MIC2920A/29201/29202/29204
Micrel
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.
Applications Information
External Capacitors
A 10µF (or greater) capacitor is required between the
MIC2920A 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 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
2.2µF for current below 10mA or 1µF for currents below
1mA. Adjusting the MIC29202/29204 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
500mA load at 1.23V output (Output shorted to Adjust) a 47µF
(or greater) capacitor should be used.
The MIC2920A/29201 will remain in regulation with a minimum
load of 1mA. When setting the output voltage of the MIC29202/
29204 versions 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 MIC2920A 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.
Programming the Output Voltage (MIC29202/29204)
The MIC29202/29204 may be programmed for any output
voltage between its 1.235V reference and its 26V maximum
rating, using an external pair of resistors, as shown in
Figure 3.
The complete equation for the output voltage is
V
OUT
= V
REF
×
{ 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
recommended 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
MIC29202/29204 typically draws 110µA at no load with
SHUTDOWN open-circuited, this is a negligible addition. The
MIC29204 may be pin-strapped for 5V using the internal
voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin
7 (Adjust) to Pin 6 (V Tap).
Configuring the MIC29201-3.3BM
For the MIC29201-3.3BM, the output (Pin 1) and sense pin
(pin 2),
must
be connected to ensure proper operation. They
are not connected internally.
Error Detection Comparator Output
(MIC29201/MIC29204)
A logic low output will be produced by the comparator whenever
the MIC29201/29204 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 MIC29201/29204. 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 MIC29201/29204 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 MIC29201/29204’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 (approximately 4.75V) does not vary with load.
The error comparator has an NPN open-collector output
which requires an external pull-up resistor. Depending on
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
4.75V
OUTPUT
VOLTAGE
ERROR
NOT
VALID
*
5V
1.3V
NOT
VALID
*
INPUT
VOLTAGE
* SEE APPLICATIONS INFORMATION
Figure 1. ERROR Output Timing
3-16
January 1998
MICREL [ MICREL SEMICONDUCTOR ]
