MIC28303 datasheet(27/38 Pages) MICROCHIP

27 / 38 page

 2016 Microchip Technology Inc.

DS20005464A-page 27

MIC28303

FIGURE 5-7:

Invisible Ripple at FB.

With the feed-forward capacitor, the feedback

voltage ripple is very close to the output voltage

ripple.

EQUATION 5-13:

• Virtually no ripple at the FB pin voltage due to the

very-low ESR of the output capacitors.

In this situation, the output voltage ripple is less than

20 mV. Therefore, additional ripple is injected into the

FB pin from the switching node SW via a resistor Rinj

and a capacitor Cinj, as shown in Figure 5-7. The

injected ripple is:

EQUATION 5-14:

EQUATION 5-15:

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

the time constant associated with Cff must be much

greater than the switching period:

EQUATION 5-16:

If the voltage divider resistors R1 and R11 are in the kΩ

range, then a Cff of 1 nF to 100 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:

Select Cff to feed all output ripples into the feed-

back pin and make sure the large time constant

assumption is satisfied. Typical choice of Cff is

1 nF to 100 nF if R1 and R11 are in the kΩ

range.

Select Rinj according to the expected feedback

voltage ripple using Equation 5-17:

EQUATION 5-17:

Then the value of Rinj is obtained as:

EQUATION 5-18:

Select Cinj as 100 nF, which could be considered

as short for a wide range of the frequencies.

Table 2-2 summarizes the typical value of components

for particular input and output voltage, and 600 kHz

switching frequency design.

5.7

Thermal Measurements and Safe

Operating Area

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 heat sink, resulting in a

lower case measurement.

Two methods of temperature measurement use a

smaller

thermal

couple

wire

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 makes 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 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.

MIC28303

R11

ESR

OUT

inj

VOUT

FB PP





ESR

LPP









FB PP





div



–









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



div

R1 R11



inj

R1 R11



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

Where:

VIN

Power Stage Input Voltage

Duty Cycle

fSW

Switching Frequency



(R1||R11||Rinj) x Cff





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



---1

div

FB PP





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





–





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



inj

R1 R11





div

----------

–







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