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AMS2027P

SINGLE POWER DISTRIBUTION SWITCH

Advanced Monolithic Systems Inc 

APPLICATION HINTS

Power Supply Considerations

A 0.047µF ceramic bypass capacitor close to the device, between

input and ground is recommended. When the output load is heavy

or has large paralleled capacitors, a high value electrolytic

capacitor should be used. To improve the immunity of the device

to ESD, use a 0.1µF ceramic capacitor to bypass the output.

Current Limit

A sense FET monitors overcurrent conditions. When an

overcurrent condition is detected the device maintains a constant

output current and decreases the voltage accordingly. If the

condition is present long enough to activate the thermal limiting

the result is the shutdown of the device.

There are three situation in which overload can occur. In the first

case, the output has been shorted before the device is enabled or

before VI has been applied. The device senses the short and

switches into a constant-current output.

In the second case, the short occurs while the device is enabled.

When this happens, very high currents flow for a short time before

the current-limit circuit can react. After the current-limit circuit

has tripped, the device limits normally.

In the third case, the load has been gradually increased beyond the

recommended operating current. The current will rise until the

current-limit threshold is reached. The AMS2027 is able to deliver

current up to the current-limit threshold without damage. When

the threshold has been reached the device switches into the

constant current mode. When over current condition is detected

the error flag logic output remains low until the condition is

removed.

Power Dissipation and Junction Temperature

The thermal resistance of the surface-mount packages such as

SOIC is high compared to that of power packages. The use of the

N-cannel MOSFET which has low on-resistance, makes it possible

for small surface-mount packages to pass large currents. To

determine the power dissipation and junction temperature the first

step is to find rON at the input voltage and operating temperature.

TYPICAL APPLICATIONS

AMS2027

As an initial estimate use the highest operating ambient

temperature of interest and read rON from Figure1. Power

dissipation is equal to:

PD = rON X I2

Calculate the junction temperature:

TJ = PD X RθJA + TA

Where RθJA is the thermal resistance and is 172°C/W for the SOIC

package. Compare the calculated junction temperature with the

initial estimate and if they don’t mach within a few degrees,

repeat the calculations using the calculated value as the new

estimate. A few repetitions will be sufficient to give a reasonable

answer.

Thermal Protection

Thermal protection prevents damage to the device when over load

or short circuits conditions are present for extended periods of

time. These conditions force the AMS2027 into the constant

current mode. As a result the voltage across the high-side switch

will increase. Under short-circuit conditions the voltage across the

switch is equal to the input voltage. Continuous heavy over loads

and short circuits increase the power dissipation in the switch and

causes the junction temperature to rise to dangerously high levels.

The protection circuit shuts off the switch when it senses the high

junction temperature. The switch remains off until the device has

cooled about 20°C. The switch continues to cycle off and on until

the fault is removed.

Undervoltage Lock-out

An undervoltage lock-out is provided to insure that the switch is

in the off state at start-up. When the input voltage falls below

3.2V the switch will be turned off immediately. This will make it

easy for designers of hot plug-in systems that may not be able to

turn the switch off before removing the input power. In such cases

when the device is reinserted, the turn on will have a controlled

rise time to reduce EMI and voltage overshoots.

Figure 1

POWER SUPPLY

4.0V - 5.0V

ENABLE

ERROR FLAG

1µF

10k

7

IN

0.1µF

8

OUT

1 EN

2 ERROR

OUT

6

GND

3

EXTERNAL LOAD

0.1µF

Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723

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