Ver 0.0 Preliminary

Mar 27, 2001

TEL: 886-3-5788833

http://www.gmt.com.tw

6

G920

Global Mixed-mode Technology Inc.

Over Current Protection

The G920 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 G920 is set to 250mA. 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. Other-

wise, the actual voltage at the IN pin may exceed the

absolute maximum rating.

Dynamic Current Feedback

The G920 is designed to work with both low and high

ESR output capacitors. Since a PMOS transistor is

used as the output transistor, an output capacitor

greater than 1 µF is needed to stabilize the feedback

loop of the regulator. Due to the large value of the out-

put capacitor, the dominant pole is the pole caused by

the output node. The pole cause by the error ampli-

fier’s output node is the second pole. With a high ESR

output capacitor, the zero caused by the ESR is typi-

cally near the second pole so that the second pole is

cancelled by the zero, and the loop is stable. However,

when the output capacitor has a low ESR, the zero will

be much larger than the second pole. When the zero

is near or larger than the unity-gain frequency, it can

no longer cancel the phase shift caused by the second

pole, and the loop becomes unstable. The G920 uses

dynamic current feedback to stabilize the loop. The

output impedence of the error amplifier is reduced

when the output current increases. Thus, the second

pole is pushed outward in accordance with the output

current so that the second pole can be cancelled by

the ESR’s zero to maintain regulator stability.

Over Temperature Protection

To prevent abnormal temperature from occurring, the

G920 has a built-in temperature monitoring circuit.

When it detects the temperature is above 170

output transistor is turned off. When the IC is cooled

down to below 150

this way, the G920 will be protected against abnormal

junction temperature during operation.

Shutdown Mode

When the EN pin is connected a logic low voltage, the

G920 enters shutdown mode. All the analog circuits

are turned off completely, which reduces the current

consumption to only the leakage current. The output 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 G920 is a linear regulator, its power dissipa-

maximum power dissipation is given by:

sistance of the chosen package to the ambient air. In

the case of a SOT23-5 package, the thermal resis-

tance is typically 140

Applications Information

Capacitor Selection and Regulator Stability

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

capacitor on the output of the G920. 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 sta-

ble operation over the full temperature range, with load

currents up to 120mA, a minimum of 1µF is recom-

mended.

Power-Supply

Rejection

and

Operation

from

Sources Other than Batteries

The G920 is designed to deliver low dropout voltages

and low quiescent currents in battery powered sys-

tems. Power-supply rejection is 53dB 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 G920 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 G920 use a P-channel MOSFET pass tran-

multiplied by the load current.