Global Mixed-mode Technology Inc.
G924
Over Current Protection
Refer to “Safe Operating Area” of the Typical Per-
formance Characteristics is the G924 valid operating
region & refer to “Maximum Power Dissipation vs.
Temperature” is the maximum power dissipation of
SOT-23-5.
The G924 uses a current mirror to monitor the output
current. A small portion of the PMOS output transis-
tor’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 1.20V
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 protection circuit will be triggered
again. Thus, when the output is shorted to ground, the
output current will be alternating between 0 and the
over current limit. The typical over current limit of the
G924 is set to 550mA. Note that the input bypass ca-
pacitor 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 ac-
tual voltage at the IN pin may exceed the absolute
maximum rating.
The die attachment area of the G924’s lead frame is
connected to pin 2, which is the GND pin. Therefore,
the GND pin of G924 can carry away the heat of the
G924 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 G924. 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 lo-
cated several inches from the power source.
Power-Supply Rejection and Operation from
Sources Other than Batteries
Over Temperature Protection
To prevent abnormal temperature from occurring, the
G924 has a built-in temperature monitoring circuit.
When it detects the temperature is above 145oC, the
output transistor is turned off. When the IC is cooled
down to below 120oC, the output is turned on again. In
this way, the G924 will be protected against abnormal
junction temperature during operation.
The G924 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.
Shutdown Mode
When the SHDN pin is connected a logic low voltage,
the G924 enters shutdown mode. All the analog cir-
cuits 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 dis-
charged to ground through the internal resistor voltage
divider.
Load Transient Considerations
The G924 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 10mA to 300mA is
30mV. Increasing the output capacitor's value and
decreasing its ESR attenuates transient spikes.
Operating Region and Power Dissipation
Since the G924 is a linear regulator, its power dissipa-
tion is always given by P = IOUT (VIN – VOUT). The
maximum power dissipation is given by:
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 G924 use a P-channel MOSFET pass tran-
sistor, their dropout voltage is a function of RDS(ON)
multiplied by the load current cause the G924 use a
P-channel MOSFET pass transistor, their dropout
voltage is a function of RDS(ON) multiplied by the load
current.
PDMAX = (TJ – TA)/θJA = (150-25) / 240 = 520mW
Where (TJ – TA) is the temperature difference the
G924 die and the ambient air, θJA, is the thermal re-
sistance of the chosen package to the ambient air. For
surface mount device, heat sinking is accomplished 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
240oC/Watt. (See Recommended Minimum Footprint).
TEL: 886-3-5788833
http://www.gmt.com.tw
Ver: 0.2 Preliminary
Oct 01, 2004
7
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
