G914X datasheet(8/10 Pages) GMT

8 / 10 page

Ver: 1.2

Jun 30, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

G914X

Global Mixed-mode Technology Inc.

Over Current Protection

The G914X use a current mirror to monitor the output

current. A small portion of the PMOS output transistor’s

current is mirrored onto a resistor such that the voltage

across this resistor is proportional to the output current.

This voltage is compared against the 1.25V reference.

Once the output current exceeds the limit, the PMOS

output transistor is turned off. Once the output transistor

is turned off, the current monitoring voltage decreases

to zero, and the output PMOS is turned on again. If the

over current condition persist, the over current protec-

tion circuit will be triggered again. Thus, when the output

is shorted to ground, the output current will be alternat-

ing between 0 and the over current limit. The typical

over current limit of the G914X is set to 500mA. Note

that the input bypass capacitor of 1µF must be used in

this case to filter out the input voltage spike caused by

the surge current due to the inductive effect of the

package pin and the printed circuit board’s routing wire.

Otherwise, the actual voltage at the IN pin may exceed

the absolute maximum rating.

Over Temperature Protection

To prevent abnormal temperature from occurring, the

G914X has a built-in temperature monitoring circuit.

When it detects the temperature is above 150

oC, the

output transistor is turned off. When the IC is cooled

down to below 135

oC, the output is turned on again. In

this way, the G914X will be protected against abnor-

mal junction temperature during operation.

Shutdown Mode

When the SHDN pin is connected a logic low voltage,

the G914X enters shutdown mode. All the analog cir-

cuits are turned off completely, which reduces the cur-

rent consumption to only the leakage current. The out-

put is disconnected from the input. When the output has

no load at all, the output voltage will be discharged to

ground through the internal resistor voltage divider.

Operating Region and Power Dissipation

Since the G914X is a linear regulator, its power dissi-

pation is always given by P = IOUT (VIN – VOUT). The

maximum power dissipation is given by:

PDMAX = (TJ – TA)/θJA = (150-25) / 240 = 520mW

Where (TJ – TA) is the temperature difference the

G914X die and the ambient air, θJA, is the thermal

resistance of the chosen package to the ambient air.

For surface mount device, heat sinking is accom-

plished by using the heat spreading capabilities of the

PC board and its copper traces. In the case of a

SOT23-5 package, the thermal resistance is typically

240

oC/Watt. (See Recommended Minimum Footprint)

[Figure 2]. Refer to Figure 3 is the G914X valid oper-

ating region (Safe Operating Area) & refer to Figure 4

is maximum power dissipation of SOT 23-5.

The die attachment area of the G914X’s lead frame is

connected to pin 2, which is the GND pin. Therefore,

the GND pin of G914X can carry away the heat of the

G914X die very effectively. To improve the power dis-

sipation, connect the GND pin to ground using a large

ground plane near the GND pin.

Applications Information

Capacitor Selection and Regulator Stability

Normally, use a 1µF capacitor on the input and a 1µF

capacitor on the output of the G914X. Larger input ca-

pacitor values and lower ESR provide better sup-

ply-noise rejection and transient response. A higher-

value input capacitor (10µF) may be necessary if large,

fast transients are anticipated and the device is located

several inches from the power source. For stable opera-

tion over the full temperature range, with load currents

up to 120mA, a minimum of 1µF is recommended.

Power-Supply Rejection and Operation from

Sources Other than Batteries

The G914X is designed to deliver low dropout voltages

and low quiescent currents in battery powered sys-

tems. Power-supply rejection is 57dB at low frequen-

cies as the frequency increases above 20kHz; the

output capacitor is the major contributor to the rejec-

tion of power-supply noise.

When operating from sources other than batteries,

improve supply-noise rejection and transient response

by increasing the values of the input and output ca-

pacitors, and using passive filtering techniques.

Load Transient Considerations

The G914X load-transient response graphs show two

components of the output response: a DC shift of the

output voltage due to the different load currents, and

the transient response. Typical overshoot for step

changes in the load current from 0mA to 100mA is

12mV. Increasing the output capacitor's value and

decreasing its ESR attenuates transient spikes.

Input-Output (Dropout) Voltage

A regulator's minimum input-output voltage differential

(or dropout voltage) determines the lowest usable

supply voltage. In battery-powered systems, this will

determine the useful end-of-life battery voltage. Be-

cause the G914X use a P-channel MOSFET pass

transistor, their dropout voltage is a function of RDS(ON)

multiplied by the load current cause the G914X use a

P-channel MOSFET pass transistor, their dropout

voltage is a function of RDS(ON) multiplied by the load

current.

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