GSC2138

Page: 3/7

ISSUED DATE :2006/04/13

REVISED DATE :

Ordering Information ( contd. )

Part Number

Marking

Output Voltage

Part Number

Marking

Output Voltage

GSC2138-AD

8CAD2

XXXX

Adjustable

Detailed Description

The GSC2138 of COMS regulator contains 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

140 : , 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 GSC2138 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 GSC2138 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.

Enable

When 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.

(Note: there is no internal pull-up for EN PIN)

Adjustable Version

The adjustable version uses external feedback resistors to generate an output voltage anywhere from 1.5V to

5.0V. Vadj is trimmed to 1.242V and Vout is given by the equation:

Feedback resistors R1 and R2 should be high enough to keep quiescent current low, but increasing R1+R2 will

reduce stability. In general, R1 and R2 in the 10’s of k will produce adequate stability, given reasonable

layout precautions. To improve stability characteristics, keep parasitic on the ADJ pin to minimum, and lower

R1 and R2 values.