MIC2937A Micrel Semiconductor, MIC2937A Datasheet - Page 8

• Manufacturer Part Number: MIC2937A
• Description: 750mA Low-Dropout Voltage Regulator
• Manufacturer: Micrel Semiconductor

MIC2937A/29371/29372
Applications Information
External Capacitors
A 10 F (or greater) capacitor is required between the
MIC2937A 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 electrolytes 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 5 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
0.5 F for current below 10mA or 0.15 F for currents below
1 mA. Adjusting the MIC29372 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 750mA load at
1.23V output (Output shorted to Adjust) a 22 F (or greater)
capacitor should be used.
The MIC2937A/29371 will remain in regulation with a minimum
load of 5mA. When setting the output voltage of the MIC29372
version with external resistors, the current through these
resistors may be included as a portion of the minimum load.
A 0.1 F capacitor should be placed from the 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 (MIC29371)
A logic low output will be produced by the comparator whenever
the MIC29371 output falls out of regulation by more than
approximately 5%. This figure is the comparator’s built-in
offset of about 75mV 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 MIC29371. 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,extremely high input voltage, current limiting, or thermal
limiting.
Figure 1 is a timing diagram depicting the ERROR signal and
the regulated output voltage as the MIC29371 input is ramped
up and down. The ERROR signal becomes valid (low) at
about 1.3V input. It goes high at about 5V input (the input
voltage at which V
voltage is load-dependent (see curve in Typical Performance
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.
October 1999
OUT
= 4.75). Since the MIC29371’s dropout
25
The error comparator has an NPN 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
250 A, this sink current adds to battery drain in a low battery
condition. Suggested values range from 100k to 1M . The
resistor is not required if this output is unused.
Programming the Output Voltage (MIC29372)
The MIC29372 may programmed for any output voltage
between its 1.235V reference and its 26V maximum rating. An
external pair of resistors is required, as shown in Figure 3.
The complete equation for the output voltage is
where V
the Adjust pin bias current, nominally 20nA. The minimum
recommended load current of 1 A forces an upper limit of
1.2M on the value of R
load (a condition often found in CMOS in standby), I
produce a –2% typical error in V
at room temperature by trimming R
choosing R
the resistor program current to 12 A. Since the MIC29372
typically draws 100 A at no load with SHUTDOWN 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 1 F to 220 F only decreases the noise from
430 V to 160 V
Noise can be reduced by a factor of four with the adjustable
* SEE APPLICATIONS INFORMATION
V
OUT
VOLTAGE
VOLTAGE
REF
OUTPUT
ERROR
= V
INPUT
2
= 100k reduces this error to 0.17% while increasing
is the nominal 1.235 reference voltage and I
REF
Figure 1. ERROR Output Timing
VALID
RMS
x { 1 + R
NOT
*
4.75V
for a 100kHz bandwidth at 5V output.
1.3V
2
, if the regulator must work with no
1
/R
5V
2
} –
OUT
|
I
MIC2937A/29371/29372
FB
which may be eliminated
1
|
. For better accuracy,
R
1
VALID
NOT
*
Micrel
FB
FB
will
is