MIC29311

Micrel

MIC29311

8

December 1998

Applications Information

The MIC29311 is a high-performance low-dropout voltage

regulator suitable for all moderate to high-current voltage

regulator applications. The 600mV dropout voltage at full

load makes it especially valuable in battery-powered systems

and as high-efficiency noise filters in “post-regulator” applica-

tions. Its unique output voltage makes the MIC39311 ideal for

Universal Serical Bus (USB) power switching applications.

Unlike older NPN-pass transistor designs, where the mini-

mum dropout voltage is limited by the base-emitter voltage

drop and collector-emitter saturation voltage, dropout perfor-

mance of the PNP output of these devices is limited merely

by the low V

A trade-off for the low dropout voltage is a varying base drive

requirement. But Micrel’s Super

βeta PNP™ process re-

duces this drive requirement to merely 1% to 5% of the load

current.

The MIC29311 regulator is fully protected from damage due

to fault conditions. Current limiting is provided. This limiting is

linear; output current under overload conditions is constant.

Thermal shutdown disables the device when the die tem-

perature exceeds the maximum safe operating temperature.

Transient protection allows device (and load) survival even

when the input voltage spikes above and below nominal. The

output structure of these regulators allows voltages in excess

of the desired output voltage to be applied without reverse

current flow. The MIC29311 version offers a logic level on-off

control: when disabled, the device draws nearly zero current.

MIC29311-5.1

IN

OUT

GND

EN

FLG

Flag

Enable

Shutdown

Figure 1. Input and Output Capacitors

Thermal Design

Linear regulators are simple to use. The most complicated

design parameters to consider are thermal characteristics.

Thermal design requires the following application-specific

parameters:

Maximum ambient temperature, T

A

Output Current, I

OUT

Output Voltage, V

OUT

Input Voltage, V

IN

First, calculate the power dissipation of the regulator from

these numbers and the device parameters from this datasheet.

P

I

1.02V

V

D

OUT

IN

OUT

=−

()

Where the ground current is approximated by 2% of I

Then the heat sink thermal resistance is determined by:

θθ

θ

SA

J(max)

A

D

JC

CS

TT

P

=

−

−+

Where T

The heat sink may be significantly reduced in applications

where the minimum input voltage is known and is large

compared with the dropout voltage. Use a series input

resistor to drop excessive voltage and distribute the heat

between this resistor and the regulator. The low dropout

properties of Micrel Super ßeta PNP regulators allow very

significant reductions in regulator power dissipation and the

associated heat sink without compromising performance.

When this technique is employed, a capacitor of at least 1

µF

is needed directly between the input and regulator ground.

Please refer to Application Note 9 for further details and

examples on thermal design and heat sink specification.

Capacitor Requirements

For stability and minimum output noise, a capacitor on the

regulator output is necessary. The value of this capacitor is

dependent upon the output current; lower currents allow

smaller capacitors. The MIC29311 regulator is stable with a

minimum capacitor value of 10

µF at full load.

This capacitor need not be an expensive low ESR type:

aluminum electrolytics are adequate. In fact, extremely low

ESR capacitors may contribute to instability. Tantalum ca-

pacitors are recommended for systems where fast load

transient response is important.

Where the regulator is powered from a source with a high AC

impedance, a 0.1

µF capacitor connected between Input and

GND is recommended. This capacitor should have good

characteristics to above 250kHz. When the regulator is

located more than 3 inches from the ac bulk supply capaci-

tors, a 1

µF or greater input capacitor is recommended.

Minimum Load Current

The MIC29311 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.

Enable Input

The MIC29311 version features an enable (EN) input that

allows on-off control of the device. Special design allows

“zero” current drain when the device is disabled—only micro-

amperes of leakage current flows. The EN input has TTL/

CMOS compatible thresholds for simple interfacing with

logic, or may be directly tied to V

requires approximately 20

µA of current into the EN pin.

Error Flag

The MIC29311 features an error flag which looks at the output

voltage and signals an error condition when this voltage

drops 5% below its expected value. The error flag is an open-

collector output that pulls low under fault conditions. It may

sink 10mA. Low output voltage signifies a number of possible

problems, including an overcurrent fault (the device is in

current limit) and low input voltage. The flag output is inopera-

tive during overtemperature shutdown conditions.

USB Applications

The main application of the MIC29311 is to control power

distribution in a self-powered Universal Serial Bus hub. For

self-powered hubs, the MIC29311 provides 5.1V

±3% to

downstream ports from an unregulated supply voltage (see