3/8

ISSUED DATE :2005/01/24

REVISED DATE :

Ordering Information ( contd. )

Part Number

Marking

Output Voltage

Part Number

Marking

Output Voltage

GT2123-AD

3BAD2

XXXX

Adjustable

Detailed Description

The GT2123 of COMS regulators contain a PMOS pass transistor, voltage reference, error amplifier,

over-current protection, thermal shutdown and Power Good detection circuitry.

The P-channel pass transistor receives data from the error amplifier, over-current shutdown, and thermal

protection circuits. During normal operation, the error amplifier compares the output voltage to a precision

reference. Over-current and Thermal shutdown circuits become active when the junction temperature exceeds

150 : , or the current exceeds 300mA. During thermal shutdown, the output voltage remains low. Normal

operation is restored when the junction temperature drops below 120 : .

The GT2123 switches from voltage mode to current mode when the load exceeds the rated output current.

This prevents over-stress. The GT2123 also incorporates current fold-back to reduce power dissipation when

the output is short circuited. This feature becomes active when the output drops below 0.8 volts, and reduces

the current flow by 65%. Full current is restored when the voltage exceeds 0.8 volts.

External Capacitors

The GT2123 is stable with an output capacitance to ground of 2.2 F or greater. Ceramic capacitors have the

lowest ESR, and will offer the best AC performance. Conversely, Aluminum Electrolytic capacitors exhibit the

highest ESR, resulting in the poorest AC response. Unfortunately, large value ceramic capacitors are

comparatively expensive. One option is to parallel a 0.1 F ceramic capacitor with a 10 F Aluminum Electrolytic.

The benefit is low ESR, high capacitance, and low overall cost.

A second capacitor is recommended between the input and ground to stabilize Vin. The input capacitor should

be at least 0.1 F to have a beneficial effect.

All capacitors should be placed in close proximity to the pins. A “Quiet” ground termination is desirable. This

can be achieved with a “Star” connection.

Enable

The Enable pin normally floats high. When actively, pulled low, the PMOS pass transistor shuts off, and all

internal circuits are powered down. In this state, the quiescent current is less than 1 A. This pin behaves much

like an electronic switch.

Power Good

The GT2123 includes the Power Good feature. When the output is not within D10% of the specified voltage, it

pulls low. This can occur under the following conditions:

1) Input Voltage too low.

2) During Over-Temperature.

3) During Over-Current.

4) If output is pulled up.

(Note: PG pin is an open-drain output.)