Micrel, Inc.
Transient Response and 3.3V to 2.5V and 2.5V to
1.8V Conversions
The MIC39300/1/2 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 47μF 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 MIC39300/1 regulator will
provide excellent performance with an input as low as
3.0V or 2.5V. This gives the PNP-based regulators a
distinct advantage over older, NPN-based linear
regulators.
Minimum Load Current
The MIC39300/1/2 regulators are 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.
Error Flag
The MIC39301 version features an error flag circuit
which monitors the output voltage and signals an error
condition when the voltage drops 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.
MIC39300/39301/39302
Enable Input
The MIC39301/2 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 MIC39302 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 ]
