Capacitor Types
C
BYPASS
: Noise and Stability Improvement
f(CBYPASS) ]200 Hz ³CBYPASS +1
2p R1 200 Hz
(1)
ERROR Function (LP2951 Only)
LP2950 , , LP2951ADJUSTABLE MICROPOWER VOLTAGE REGULATORSWITH SHUTDOWN
SLVS582C – APRIL 2006 – REVISED SEPTEMBER 2006
APPLICATION INFORMATION (continued)
Most tantalum or aluminum electrolytics are suitable for use at the input. Film-type capacitors also work, but athigher cost. When operating at low temperature, care should be taken with aluminum electrolytics, as theirelectrolytes often freeze at –30 °C. For this reason, solid tantalum capacitors should be used at temperaturesbelow –25 °C.
Ceramic capacitors can be used, but due to their low ESR (as low as 5 m Ωto 10 m Ω), they may not meet theminimum ESR requirement previously discussed. If a ceramic capacitor is used, a series resistor between 0.1 Ωto 2 Ωmust be added to meet the minimum ESR requirement. In addition, ceramic capacitors have one glaringdisadvantage that must be taken into account — a poor temperature coefficient, where the capacitance can varysignificantly with temperature. For instance, a large-value ceramic capacitor ( ≥2.2 µF) can lose more than half ofits capacitance as temperature rises from 25 °C to 85 °C. Thus, a 2.2- µF capacitor at 25 °C drops well below theminimum C
OUT
required for stability as ambient temperature rises. For this reason, select an output capacitorthat maintains the minimum 2.2- µF required for stability for the entire operating temperature range.
In the LP2951, an external FEEDBACK pin directly connected to the error amplifier noninverting input can allowstray capacitance to cause instability by shunting the error amplifier feedback to GND, especially at highfrequencies. This is worsened if high-value external resistors are used to set the output voltage, because a highresistance allows the stray capacitance to play a more significant role; i.e., a larger RC time delay is introducedbetween the output of the error amplifier and its FEEDBACK input, leading to more phase shift and lower phasemargin. A solution is to add a 100-pF bypass capacitor (C
BYPASS
) between OUTPUT and FEEDBACK; becauseC
BYPASS
is in parallel with R1, it lowers the impedance seen at FEEDBACK at high frequencies, in effectoffsetting the effect of the parasitic capacitance by providing more feedback at higher frequencies. Morefeedback forces the error amplifier to work at a lower loop gain, so C
OUT
should be increased to a minimum of3.3 µF to improve the regulator’s phase margin.
C
BYPASS
can be also used to reduce output noise in the LP2951. This bypass capacitor reduces the closed loopgain of the error amplifier at the high frequency, so noise no longer scales with the output voltage. Thisimprovement is more noticeable with higher output voltages, because loop gain reduction is greatest. A suitableC
BYPASS
is calculated as shown in Equation 1 :
On the 3-pin LP2950, noise reduction can be achieved by increasing the output capacitor, which causes theregulator bandwidth to be reduced, therefore, eliminating high-frequency noise. However, this method isrelatively inefficient, as increasing C
OUT
from 1 µF to 220 µF only reduces the regulator’s output noise from430 µV to 160 µV (over a 100-kHz bandwidth).
The LP2951 has a low-voltage detection comparator that outputs a logic low when the output voltage drops by≈6% from its nominal value, and outputs a logic high when V
OUT
has reached ≈95% of its nominal value. This95% of nominal figure is obtained by dividing the built-in offset of ≈60 mV by the 1.235-V bandgap reference,and remains independent of the programmed output voltage. For example, the trip-point threshold ( ERRORoutput goes high) typically is 4.75 V for a 5-V output and 11.4 V for a 12-V output. Typically, there is a hysteresisof 15 mV between the thresholds for high and low ERROR output.
A timing diagram is shown in Figure 1 for ERROR vs V
OUT
(5 V), as V
IN
is ramped up and down. ERRORbecomes valid (low) when V
IN
≈1.3 V. When V
IN
≈5 V, V
OUT
= 4.75 V, causing ERROR to go high. Because thedropout voltage is load dependent, the output trip-point threshold is reached at different values of V
IN
, dependingon the load current. For instance, at higher load current, ERROR goes high at a slightly higher value of V
IN
, andvice versa for lower load current. The output-voltage trip point remains at ≈4.75 V, regardless of the load. Notethat when V
IN
≤1.3 V, the ERROR comparator output is turned off and pulled high to its pullup voltage. If V
OUT
isused as the pullup voltage, rather than an external 5-V source, ERROR typically is ≈1.2 V. In this condition, anequal resistor divider (10 k Ωis suitable) can be tied to ERROR to divide down the voltage to a valid logic lowduring any fault condition, while still enabling a logic high during normal operation.
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