CS8147 데이터 시트보기 (PDF) - Cherry semiconductor

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CS8147

10V/5V Low Dropout Dual Regulator with ENABLE

Cherry semiconductor 

Applications

Since both outputs are controlled by the same ENABLE ,

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.

Stability Considerations

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.

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.

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.

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.

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.

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 condi-

tions.

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.

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.

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.

Calculating Power Dissipation

in a Dual Output Linear Regulator

The maximum power dissipation for a dual output regula-

tor (Figure 1) is

PD(max) = {VIN(max) Ð VOUT1(min)}IOUT1(max) +

{VIN(max) Ð VOUT2(min)}IOUT2(max) + VIN(max)IQ

(1)

Where:

VIN(max) is the maximum input voltage,

VOUT1(min) is the minimum output voltage from VOUT1,

VOUT2(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).

Once the value of PD(max) is known, the maximum permissi-

ble value of RQJA can be calculated:

RQJA =

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.

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