Datasheet

V
OUT
V
IN
LP2985
www.ti.com SLVS522N –JULY 2004– REVISED JUNE 2011
Noise Bypass Capacitor (C
BYPASS
)
The LP2985 allows for low-noise performance with the use of a bypass capacitor that is connected to the internal
bandgap reference via the BYPASS pin. This high-impedance bandgap circuitry is biased in the microampere
range and, thus, cannot be loaded significantly, otherwise, its output – and, correspondingly, the output of the
regulator – changes. Thus, for best output accuracy, dc leakage current through C
BYPASS
should be minimized as
much as possible and never should exceed 100 nA.
A 10-nF capacitor is recommended for C
BYPASS
. Ceramic and film capacitors are well suited for this purpose.
Capacitor Characteristics
Ceramics
Ceramic capacitors are ideal choices for use on the output of the LP2985 for several reasons. For capacitances
in the range of 2.2 μF to 4.7 μF, ceramic capacitors have the lowest cost and the lowest ESR, making them
choice candidates for filtering high-frequency noise. For instance, a typical 2.2-μF ceramic capacitor has an ESR
in the range of 10 mΩ to 20 mΩ and, thus, satisfies minimum ESR requirements of the regulator.
Ceramic capacitors have one major disadvantage that must be taken into account – a poor temperature
coefficient, where the capacitance can vary significantly with temperature. For instance, a large-value ceramic
capacitor (≥2.2 μF) can lose more than half of its capacitance as the temperature rises from 25°C to 85°C. Thus,
a 2.2-μF capacitor at 25°C drops well below the minimum C
OUT
required for stability, as ambient temperature
rises. For this reason, select an output capacitor that maintains the minimum 2.2 μF required for stability over the
entire operating temperature range. Note that there are some ceramic capacitors that can maintain a ±15%
capacitance tolerance over temperature.
Tantalum
Tantalum capacitors can be used at the output of the LP2985, but there are significant disadvantages that could
prohibit their use:
• In the 1-μF to 4.7-μF range, tantalum capacitors are more expensive than ceramics of the equivalent
capacitance and voltage ratings.
• Tantalum capacitors have higher ESRs than their equivalent-sized ceramic counterparts. Thus, to meet the
ESR requirements, a higher-capacitance tantalum may be required, at the expense of larger size and higher
cost.
• The ESR of a tantalum capacitor increases as temperature drops, as much as double from 25°C to –40°C.
Thus, ESR margins must be maintained over the temperature range to prevent regulator instability.
ON/OFF Operation
The LP2985 allows for a shutdown mode via the ON/OFF pin. Driving the pin LOW (≤0.3 V) turns the device
OFF; conversely, a HIGH (≥1.6 V) turns the device ON. If the shutdown feature is not used, ON/OFF should be
connected to the input to ensure that the regulator is on at all times. For proper operation, do not leave ON/OFF
unconnected, and apply a signal with a slew rate of ≥40 mV/μs.
Reverse Input-Output Voltage
There is an inherent diode present across the PNP pass element of the LP2985.
With the anode connected to the output, this diode is reverse biased during normal operation, since the input
voltage is higher than the output. However, if the output is pulled higher than the input for any reason, this diode
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