Application Notes: continued
increase the load current slowly from zero to full load
Table 1. Logic Control of CS8101 Output
Microprocessor
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.
I/O drive
ON
Switch
Closed
Open
ENABLE
LOW
LOW
Output
ON
ON
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.
OFF
Closed
Open
LOW
HIGH
ON
OFF
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 conditions.
The I/O port of the microprocessor typically provides
50µA to Q1. In automotive applications the SWITCH is
connected to the ignition switch.
Step 5: If the capacitor is adequate, repeat steps 3 and 4
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.
Stability Considerations
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
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.
VIN
VOUT
Step 7: Remove the unit from the environmental chamber
and heat the IC with a heat gun. Vary the load current as
instructed in step 5 to test for any oscillations.
C
*
IN
0.1mF
C
**
OUT
R
RST
10mF
CS8101
RESET
Once the minimum capacitor value with the maximum
ESR is found, a safety factor should be added to allow for
the tolerance of the capacitor and any variations in regula-
tor 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 tem-
peratures. The ESR of the capacitor should be less than
50% of the maximum allowable ESR found in step 3 above.
ENABLE
*C required if regulator is located far from the power supply filter.
IN
OUT
**C
required for stability. Capacitor must operate at minimum
temperature expected.
Figure 5. Test and application circuit showing output compensation.
Calculating Power Dissipation
in a Single Output Linear Regulator
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
capacitor will vary considerably. The capacitor manufac-
turers data sheet usually provides this information.
The maximum power dissipation for a single output regu-
lator (Figure 6) is:
PD(max) = VIN(max) - VOUT(min)}
I
OUT(max) + VIN(max) Q
I
(1)
{
where:
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
The value for the output capacitor COUT shown in Figure 5
should work for most applications, however it is not nec-
essarily the optimized solution.
IOUT(max) is the maximum output current for the applica-
tion, and
IQ is the quiescent current the regulator consumes at
IOUT(max)
.
To determine an acceptable value for COUT for a particular
application, start with a tantalum capacitor of the recom-
mended value and work towards a less expensive alterna-
tive part.
Once the value of PD(max) is known, the maximum permis-
sible value of RQJA can be calculated:
Step 1: Place the completed circuit with a tantalum capac-
itor 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.
150¡C - TA
(2)
RQJA
=
PD
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages with
QJA's less than the calculated value in equation 2 will keep
the die temperature below 150¡C.
R
Step 2: With the input voltage at its maximum value,
5
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