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°C, the
output transistor is turned off. When the IC is cooled
down to below 135°C, 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 = I
OUT
(V
IN
–
V
OUT
). The
maximum power dissipation is given by:
G914X
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 1
µ
F capacitor on the input and a 4.7
µ
F
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 (10
µ
F) 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 4.7
µ
F 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 70dB 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 R
DS(ON)
multiplied by the load current cause the G914X use a
P-channel MOSFET pass transistor, their dropout
voltage is a function of R
DS(ON)
multiplied by the load
current.
P
DMAX
= (T
J
–
T
A
)/
θ
JA
= (150-25) / 240 = 520mW
Where (T
J
–
T
A
) is the temperature difference the
G914X die and the ambient air,
θ
JA
, 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
SOT-23-5 package, the thermal resistance is typically
240
°
C/Watt. (See Recommended Minimum Footprint).
Refer to Figure 2 is the G914X valid operating region
(Safe Operating Area) & refer to Figure 3 is maximum
power dissipation of SOT-23-5.
Ver: 1.8
Jul 24, 2007
TEL: 886-3-5788833
http://www.gmt.com.tw
9
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
