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CS8120

5.0 V, 300 mA Linear Regulator with RESET and ENABLE

ON Semiconductor 

CS8120

STABILITY CONSIDERATIONS

The output or compensation capacitor, C2, 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 C2 shown in Figure 14

should work for most applications, however it is not

necessarily the optimized solution.

VIN

C1*

0.1 mF

VOUT

CS8120

RESET

ENABLE

5.0 V to mP and

System Power

RRST

to mP

RESET

port

CRST

C2**

10 mF

*C1 is required if regulator is far from the power source filter.

**C2 is required for stability.

Figure 14. Circuit Showing Output

Compensation Capacitor

To determine an acceptable value for C2 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 15) is:

PD(max) + NJVIN(max) * VOUT(min)NjIOUT(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:

RQJA

+

150°C *

PD

TA

(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.

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