LP2950 datasheet(17/32 Pages) TI

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p ´

;

(CBYPASS)

(BYPASS)

200 Hz

R1 200 Hz

LP2950, LP2951

www.ti.com

SLVS582I – APRIL 2006 – REVISED NOVEMBER 2014

Typical Application (continued)

8.2.2 Detailed Design Procedure

8.2.2.1 Capacitance Value

For VOUT ≥ 5 V, a minimum of 1 μF is required. For lower VOUT, the regulator’s loop gain is running closer to unity

gain and, thus, has lower phase margins. Consequently, a larger capacitance is needed for stability.

For VOUT = 3 V or 3.3 V, a minimum of 2.2 μF is recommended. For worst case, VOUT = 1.23 V (using the ADJ

version), a minimum of 3.3

μF is recommended. COUT can be increased without limit and only improves the

regulator stability and transient response. Regardless of its value, the output capacitor should have a resonant

frequency greater than 500 kHz.

The minimum capacitance values given above are for maximum load current of 100 mA. If the maximum

expected load current is less than 100 mA, then lower values of COUT can be used. For instance, if IOUT < 10 mA,

then only 0.33

μF is required for COUT. For IOUT < 1 mA, 0.1 μF is sufficient for stability requirements. Thus, for a

worst-case condition of 100-mA load and VOUT = VREF = 1.235 V (representing the highest load current and

lowest loop gain), a minimum COUT of 3.3 μF is recommended.

For the LP2950/51, no load stability is inherent in the design — a desirable feature in CMOS circuits that are put

in standby (such as RAM keep-alive applications). If the LP2951 is used with external resistors to set the output

voltage, a minimum load current of 1

μA is recommended through the resistor divider.

8.2.2.2 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.

8.2.2.3 CBYPASS: Noise and Stability Improvement

In the LP2951 device, 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 device. 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, where loop gain reduction is greatest. A

suitable CBYPASS is calculated as shown in Equation 2:

(2)

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

the regulator bandwidth to be reduced, thus 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).

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