Applications
Step 5: If the capacitor is adequate, repeat steps 3 and 4
ENABLE
Since both outputs are controlled by the same
,
with the next smaller valued capacitor. A smaller capacitor
will usually cost less and occupy less board space. If the
output oscillates within the range of expected operating
conditions, repeat steps 3 and 4 with the next larger stan-
dard capacitor value.
the CS8147 is ideal for applications where a sleep mode is
required. Using the CS8147, a section of circuitry such as a
display and nonessential 5V circuits can be shut down
under microprocessor control to conserve energy.
The test applications circuit diagram shows an automotive
radio application where the display is powered by 10V
from VOUT1 and the Tuner IC is powered by 5V from
VOUT2. Neither output is required unless both the ignition
and the Radio On/OFF switch are on.
Step 6: Test the load transient response by switching in
various loads at several frequencies to simulate its real
working environment. Vary the ESR to reduce ringing.
Step 7: Raise the temperature to the highest specified oper-
ating temperature. Vary the load current as instructed in
step 5 to test for any oscillations.
Stability Considerations
Once the minimum capacitor value with the maximum
ESR is found for each output, a safety factor should be
added to allow for the tolerance of the capacitor and any
variations in regulator performance. Most good quality
aluminum electrolytic capacitors have a tolerance of ±20%
so the minimum value found should be increased by at
least 50% to allow for this tolerance plus the variation
which will occur at low temperatures. The ESR of the
capacitors should be less than 50% of the maximum allow-
able ESR found in step 3 above.
The secondary output VOUT2 is inherently stable and does
not require a compensation capacitor. However a compen-
sation capacitor connected between VOUT1 and ground is
required for stability in most applications.
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
The capacitor value and type should be based on cost,
availability, size and temperature constraints. A tantalum
or aluminum electrolytic capacitor is best, since a film or
ceramic capacitor with almost zero ESR can cause instabili-
ty. The aluminum electrolytic capacitor is the least expen-
sive solution, but, if the circuit operates at low tempera-
tures (-25¡C to -40¡C), both the value and ESR of the capac-
itor will vary considerably. The capacitor manufacturers
data sheet usually provides this information.
Calculating Power Dissipation
in a Dual Output Linear Regulator
The maximum power dissipation for a dual output regula-
tor (Figure 1) is
The value for the output capacitor C2 shown in the test
and applications circuit should work for most applications,
however it is not necessarily the optimized solution.
PD(max) = {VIN(max) Ð VOUT1(min)}IOUT1(max)
+
{VIN(max) Ð VOUT2(min)}IOUT2(max) + VIN(max)IQ
(1)
To determine acceptable value for C2 for a particular
application, start with a tantalum capacitor of the recom-
mended value and work towards a less expensive alterna-
tive part.
Where:
VIN(max) is the maximum input voltage,
VOUT1(min) is the minimum output voltage from VOUT1
,
Step 1: Place the completed circuit with a tantalum capaci-
tor of the recommended value in an environmental cham-
ber at the lowest specified operating temperature and
monitor the outputs with an oscilloscope. A decade box
connected in series with the capacitor will simulate the
higher ESR of an aluminum capacitor. Leave the decade
box outside the chamber, the small resistance added by the
longer leads is negligible.
V
OUT2(min) is the minimum output voltage from VOUT2
,
IOUT1(max) is the maximum output current, for the appli-
cation,
IOUT2(max) is the maximum output current, for the appli-
cation, and
IQ is the quiescent current the regulator consumes at
IOUT(max)
.
Step 2: With the input voltage at its maximum value,
increase the load current slowly from zero to full load
while observing the output for any oscillations. If no oscil-
lations are observed, the capacitor is large enough to
ensure a stable design under steady state conditions.
Once the value of PD(max) is known, the maximum permissi-
ble value of RQJA can be calculated:
Step 3: Increase the ESR of the capacitor from zero using
the decade box and vary the load current until oscillations
appear. Record the values of load current and ESR that
cause the greatest oscillation. This represents the worst
case load conditions for the regulator at low temperature.
150¡C - TA
RQJA
=
(2)
PD
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages with
R
QJA's less than the calculated value in equation 2 will keep
Step 4: Maintain the worst case load conditions set in step
3 and vary the input voltage until the oscillations increase.
This point represents the worst case input voltage condi-
tions.
the die temperature below 150¡C.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
6
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