Micrel, Inc.

MIC39300/39301/39302

October 2009

11

M9999-102309-A

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

required for proper operation.

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

1

1.240

V

R2

R1

OUT

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).