RT9170
range of 125°C to -40°C. ESR will vary only about 2X
Aluminum electrolytics also typically have large
temperature variation of capacitance value.
going from the high to low temperature limits.
The increasing ESR at lower temperatures can cause
oscillations when marginal quality capacitors are used (if
the ESR of the capacitor is near the upper limit of the
stability range at room temperature).
Equally important to consider is a capacitor's ESR change
with temperature: this is not an issue with ceramics, as
their ESR is extremely low. However, it is very important
in Tantalum and aluminum electrolytic capacitors. Both
show increasing ESR at colder temperatures, but the
increase in aluminum electrolytic capacitors is so severe
they may not be feasible for some applications.
Aluminum :
This capacitor type offers the most capacitance for the
money. The disadvantages are that they are larger in
physical size, not widely available in surface mount, and
have poor AC performance (especially at higher
frequencies) due to higher ESR and ESL.
Ceramic :
For values of capacitance in the 10μF to 100μF range,
ceramics are usually larger and more costly than
tantalums but give superior AC performance for by-
passing high frequency noise because of very low ESR
(typically less than 10mΩ). However, some dielectric types
do not have good capacitance characteristics as a function
of voltage and temperature.
Compared by size, the ESR of an aluminum electrolytic
is higher than either Tantalum or ceramic, and it also varies
greatly with temperature. A typical aluminum electrolytic
can exhibit an ESR increase of as much as 50X when
going from 25°C down to -40°C.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. Atypical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with half
of the rated voltage applied to it. The Z5U and Y5V also
exhibit a severe temperature effect, losing more than 50%
of nominal capacitance at high and low limits of the
temperature range.
It should also be noted that many aluminum electrolytics
only specify impedance at a frequency of 120Hz, which
indicates they have poor high frequency performance.
Only aluminum electrolytics that have an impedance
specified at a higher frequency (between 20kHz and
100kHz) should be used for the device. Derating must be
applied to the manufacturer's ESR specification, since it
is typically only valid at room temperature.
X7R and X5R dielectric ceramic capacitors are strongly
recommended if ceramics are used, as they typically
maintain a capacitance range within 20% of nominal over
full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than
Z5U/Y5U types for a given voltage and capacitance.
Any applications using aluminum electrolytics should be
thoroughly tested at the lowest ambient operating
temperature where ESR is maximum.
Thermal Considerations
The RT9170 series can deliver a current of up to 300mA
over the full operating junction temperature range. However,
the maximum output current must be derated at higher
ambient temperature to ensure the junction temperature
does not exceed 125°C. With all possible conditions, the
junction temperature must be within the range specified
under operating conditions. Power dissipation can be
calculated based on the output current and the voltage
drop across regulator.
Tantalum :
Solid tantalum capacitors are recommended for use on
the output because their typical ESR is very close to the
ideal value required for loop compensation. They also
work well as input capacitors if selected to meet the ESR
requirements previously listed.
Tantalums also have good temperature stability: a good
quality tantalum will typically show a capacitance value
that varies less than 10-15% across the full temperature
PD = (VIN - VOUT) IOUT + VIN IGND
DS9170-11 March 2007
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