RT9178
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 to 15% across the full temperature
range of 125°C to
−40°C.
ESR will vary only about 2X
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).
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.
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.
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.
Any applications using aluminum electrolytics should be
thoroughly tested at the lowest ambient operating
temperature where ESR is maximum.
Thermal Considerations
The RT9178 series can deliver a current of up to 200mA
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.
P
D
= (V
IN
−
V
OUT
) I
OUT
+ V
IN
I
GND
The final operating junction temperature for any set of
conditions can be estimated by the following thermal
equation:
P
D (MAX)
= ( T
J (MAX)
−
T
A
) /
θ
JA
Where T
J (MAX)
is the maximum junction temperature of
the die (125°C) and T
A
is the maximum ambient
temperature. The junction to ambient thermal resistance
(θ
JA
) for
SOT-23-5
package at recommended minimum
footprint is 250°C/W (θ
JA
is layout dependent). Visit our
website in which
“Recommended
Footprints for Soldering
Surface Mount Packages” for detail.
PCB Layout
Good board layout practices must be used or instability
can be induced because of ground loops and voltage drops.
The input and output capacitors
MUST
be directly
connected to the input, output, and ground pins of the
device using traces which have no other currents flowing
through them.
The best way to do this is to layout C
IN
and C
OUT
near the
device with short traces to the V
IN
, V
OUT
, and ground pins.
The regulator ground pin should be connected to the
external circuit ground so that the regulator and its
capacitors have a
“single
point ground”.
DS9178-16 April 2008
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
