Micrel, Inc. MIC5206
May 2006
8
M9999-051506
(408) 955-1690
Application Information
Enable/Shutdown
Forcing EN (enable/shutdown) high (> 2V) enables the
regulator. EN is compatible with CMOS logic gates.
If the enable/shutdown feature is not required, connect
EN (enable) to IN (supply input). Refer to the text with
Figures 1aand 2.
Input Capacitor
A 1µF capacitor should be placed from IN to GND 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.
Reference Bypass Capacitor
BYP (reference bypass) is connected to the internal
voltage reference. A 470pF capacitor (C
BYP
) connected
from BYP to GND quiets this reference, providing a
significant reduction in output noise. See Figure 2. C
BYP
reduces the regulator phase margin; when using C
BYP
,
output capacitors of 2.2µF or greater are generally
required to maintain stability.
The start-up speed of the MIC5206 is inversely
proportional to the size of the reference bypass
capacitor. Applications requiring a slow ramp-up of
output voltage should consider larger values of C
BYP
.
Likewise, if rapid turn-on is necessary, consider omitting
C
BYP
.
If output noise is not a major concern, omit C
BYP
and
leave BYP open.
Output Capacitor
An output capacitor is required between OUT and GND
to prevent oscillation. The minimum size of the output
capacitor is dependent upon whether a reference bypass
capacitor is used. 1.0µF minimum is recommended
when C
BYP
is not used (see Figure 2). 2.2µF minimum is
recommended when C
BYP
is 470pF (see Figure 2).
Larger values improve the regulator’s transient
response. The output capacitor value may be increased
without limit.
The output capacitor should have an ESR (effective
series resistance) of about 5Ω or less and a resonant
frequency above 1MHz. Most tantalum or aluminum
electrolytic capacitors are adequate; film types will work,
but are more expensive. Since many aluminum
electrolytics have electrolytes that freeze at about
–30°C, solid tantalums are recommended for operation
below –25°C.
At lower values of output current, less output
capacitance is required for output stability. The capacitor
can be reduced to 0.47µF for current below 10mA or
0.33µF for currents below 1mA.
No-Load Stability
The MIC5205 will remain stable and in regulation with no
load (other than the internal voltage divider) unlike many
other voltage regulators. This is especially important in
CMOS RAM keep-alive applications.
Error Flag Output
The error flag is an open-collector output and is active
(low) when an undervoltage of approximately 5% below
the nominal output voltage is detected. A pull-up resistor
from IN to FLAG is shown in all schematics.
If an error indication is not required, FLAG may be left
open and the pull-up resistor may be omitted.
Enable Pin Ramp and the Error Flag
To prevent indeterminate behavior on the error flag
during power down of the device, ensure that the fall
time of the enable pin signal, from logic high to logic low,
is faster than 100µs.
Fixed Regulator Applications
Figure 1a. Low-Noise Fixed Voltage Application
EN (pin 3) is shown connected to IN (pin 1) for an
application where enable/shutdown is not required. The
error flag is shown with a 100kΩ pull-up resistor.
Figure 1b. Low-Noise Fixed Voltage Application
Figure 1b is an example of a basic configuration where
the lowest-noise operation is not required. C
OUT
= 1µF
minimum. The error flag is shown with a 47kΩ pull-up
resistor.
