AMS112M1-3.5 Просмотр технического описания (PDF) - Advanced Monolithic Systems Inc
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AMS112M1-3.5 Datasheet PDF : 4 Pages
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APPLICATION HINTS
Package Power Dissipation
The package power dissipation is the level at which the thermal
sensor monitoring the junction temperature is activated. The
AMS112 shuts down when the junction temperature exceeds the
limit of 150°C. The junction temperature rises as the difference
between the input power and output power increases. The
mounting pad configuration on the PCB, the board material, as
well as the ambient temperature affect the rate of temperature
rise. The junction temperature will be low, even if the power
dissipation is high, when the mounting of the device has good
thermal conductivity. When mounted on the recommended
mounting pad the power dissipation for the SOT-23 package is
400mW. For operation above 25°C derate the power dissipation
at 3.2 mW/°C. To determine the power dissipation for shutdown
when mounted, attach the device on the PCB and increase the
input-to-output voltage until the thermal protection circuit is
activated. Calculate the power dissipation of the device by
subtracting the output voltage from the input voltage and multiply
by the output current. The measurements should allow for the
ambient temperature of the PCB. The value obtained from PD/
(150°C - TA) is the derating factor. The PCB mounting pad
should provide maximum thermal conductivity in order to
maintain low device temperatures. As a general rule, the lower
the temperature, the better the reliability of the device.
The thermal resistance when the device is mounted is equal to:
TJ = θJA x PD + TA
The internal limit for junction temperature is 150°C. If the
ambient temperature is 25°C, then:
150°C = θJA x PD + 25°C
θJA = 125°C/ PD
A simple way to determine PD is to calculate VIN x IIN when the
output is shorted. As the temperature rises, the input gradually
will decrease. The PD value obtained when the thermal
equilibrium is reached, is the value that should be used.
The range of usable currents can be found from the graph in
figure 2.
(mW)
PD
3
DPD
6
4
5
25
50
75
150
T (°C)
Figure 2
Procedure:
1. Find PD.
2. PD1 is calculated as PD x (0.8 - 0.9).
3. Plot PD1 against 25°C.
4. Connect PD1 to the point corresponding to the 150°C.
AMS112
5. Take a vertical line from the maximum operating
temperature (75°C) to the derating curve.
6. Read the value of PD at the point where the vertical line
intersects the derating curve. This is the maximum power
dissipation, DPD.
The maximum operating current is:
IOUT = (DPD/ (VIN(MAX) - VO)
External Capacitors
The AMS112 series require input and output decoupling
capacitors. The required value of these capacitors depends on the
application circuit and other factors.
Because high frequency characteristics of electrolytic capacitors
depend greatly on the type and even the manufacturer, the value
of capacitance that works well with AMS112 for one brand or
type may not necessary be sufficient with an electrolytic of
different origin. Sometimes actual bench testing will be the only
means to determine the proper capacitor type and value. To
obtain stability in all general applications a high quality 4.7µF
aluminum electrolytic or a 2.2µF tantalum electrolytic can be
used.
A critical characteristic of the electrolytic capacitors is their
performance over temperature. The AMS112 is designed to
operate to -30°C, but some electrolytics will freeze around -30°C
therefore becoming ineffective. In such case the result is
oscillation at the regulator output. For all application circuits
where cold operation is necessary, the output capacitor must be
rated to operate at the minimum temperature.
In order to determine the minimum value of the output capacitor,
for an application circuit, the entire circuit including the capacitor
should be bench tested at minimum operating temperatures and
maximum operating currents. After the minimum capacitance
value has been found, the value should be doubled for actual use
to cover for production variations both in the regulator and the
capacitor. The recommended minimum capacitance for AMS112
is 2.2µF. As a general rule, with higher output voltages the value
of the output capacitance decreases, since the internal loop gain
is reduced.
Noise Bypass Capacitor
The noise bypass capacitor should be connected as close as
possible to pin 3 and ground. The recommended value for this
capacitor is 0.01µF. The noise bypass terminal is susceptible to
external noise, and oscillation can occur when the bypass
capacitor is not used and the solder pad for this pin is too large.
Because of the high impedance of the noise bypass terminal, care
should be taken if the bypass capacitor is not used.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
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