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参数资料
| 型号： | G920 |
| 厂商： | Electronic Theatre Controls, Inc. |
| 元件分类： | 基准电压源/电流源 |
| 英文描述： | 150mA Micro-power LDO Regulators |
| 中文描述： | 150mA的微功率LDO稳压器 |
| 文件页数： | 6/7页 |
| 文件大小： | 122K |
| 代理商： | G920 |
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
o
C, the
output transistor is turned off. When the IC is cooled
down to below 150
o
C, the output is turned on again. In
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-
tion is always given by P = I
OUT
(V
IN
– V
OUT
). The
maximum power dissipation is given by:
P
MAX
= (T
J
– T
A
)/
θ
JA
,
Where (T
J
– T
A
) is the temperature difference the
G920 die and the ambient air,
θ
JA
, is the thermal re-
sistance of the chosen package to the ambient air. In
the case of a SOT23-5 package, the thermal resis-
tance is typically 140
o
C/Watt.
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
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.
transient
response.
A
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-
sistor, their dropout voltage is a function of R
DS(ON)
multiplied by the load current.
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