MIC28514 datasheet(22/34 Pages) MICROCHIP

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DS20005693G-page 22

MIC28514

If the voltage divider resistors, R1 and R2, are in the kΩ

range, a Cff of 1 nF to 22 nF can easily satisfy the large

time constant requirements. Also, a 100 nF Injection

Capacitor, CINJ, is used in order to be considered as

short for a wide range of the frequencies.

The process of sizing the ripple injection resistor and

capacitors is as follows:

Select Cff to feed all output ripples into the Feed-

back pin and make sure the large time constant

assumption is satisfied. A typical choice for Cff is

1 nF to 22 nF if R1 and R2 are in the kΩ range.

Select RINJ according to the expected feedback

voltage ripple using Equation 5-24:

EQUATION 5-24:

Then, the value of RINJ is obtained as:

EQUATION 5-25:

Select CINJ as 100 nF, which could be considered

as short for a wide range of the frequencies.

5.9

Thermal Measurements

Measuring the IC’s case temperature is recommended

to ensure it is within its operating limits. Although this

might seem like a very elementary task, it is easy to get

erroneous results. The most common mistake is to use

the standard thermocouple that comes with a thermal

meter. This thermocouple wire gauge is large, typically

22 gauge, and behaves like a heat sink, resulting in a

lower case measurement.

There are two methods of temperature measurement:

using a smaller thermocouple wire or an infrared

thermometer. If a thermocouple wire is used, it must be

of 36-gauge or higher (smaller wire size) to minimize

the

wire

heat-sinking

effect.

addition,

the

thermocouple tip must be covered in either thermal

grease or thermal glue to make sure that the

thermocouple junction makes good contact with the

case of the IC.

Wherever possible, an infrared thermometer is

recommended. An optional stand makes it easy to hold

the beam on the IC for long periods of time.

KDIV

VFB PP





VIN

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

fSW 



D1 D

–





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



RINJ

R1//R2



KDIV

------------ 1

–







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