July 2005

9

MIC5236

MIC5236

Micrel, Inc.

Application Information

The MIC5236 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 ad-

vantages of reduced quiescent current and smaller package.

Additionally, when disabled, quiescent current is reduced to

0.1µA.

Enable

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

cent current to less than 0.1µA. Thermal shutdown and the

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

maximum enable bias current is 2µAfor 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

MIC5236

EN

200k

200k

OUT

GND

SHUTDOWN

ENABLE

ERR

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 bat-

tery. 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 MIC5236 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 10µF capacitor. Virtually

any 10µF 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 1µF film or ceramic capacitors.

0

1

2

3

4

5

5

10

15

20

25

30

INPUT VOLTAGE (V)

Stable Region

Figure 2. Output Capacitor ESR

Error Detection Comparator Output

The ERR pin is an open collector output which goes low when

the output voltage drops 5% below it’s internally programmed

level. It senses conditions such as excessive load (current

limit), low input voltage, and over temperature conditions.

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

200µA. In order to minimize the drain on the source used

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

Output

Input

Voltage

Output

Voltage

4.75V

0V

0V

5V

1.3V

Figure 3. Error Output Timing

Reverse Current Protection

The MIC5236 is designed to limit the reverse current flow

from output to input in the event that the MIC5236 output

has been tied to the output of another power supply. See

the graphs detailing the reverse current flow with the input

grounded and open.

Thermal Shutdown

The MIC5236 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 ther-

mal 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 4.7µF capacitor tied to the enable pin. The

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

capacitor, enabling the regulator. This circuit reduces the

short circuit current from 280mA to 15mA while allowing for

regulator restart once the short is removed.