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CS8122 Datasheet(PDF) 7 Page - ON Semiconductor

Part No. CS8122
Description  2.0% 5.0 V, 750 mA Low Dropout Linear Regulator with Delayed RESET
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS8122 Datasheet(HTML) 7 Page - ON Semiconductor

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CS8122
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7
APPLICATION NOTES
STABILITY CONSIDERATIONS
The output or compensation capacitor, COUT, 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
instability. The 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 COUT shown in Figure
13 should work for most applications, however it is not
necessarily the optimized solution.
To determine an acceptable value for COUT 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: Raise 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 14) is:
PD(max) + VIN(max) * VOUT(min) IOUT(max) ) VIN(max)IQ
(1)
where:
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
IOUT(max) is the maximum output current for the
application, and
IQ is the quiescent current the regulator consumes at
IOUT(max).
Once the value of PD(max) is known, the maximum
permissible value of RqJA can be calculated:
R
qJA +
150
°C * TA
PD
(2)
The value of RqJA can then be compared with those in the
package section of the data sheet. Those packages with
RqJA’s less than the calculated value in equation 2 will keep
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.
Figure 14. Single Output Regulator With Key
Performance Parameters Labeled
SMART
REGULATOR®
Control
Features
IOUT
IIN
IQ
VIN
VOUT


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