Advanced Monolithic Systems, Inc.

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS2026

APPLICATION HINTS

Power Supply Considerations

A 0.047

µF 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 0.1

µF 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

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 AMS2026 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

As an initial estimate use the highest operating ambient

dissipation is equal to:

2

Calculate the junction temperature:

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 AMS2026 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 short

circuits and heavy over loads 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

3.0V 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.

POWER SUPPLY

4.0V - 5.0V

EXTERNAL LOAD A

0.1

µF

0.1

µF

1

µF

10k

2

3

6

ERROR FLAG A

ENABLE A

10k

EXTERNAL LOAD B

0.1

µF

1

4

ENABLE B

ERROR FLAG B

7

8

5

IN

OUT A

OUT B

EN

EN

GND

ERROR A

ERROR B

Figure 1

TYPICAL APPLICATIONS