Micrel, Inc.
promote stability. Ultralow ESR capacitors (<100mΩ),
such as ceramic chip capacitors may promote instability.
These very low ESR levels may cause an oscillation
and/or underdamped transient response. A low-ESR
solid tantalum capacitor works extremely well and
provides good transient response and stability over
temperature. Aluminum electrolytics can also be used,
as long as the ESR of the capacitor is < 2Ω.
The value of the output capacitor can be increased
without limit. Higher capacitance values help to improve
transient response and ripple rejection and reduce
output noise.
Input Capacitor
An input capacitor of 1µF or greater is recommended
when the device is more than 4 inches away from the
bulk ac supply capacitance, or when the supply is a
battery. Small, surface-mount, ceramic chip capacitors
can be used for the bypassing. The capacitor should be
placed within 1" of the device for optimal performance.
Larger values will help to improve ripple rejection by
bypassing the input to the regulator, further improving
the integrity of the output voltage.
Transient Response and 3.3V to 2.5Vor 2.5V to 1.8V
Conversion
The MIC39150/1/2 has excellent transient response to
variations in input voltage and load current. The device
has been designed to respond quickly to load current
variations and input voltage variations. Large output
capacitors are not required to obtain this performance. A
standard 10µF output capacitor, preferably tantalum, is
all that is required. Larger values help to improve
performance even further.
By virtue of its low-dropout voltage, this device does not
saturate into dropout as readily as similar NPN-based
designs. When converting from 3.3V to 2.5V, or 2.5V to
1.8V, the NPN-based regulators are already operating in
dropout, with typical dropout requirements of 1.2V or
greater. To convert down to 2.5V without operating in
dropout, NPN-based regulators require an input voltage
of 3.7V at the very least. The MIC39150/1 regulator will
provide excellent performance with an input as low as
3.0V or 2.5V, respectively. This gives the PNP-based
regulators a distinct advantage over older, NPN-based
linear regulators.
Minimum Load Current
The MIC39150 regulator is specified between finite
loads. If the output current is too small, leakage currents
dominate and the output voltage rises. A 10mA minimum
load current is necessary for proper regulation.
MIC39150/39151/39152
Error Flag
The MIC39151 version features an error flag circuit
which monitors the output voltage and signals an error
condition when the voltage 5% below the nominal output
voltage. The error flag is an open-collector output that
can sink 10mA during a fault condition.
Low output voltage can be caused by a number of
problems, including an overcurrent fault (device in
current limit) or low input voltage. The flag is inoperative
during overtemperature shutdown.
When the error flag is not used, it is best to leave it
open. A pull-up resistor from FLG to either V
IN
or V
OUT
is
required for proper operation.
Enable Input
The MIC39151/2 features an enable input for on/off
control of the device. The enable input’s shutdown state
draws “zero” current (only microamperes of leakage).
The enable input is TTL/CMOS compatible for simple
logic interface, but can be connected to up to 20V. When
enabled, it draws approximately 15µA.
Adjustable Regulator Design
Figure 2. Adjustable Regulator with Resistors
The MIC39152 allows programming the output voltage
anywhere between 1.24V and 15.5V. Two resistors are
used. The resistor values are calculated by:
⎛
V
⎞
R1
=
R2
× ⎜
OUT
−
1
⎟
⎜
1.240
⎟
⎝
⎠
Where V
OUT
is the desired output voltage. Figure 2
shows component definition. Applications with widely
varying load currents may scale the resistors to draw the
minimum load current required for proper operation (see
Minimum Load Current
section).
October 2009
11
M9999-102309-A
MICREL [ MICREL SEMICONDUCTOR ]
