3/5

ISSUED DATE :2004/11/05

REVISED DATE :2006/04/18B

Detailed Description

The GL2138 series of COMS regulators contain a PMOS pass transistor, voltage reference, error amplifier,

over-current protection, and thermal shutdown.

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 2.2A. During thermal shutdown, the output voltage remains low. Normal

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

The GL2138 behaves like a current source when the load reaches 2.2A. However, if the load impedance drops

below 0.3 , the current drops back to 600mA to prevent excessive power dissipation. Normal operation is

restored when the load resistance exceeds 0.75 .

External Capacitors

The GL2138 is stable with an output capacitance to ground of 4.7 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.

Characteristics Curve