CS8361 데이터 시트보기 (PDF) - Cherry semiconductor

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CS8361

5V Dual Micropower Low Dropout Regulator with ENABLE and RESET

Cherry semiconductor 

Circuit Description

ENABLE Function

The ENABLE function switches the output transistor for

VTRK on and off. When the ENABLE lead voltage exceeds

1.4V(typ), VTRK turns off. This input has several hundred

millivolts of hysteresis to prevent spurious output activity

during power-up or power-down.

RESET Function

The RESET is an open collector NPN transistor, con-

trolled by a low voltage detection circuit sensing the VSTBY

(5V) output voltage. This circuit guarantees the RESET

output stays below 1V (0.1V typ) when VSTBY is as low as

1V to ensure reliable operation of microprocessor-based

systems.

VTRK Output Voltage

This output uses the same type of output device as VSTBY,

but is rated for 250mA. The output is configured as a

tracking regulator of the standby output. By using the

standby output as a voltage reference, giving the user an

external programming lead (Adj lead), output voltages

from 5V to 20V are easily realized. The programming is

done with a simple resistor divider (Figure 2), and follow-

ing the formula:

VTRK = VSTBY ´ (1 + R1/R2) + IAdj ´ R1

If another 5V output is needed, simply connect the Adj

lead to the VTRK output lead.

Application Notes

External Capacitors

Output capacitors for the CS8361 are required for stability.

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

conditions of negative input transients that might be char-

acteristic of a particular system.

Capacitors must also be rated at all ambient temperatures

expected in the system. To maintain regulator stability

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, ÒCompensation for Linear Regulators.Ó

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)

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

application

IOUT2(max) is the maximum output current, for the

application

IQ is the quiescent current the regulator consumes at

IOUT(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.

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

IQ

IOUT1

IOUT2

VOUT1

VOUT2

Figure 1: Dual output regulator with key performance parameters

labeled.

4

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