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(CBYPASS)

] 200 Hz ³ CBYPASS +

200 Hz

LP2950

LP2951

SLVS582F – APRIL 2006 – REVISED DECEMBER 2009

www.ti.com

Capacitor Types

Most tantalum or aluminum electrolytics are suitable for use at the input. Film-type capacitors also work, but at

higher cost. When operating at low temperature, care should be taken with aluminum electrolytics, as their

electrolytes often freeze at –30°C. For this reason, solid tantalum capacitors should be used at temperatures

below –25°C.

Ceramic capacitors can be used, but due to their low ESR (as low as 5 m

Ω to 10 mΩ), they may not meet the

minimum ESR requirement previously discussed. If a ceramic capacitor is used, a series resistor between 0.1

Ω

to 2

Ω must be added to meet the minimum ESR requirement. In addition, ceramic capacitors have one glaring

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 temperature rises from 25°C to 85°C. Thus, a 2.2-

μF capacitor at 25°C drops well below the

minimum COUT required for stability as ambient temperature rises. For this reason, select an output capacitor that

maintains the minimum 2.2

μF required for stability for the entire operating temperature range.

CBYPASS: Noise and Stability Improvement

In the LP2951, an external FEEDBACK pin directly connected to the error amplifier noninverting input can allow

stray capacitance to cause instability by shunting the error amplifier feedback to GND, especially at high

frequencies. This is worsened if high-value external resistors are used to set the output voltage, because a high

resistance allows the stray capacitance to play a more significant role; i.e., a larger RC time delay is introduced

between the output of the error amplifier and its FEEDBACK input, leading to more phase shift and lower phase

margin. A solution is to add a 100-pF bypass capacitor (CBYPASS) between OUTPUT and FEEDBACK; because

CBYPASS is in parallel with R1, it lowers the impedance seen at FEEDBACK at high frequencies, in effect

offsetting the effect of the parasitic capacitance by providing more feedback at higher frequencies. More

feedback forces the error amplifier to work at a lower loop gain, so COUT should be increased to a minimum of

3.3

μF to improve the regulator’s phase margin.

CBYPASS can be also used to reduce output noise in the LP2951. This bypass capacitor reduces the closed loop

gain of the error amplifier at the high frequency, so noise no longer scales with the output voltage. This

improvement is more noticeable with higher output voltages, because loop gain reduction is greatest. A suitable

CBYPASS is calculated as shown in Equation 1:

(1)

On the 3-pin LP2950, noise reduction can be achieved by increasing the output capacitor, which causes the

regulator bandwidth to be reduced, therefore, eliminating high-frequency noise. However, this method is relatively

inefficient, as increasing COUT from 1 μF to 220 μF only reduces the regulator’s output noise from 430 μV to

160

μV (over a 100-kHz bandwidth).

ERROR Function (LP2951 Only)

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 VOUT has reached

95% of its nominal value. This

95% 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 (ERROR

output 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 hysteresis

of 15 mV between the thresholds for high and low ERROR output.

A timing diagram is shown in Figure 1 for ERROR vs VOUT (5 V), as VIN is ramped up and down. ERROR

becomes valid (low) when VIN

1.3 V. When VIN

5 V, VOUT = 4.75 V, causing ERROR to go high. Because the

dropout voltage is load dependent, the output trip-point threshold is reached at different values of VIN, depending

on the load current. For instance, at higher load current, ERROR goes high at a slightly higher value of VIN, and

vice versa for lower load current. The output-voltage trip point remains at

4.75 V, regardless of the load. Note

that when VIN ≤ 1.3 V, the ERROR comparator output is turned off and pulled high to its pullup voltage. If VOUT is

used as the pullup voltage, rather than an external 5-V source, ERROR typically is

1.2 V. In this condition, an

equal resistor divider (10 k

Ω is suitable) can be tied to ERROR to divide down the voltage to a valid logic low

during any fault condition, while still enabling a logic high during normal operation.

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