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CS8391YDP5

5V/250mA, 5V/100mA Micropower Low Dropout Regulator with ENABLE

Cherry semiconductor 

Application Notes

General

The CS8391 is a micropower dual 5V regulator. All bias

required to operate the internal circuitry is derived from

the standby output, VOUT2. If this output experiences an

over current situation and collapses, then VOUT1 will also

collapse (see timing diagrams).

If there is critical circuitry that must remain active under

most conditions it should be connected to VOUT2. Any cir-

cuitry that is likely to be subjected to a short circuit, e.g.,

circuitry outside the module, should be connected to VOUT1.

External Capacitors

IOUT2(max) is the maximum output current for the appli-

cation, and

IQ is the quiescent current the regulator consumes at both

IOUT1(max) and IOUT2(max).

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

missible 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.

Output capacitors are required for stability with the CS8391.

Without them, the regulator outputs will oscillate. Actual

size and type may vary depending upon the application

load and temperature range. Capacitor effective series

resistance (ESR) is also a factor in the IC stability. Worst-

case is determined at the minimum ambient temperature

and maximum load expected.

Output capacitors can be increased in size to any desired

value above the minimum. One possible purpose of this

would be to maintain the output voltages during brief con-

ditions of negative input transients that might be character-

istic of a particular system.

In some cases, none of the packages will be sufficient to

dissipate the heat generated by the IC, and an external heat

sink will be required.

IIN

VIN

Smart

Regulator

}Control

Features

IOUT1

IOUT2

VOUT1

VOUT2

Capacitors must also be rated at all ambient temperatures

expected in the system. To maintain regulator stability

IQ

down to -40ûC, capacitors rated at that temperature must be

used.

More information on capacitor selection for Smart

Regulatorsª is available in the Smart Regulator applica-

tion note, Compensation for Linear Regulators.

Figure 1: Dual output regulator with key performance parameters

labeled.

Heat Sinks

ENABLE

The ENABLE function controls VOUT1. When ENABLE is

high, VOUT1 is on. When ENABLE is low, VOUT1 is off.

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)ÐVOUT1(min)}IOUT1(max)+

{VIN(max)ÐVOUT2(min)}IOUT2(max)+VIN(max)IQ (1)

A heat sink effectively increases the surface area of the

package to improve the flow of heat away from the IC and

into the surrounding air.

Each material in the heat flow path between the IC and the

outside environment will have a thermal resistance. Like

series electrical resistances, these resistances are summed to

determine the value of RQJA:

RQJA = RQJC + RQCS + RQSA

(3)

where:

RQJC = the junctionÐtoÐcase thermal resistance,

RQCS = the caseÐtoÐheat sink thermal resistance, and

RQSA = the heat sinkÐtoÐambient thermal resistance.

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,

RQJC appears in the package section of the data sheet. Like

RQJA, it too is a function of package type. RQCS and RQSA are

functions of the package type, heat sink and the interface

between them. These values appear in heat sink data sheets

of heat sink manufacturers.

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