MIC45205 datasheet(21/40 Pages) MICROCHIP

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DS20005798B-page 21

MIC45205

The injected ripple is:

EQUATION 5-13:

EQUATION 5-14:

In Equation 5-14 and Equation 5-15, it is assumed that

the time constant associated with C14 must be much

greater than the switching period:

EQUATION 5-15:

If the voltage divider resistors RFB1 and RFB2 are in the

kΩ range, then a C14 of 1 nF to 100 nF can easily

satisfy the large time constant requirements.

5.6

Thermal Measurements and Safe

Operating Area (SOA)

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 thermal couple that comes with a thermal

meter. This thermal couple wire gauge is large, typically

22 gauge, and behaves like a heatsink, resulting in a

lower case measurement.

Two methods of temperature measurement are using a

smaller thermal couple wire or an infrared

thermometer. If a thermal couple wire is used, it must

be constructed of 36-gauge wire or higher (smaller wire

size) to minimize the wire heat-sinking effect. In

addition, the thermal couple tip must be covered in

either thermal grease or thermal glue to make sure that

the thermal couple junction is making good contact with

the case of the IC. Omega brand thermal couple

(5SC-TT-K-36-36) is adequate for most applications.

Wherever possible, an infrared thermometer is

recommended. The measurement spot size of most

infrared thermometers is too large for an accurate

reading on a small form factor ICs. However, an IR

thermometer from Optris has a 1 mm spot size, which

makes it a good choice for measuring the hottest point

on the case. An optional stand makes it easy to hold the

beam on the IC for long periods of time.

The safe operating area (SOA) of the MIC45205 is

shown in Figure 5-6 through Figure 5-10. These

thermal measurements were taken on MIC45205

evaluation board. Since the MIC45205 is an entire

system comprised of switching regulator controller,

MOSFETs and inductor, the part needs to be

considered as a system. The SOA curves will give

guidance to reasonable use of the MIC45205.

SOA curves should only be used as a point of

reference. SOA data was acquired using the MIC45205

evaluation board. Thermal performance depends on

the PCB layout, board size, copper thickness, number

of thermal vias, and actual airflow.

FIGURE 5-6:

MIC45205 Power Derating

vs. Airflow (5VIN to 1.5VOUT).

VFB PP







VIN Kdiv



1 D

–







fSW 



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



Where:

VIN

Power stage input voltage

Duty cycle

fSW Switching frequency

(RFB1//RFB2//RINJ) x C14

RINJ = 10 kΩ, CINJ = 0.1 µF

Kdiv

RFB1//RFB2

RINJ RFB1//RFB2

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

Where:

RINJ 10 kΩ

fSW 



----------------- T



---

100

105

110

115

120

125

AMBIENT TEMPERATURE( C)

0 LFM

200 LFM

400 LFM

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