MIC5239

Micrel

MIC5239

8

January 2002

Application Information

The MIC5239 provides all of the advantages of the MIC2950:

wide input voltage range, load dump (positive transients up to

60V), and reversed-battery protection, with the added advan-

tages of reduced quiescent current and smaller package.

Additionally, when disabled, quiescent current is reduced to

0.1

µA.

Enable

A low on the enable pin disables the part, forcing the quies-

cent current to less than 0.1

µA. Thermal shutdown and the

error flag are not functional while the device is disabled. The

maximum enable bias current is 2

µA for a 2.0V input. An open

collector pull-up resistor tied to the input voltage should be set

low enough to maintain 2V on the enable input. Figure 1

shows an open collector output driving the enable pin through

a 200k pull-up resistor tied to the input voltage.

In order to avoid output oscillations, slow transitions from low

to high should be avoided.

5V

IN

MIC5239

EN

200k

200k

OUT

GND

SHUTDOWN

ENABLE

FLG

Figure 1. Remote Enable

Input Capacitor

An input capacitor may be required when the device is not

near the source power supply or when supplied by a battery.

Small, surface mount, ceramic capacitors can be used for

bypassing. Larger values may be required if the source

supply has high ripple.

Output Capacitor

The MIC5239 has been designed to minimize the effect of the

output capacitor ESR on the closed loop stability. As a result,

ceramic or film capacitors can be used at the output. Figure 2

displays a range of ESR values for a 10

µF capacitor. Virtually

any 10

µF capacitor with an ESR less than 3.4Ω is sufficient

for stability over the entire input voltage range. Stability can

also be maintained throughout the specified load and line

conditions with 4.7

µF film or ceramic capacitors.

0

1

2

3

4

5

5

1015202530

INPUT VOLTAGE (V)

Stable Region

T

V

Figure 2. Output Capacitor ESR

Error Detection Comparator Output

The FLAG pin is an open collector output which goes low

when the output voltage drops 5% below it’s internally pro-

grammed level. It senses conditions such as excessive load

(current limit), low input voltage, and over temperature con-

ditions. Once the part is disabled via the enable input, the

error flag output is not valid. Overvoltage conditions are not

reflected in the error flag output. The error flag output is also

not valid for input voltages less than 2.3V.

The error output has a low voltage of 400mV at a current of

200

µA. In order to minimize the drain on the source used for

the pull-up, a value of 200k to 1M

Ω is suggested for the error

flag pull-up. This will guarantee a maximum low voltage of

0.4V for a 30V pull-up potential. An unused error flag can be

left unconnected.

NOT

VALID

NOT

VALID

VALID ERROR

Error FLAG

Output

Input

Voltage

Output

Voltage

4.75V

0V

0V

5V

1.3V

Figure 3. Error FLAG Output Timing

Thermal Shutdown

The MIC5239 has integrated thermal protection. This feature

is only for protection purposes. The device should never be

intentionally operated near this temperature as this may have

detrimental effects on the life of the device. The thermal

shutdown may become inactive while the enable input is

transitioning a high to a low. When disabling the device via the

enable pin, transition from a high to low quickly. This will

insure that the output remains disabled in the event of a

thermal shutdown.

Current Limit

Figure 4 displays a method for reducing the steady state

short circuit current. The duration that the supply delivers

current is set by the time required for the error flag output to

discharge the 4.7

µF capacitor tied to the enable pin. The off

time is set by the 200K resistor as it recharges the 4.7

µF

capacitor, enabling the regulator. This circuit reduces the

short circuit current from 800mA to 40mA while allowing for

regulator restart once the short is removed.

5V

IN

MIC5239

EN

200k

1N4148

200k

4.7

µF

OUT

GND

SHUTDOWN

ENABLE

FLG

Figure 4. Remote Enable with Short-Circuit

Current Foldback