LP2951-33-Q1 Datasheet, PDF (17/30 Pages) Texas Instruments – Adjustable Micropower Voltage Regulators With Shutdown

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LP2951-33-Q1, LP2951-50-Q1
SLVSAW6E – JUNE 2011 – REVISED NOVEMBER 2014
Typical Application (continued)
8.2.2 Detailed Design Procedure
8.2.2.1 Capacitance Value
For V_OUT ≥ 5 V, a minimum of 1 μF is required. For lower V_OUT, 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 V_OUT = 3 V or 3.3 V, a minimum of 2.2 μF is recommended. For worst case, V_OUT = 1.23 V (using the ADJ
version), a minimum of 3.3 μF is recommended. C_OUT 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 C_OUT can be used. For instance, if I_OUT < 10 mA,
then only 0.33 μF is required for C_OUT. For I_OUT < 1 mA, 0.1 μF is sufficient for stability requirements. Thus, for a
worst-case condition of 100-mA load and V_OUT = V_REF = 1.235 V (representing the highest load current and
lowest loop gain), a minimum C_OUT of 3.3 μF is recommended.
For the LP2951-xx-Q1 devices, 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-xx-Q1 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 mΩ 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 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 for the entire operating temperature range.
8.2.2.3 C_BYPASS: Noise and Stability Improvement
In the LP2951-xx-Q1 devices, 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 (C_BYPASS) between OUTPUT and
FEEDBACK; because C_BYPASS 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 C_OUT should be increased
to a minimum of 3.3 μF to improve the regulator’s phase margin.
C_BYPASS can be also used to reduce output noise in the LP2951-xx-Q1 devices. 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 C_BYPASS is calculated as shown in Equation 2:
f_{( CBYPASS )}
;
200 Hz ® C_(BYPASS)
=
1
2p ´ R1´ 200 Hz
(2)
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