MIC2954 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – 250mA Low-Dropout Regulator

MIC2954
Applications Information
External Capacitors
A 2.2µF (or greater) capacitor is required between the MIC2954
output and ground to prevent oscillations due to instability.
Most types of tantalum or aluminum electrolytics will be
adequate; film types will work, but are costly and therefore not
recommended. Many aluminum electrolytics have electro-
lytes that freeze at about –30°C, so solid tantalums are
recommended for operation below –25°C. The important
parameters of the capacitor are an effective series resistance
of about 5Ω or less and a resonant frequency above 500kHz.
The value of this capacitor may be increased without limit.
At lower values of output current, less output capacitance is
required for output stability. The capacitor can be reduced to
0.5µF for current below 10mA or 0.15µF for currents below
1mA. Adjusting the MIC2954-07/-08 to voltages below 5V
runs the error amplifier at lower gains so that more output
capacitance is needed. For the worst-case situation of a
250mA load at 1.23V output (output shorted to feedback) a
5µF (or greater) capacitor should be used.
The MIC2954 will remain in regulation with a minimum load
of 1mA. When setting the output voltage of the MIC2954-07/
-08 version with external resistors, the current through these
resistors may be included as a portion of the minimum load.
A 0.1µF capacitor should be placed from the MIC2954 input
to ground if there is more than 10 inches of wire between the
input and the ac filter capacitor or if a battery is used as the
input.
Error Detection Comparator Output
(MIC2954-07/-08)
A logic-low output will be produced by the comparator when-
ever the MIC2954-07/-08 output falls out of regulation by
more than approximately 5%. This figure is the comparator’s
built-in offset of about 60mV divided by the 1.235V reference
voltage. (Refer to the block diagram on page 1). This trip level
remains “5% below normal” regardless of the programmed
output voltage of the MIC2954-07/-08. For example, the error
flag trip level is typically 4.75V for a 5V output or 11.4V for a
12V output. The out of regulation condition may be due either
to low input voltage, current limiting, or thermal limiting.
Figure 1 is a timing diagram depicting the ERR signal and the
regulated output voltage as the MIC2954-07/-08 input is
ramped up and down. The ERR signal becomes valid (low) at
about 1.3V input. It goes high at about 5V input (the input
voltage at which V_OUT = 4.75). Since the MIC2954-07/-08’s
dropout voltage is load-dependent (see curve in “Typical
Characteristics”), the input voltage trip point (about 5V) will
vary with the load current. The output voltage trip point
(approximately 4.75V) does not vary with load.
The error comparator has an open-collector output which
requires an external pull-up resistor. Depending on system
requirements, this resistor may be returned to the 5V output
or some other supply voltage. In determining a value for this
resistor, note that while the output is rated to sink 400µA, this
sink current adds to battery drain in a low battery condition.
Suggested values range from 100k to 1MΩ. The resistor is
not required if this output is unused.
Micrel
Programming the Output Voltage
(MIC2954-07/-08)
The MIC2954-07/-08 may be pin-strapped for 5V using its
internal voltage divider by tying pin 1 (OUT) to pin 2 (SNS) and
pin 7 (FB) to pin 6 (TAP). Alternatively, it may be programmed
for any output voltage between its 1.235V reference and its
30V maximum rating. An external pair of resistors is required,
as shown in Figure 3.
The complete equation for the output voltage is
V_OUT = V_REF ^{}1+ _RR21^{} + I_FB R1
where:
V_REF = nominal 1.235V reference voltage
I_FB = nominal FB pin bias current (–20nA)
The minimum recommended load current of 1µA forces an
upper limit of 1.2MΩ on the value of R2, if the regulator must
work with no load (a condition often found in CMOS in
standby), I_FB will produce a 2% typical error in V_OUT which
may be eliminated at room temperature by trimming R1. For
better accuracy, choosing R2 = 100k reduces this error to
0.17% while increasing the resistor program current to 12µA.
Since the MIC2954-07/-08 typically draws 60µA at no load
with pin 2 (SNS) open-circuited, this is a negligible addition.
Reducing Output Noise
In reference applications it may be advantageous to reduce
the ac noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor. This is relatively inefficient, as increasing the
capacitor from 1µF to 220µF only decreases the noise from
430µV to 160µV_RMS for a 100kHz bandwidth at 5V output.
Noise can be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity.
Pick:
C_BYPASS
≅
1
2πR1× 200Hz
Output
Voltage
4.75V
*
ERR ^Not
Valid
Input
Voltage
5V
1.3V
*
Not
Valid
*See Application Information
Figure 1. Error Output Timing
August 1999
9
MIC2954