LP38841 Datasheet, PDF (6/18 Pages) National Semiconductor (TI) – 0.8A Ultra Low Dropout Linear Regulators Stable with Ceramic Output Capacitors

LP38841
SNVS289B – DECEMBER 2004 – REVISED SEPTEMBER 2006
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Typical Performance Characteristics (continued)
Unless otherwise specified: T_J = 25°C, C_IN = 10 µF CER, C_OUT = 22 µF CER, C_BIAS = 1 µF CER,S/D Pin is tied to V_BIAS, V_OUT
= 1.2V, I_L = 10mA, V_BIAS = 5V, V_IN = V_OUT + 1V.
V_BIAS PSRR
V_IN PSRR
100
90
C_BIAS = 1 PF Tantalum
80
I_L = 0
V_OUT = 1.2V
70
60
50
40
80
C_OUT = 4.7 PF Tantalum
70
C_BIAS = 1 PF Tantalum
60
I_L = 10 mA
50
40
30
20
30
10
20
10
0
100
1000 10000 100000 1000000
0
1000
10000
100000
FREQUENCY (Hz)
1000000
FREQUENCY (Hz)
Application Hints
EXTERNAL CAPACITORS
To assure regulator stability, input and output capacitors are required as shown in the Typical Application Circuit.
OUTPUT CAPACITOR
An output capacitor is required on the LP3884X devices for loop stability. The minimum value of capacitance
necessary depends on type of capacitor: if a solid Tantalum capacitor is used, the part is stable with capacitor
values as low as 4.7µF. If a ceramic capacitor is used, a minimum of 22 µF of capacitance must be used
(capacitance may be increased without limit). The reason a larger ceramic capacitor is required is that the output
capacitor sets a pole which limits the loop bandwidth. The Tantalum capacitor has a higher ESR than the
ceramic which provides more phase margin to the loop, thereby allowing the use of a smaller output capacitor
because adequate phase margin can be maintained out to a higher crossover frequency. The tantalum capacitor
will typically also provide faster settling time on the output after a fast changing load transient occurs, but the
ceramic capacitor is superior for bypassing high frequency noise.
The output capacitor must be located less than one centimeter from the output pin and returned to a clean
analog ground. Care must be taken in choosing the output capacitor to ensure that sufficient capacitance is
provided over the full operating temperature range. If ceramics are selected, only X7R or X5R types may be
used because Z5U and Y5F types suffer severe loss of capacitance with temperature and applied voltage and
may only provide 20% of their rated capacitance in operation.
INPUT CAPACITOR
The input capacitor is also critical to loop stability because it provides a low source impedance for the regulator.
The minimum required input capacitance is 10 µF ceramic (Tantalum not recommended). The value of C_IN may
be increased without limit. As stated above, X5R or X7R must be used to ensure sufficient capacitance is
provided. The input capacitor must be located less than one centimeter from the input pin and returned to a clean
analog ground.
BIAS CAPACITOR
The 0.1µF capacitor on the bias line can be any good quality capacitor (ceramic is recommended).
BIAS VOLTAGE
The bias voltage is an external voltage rail required to get gate drive for the N-FET pass transistor. Bias voltage
must be in the range of 4.5 - 5.5V to assure proper operation of the part.
6
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