MIC39151-1.65WU datasheet(11/14 Pages) MICREL

11 / 14 page

Micrel, Inc.

MIC39150/39151/39152

October 2009

M9999-102309-A

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.

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 VIN or VOUT 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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

1.240

OUT

Where VOUT 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).

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