MIC2937A/29371/29372
Micrel, Inc.
Applications Information
External Capacitors
A 10µF (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 recom-
mended. 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 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
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 22µF (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 resis-
tors may be included as a portion of the minimum load.
A 0.1µF 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.
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 250µA, 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.
Programming the Output Voltage (MIC29372)
The MIC29372 may programmed for any output voltage be-
tween 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
V
OUT
= V
REF
x { 1 + R
1
/R
2
} –
|
I
FB
|
R
1
where V
REF
is the nominal 1.235 reference voltage and I
FB
is
the Adjust pin bias current, nominally 20nA. The minimum rec-
ommended load current of 1µA forces an upper limit of 1.2MΩ
on the value of R
2
, 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 R
1
. For better accuracy, choosing
R
2
= 100k reduces this error to 0.17% while increasing the
resistor program current to 12µA. Since the MIC29372 typically
draws 100µA at no load with SHUTDOWN open-circuited,
this is a negligible addition.
Error Detection Comparator Output (MIC29371)
A logic low output will be produced by the comparator when-
ever 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 volt-
age. (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 limit-
ing, 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
OUT
= 4.75). Since the MIC29371’s dropout
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 (ap-
proximately 4.75V) does not vary with load.
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 capaci-
tor from 1µF to 220µF only decreases the noise from 430µV
to 160µV
RMS
for a 100kHz bandwidth at 5V output. Noise can
be reduced by a factor of four with the adjustable regulators
OUTPUT
VOLTAGE
4.75V
ERROR NOT
*
VALID
INPUT
VOLTAGE
5V
1.3V
NOT
*
VALID
* SEE APPLICATIONS INFORMATION
Figure 1. ERROR Output Timing
MIC2937A/29371/29372
8
May 2006
MICREL [ MICREL SEMICONDUCTOR ]
