NCV8141
http://onsemi.com
11
Figure 14. Application Diagram
NCV8141
Battery
Ignition
WDI
RESET
V
OUT
V
IN
ENABLE
DELAY
GND
C
1
*
0.1 mF
(optional)
0.1 mF
R***
C
2
*
10 mF*
2.7 kW
V
CC
RESET
WATCHDOG
PORT
Microprocessor
*C1 is required if regulator is located far from the power source filter.
**C2 is required for stability.
***R ≤ 80 kW.
STABILITY CONSIDERATIONS
The output or compensation capacitor C
2
in Figure 14
helps determine three main characteristics of a linear
regulator: startup delay, load transient response and loop
stability.
The capacitor value and type should be based on cost,
availability, size and temperature constraints. An aluminum
electrolytic capacitor is the least expensive solution, but, if
the circuit operates at low temperatures (−25°C to −40°C),
both the value and ESR of the capacitor will vary
considerably. The capacitor manufacturers data sheet
usually provides this information.
The value for the output capacitor C
2
shown in Figure 14
should work for most applications, however it is not
necessarily the optimized solution.
To determine an acceptable value for C
2
for a particular
application, start with a tantalum capacitor of the
recommended value and work towards a less expensive
alternative part.
Step 1: Place the completed circuit with a tantalum
capacitor of the recommended value in an environmental
chamber 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.
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 oscillations
are observed, the capacitor is large enough to ensure a stable
design under steady state conditions.
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.
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.
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 standard
capacitor value.
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: Increase the temperature to the highest specified
operating temperature. Vary the load current as instructed in
Step 5 to test for any oscillations.
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 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 capacitor should be less than
50% of the maximum allowable ESR found in Step 3 above.
CALCULATING POWER DISSIPATION IN A SINGLE
OUTPUT LINEAR REGULATOR
The maximum power dissipation for a single output
regulator (Figure 15) is:
P
D(max)
+
NJ
V
IN(max)
* V
OUT(min)
Nj
I
OUT(max)
) V
IN(max)
I
Q
(1)
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current for the
application, and
