DS116A Datasheet, PDF (3/7 Pages) –

APPLICATION HINTS
Package Power Dissipation
The package power dissipation is the level at which the thermal
sensor monitoring the junction temperature is activated. The
AMS116 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 (figure1) the power dissipation for the SOT-89
package is 600mW. For operation above 25°C derate the power
dissipation at 4.8mW/°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
P_D/ (150°C - T_A) 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:
T_J = θ_JA x P_D + T_A
The internal limit for junction temperature is 150°C. If the ambient
temperature is 25°C, then:
150°C = θ_JA x P_D + 25°C
θ_JA = 125°C/ P_D
A simple way to determine P_D is to calculate V_IN x I_IN when the
output is shorted. As the temperature rises, the input gradually will
decrease. The P_D 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)
P_D
3
D_PD
6
4
5
25
50
75
150
T (°C)
Figure 2
Procedure:
1. Find P_D.
2. P_D1 is calculated as P_D x (0.8 - 0.9).
3. Plot P_D1 against 25°C.
4. Connect P_D1 to the point corresponding to the 150°C.
AMS116
5. Take a vertical line from the maximum operating temperature
(75°C) to the derating curve.
6. Read the value of P_D at the point where the vertical line
intersects the derating curve. This is the maximum power
dissipation, D_PD.
The maximum operating current is:
I_OUT = (D_PD/ (V_IN(MAX) - V_O)
External Capacitors
The AMS116 series require an output capacitor for device
stability. The value required 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 AMS116 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 100µF
aluminum electrolytic or a 47µF tantalum electrolytic can be used.
A critical characteristic of the electrolytic capacitors is their
performance over temperature. The AMS116 is designed to
operate to -40°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 applications
where the regulator junction temperature will never be lower than
25°C the output capacitor value can be reduced by a factor of two
over the value required for the entire temperature range (47µF for
a high quality aluminum or 22µF for a tantalum electrolytic
capacitor).
With higher output currents, the stability of AMS116 decreases.
Considering the fact that in many applications the AMS116 is
operated at only a few milliamps (or less) of output current, the
output capacitor value can be reduced even further. For example,
a circuit that is required to deliver a maximum of 10mA of output
current from the regulator output will need an output capacitor of
only half the value compared to the same regulator required to
deliver the full output current of 100mA.
As a general rule, with higher output voltages the value of the
output capacitance decreases, since the internal loop gain is
reduced.
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. To maintain internal power
dissipation and die heating to a minimum, the input voltage should
be maintain at 0.6V above the output. Worst-case occurs just after
input power is applied and before the die had the chance to heat
up. After the minimum capacitance value has been found for the
specific brand and type of electrolytic capacitor, the value should
be doubled for actual use to cover for production variations both
in the regulator and the capacitor.
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