 4.7 µF range, ceramics are the least expensive and also have
the lowest ESR values (which makes them best for eliminating high-frequency noise). The ESR of a typical 2.2
µF ceramic capacitor is in the range of 10 mΩ to 20 mΩ, which easily meets the ESR limits required for stability
by the LP2985-N.
One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Most large value
ceramic capacitors (≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in
the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.
This could cause problems if a 2.2 µF capacitor were used on the output since it will drop down to approximately
1 µF at high ambient temperatures (which could cause the LM2985 to oscillate). If Z5U or Y5V capacitors are
used on the output, a minimum capacitance value of 4.7 µF must be observed.
A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within
±15%. Unfortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric.
Tantalum
Tantalum capacitors are less desirable than ceramics 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.
On/Off Input Operation
The LP2985-N is shut off by driving the ON/OFF input low, and turned on by pulling it high. If this feature is not to
be used, the ON/OFF input should be tied to VIN to keep the regulator output on at all times.
To assure proper operation, the signal source used to drive the ON/OFF input must be able to swing above and
below the specified turn-on/turn-off voltage thresholds listed in the ELECTRICAL CHARACTERISTICS section
under VON/OFF. To prevent mis-operation, the turn-on (and turn-off) voltage signals applied to the ON/OFF input
must have a slew rate which is ≥ 40 mV/µs.
CAUTION
The regulator output voltage cannot be ensured if a slow-moving AC (or DC) signal is
applied that is in the range between the specified turn-on and turn-off voltages listed
under the electrical specification VON/OFF (see ELECTRICAL CHARACTERISTICS).
12
Submit Documentation Feedback
Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LP2985-N
TI [ TEXAS INSTRUMENTS ]
