MIC5271 datasheet(11/20 Pages) MICROCHIP

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DS20005881B-page 11

MIC5271

EQUATION 4-1:

EQUATION 4-2:

The actual power dissipation of the regulator circuit can

be determined using Equation 4-3:

EQUATION 4-3:

Substituting PD(MAX), determined above, for PD and

solving for the operating conditions that are critical to

the application will give the maximum operating

conditions for the regulator circuit. The maximum

power dissipation number cannot be exceeded for

proper operation of the device. The maximum input

voltage can be determined using the output voltage of

5.0V and an output current of 100 mA. Ground current,

of 2 mA for 100 mA of output current, can be taken from

Table .

• 425 mW = (VIN – 5.0V) x 100 mA + VIN x 2 mA

• 425 mW = (100 mA x VIN + 2 mA x VIN) – 500mW

• 925 mW = 102 mA x VIN

• VIN = 9.07V (maximum)

Therefore, a –5.0V application at –100 mA of output

current can accept a maximum input voltage of –9.07V

in a SOT-23 package. For a full discussion of heat

sinking and thermal effects on voltage regulators, refer

to “Regulator Thermals” section of Microchip’s

Designing with Low Dropout Voltage Regulators

handbook and AN792, A Method to Determine How

Much Power an SOT23 Can Dissipate in an

Application.

4.6

Adjustable Regulator Application

The MIC5271YM5 can be adjusted from –1.20V to

–14V by using two external resistors (Figure 4-2). The

resistors set the output voltage based on Equation 4-4.

FIGURE 4-2:

Adjustable Voltage

Application.

EQUATION 4-4:

PD MAX





TJ MAX





–

JA

--------------------------------

PD MAX





125C 25C

–

235C/W

-----------------------------------

425mW

VIN VOUT

–





IOUT



VIN IGND







–V

MIC5271YM5

–V

OUT

GND

–IN

ADJ

–OUT

VOUT

VREF 1 R2

-------









Where:

VREF = 1.20V

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