MIC45116 datasheet(25/42 Pages) MICROCHIP

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DS20005571A-page 25

MIC45116

• Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors.

The output voltage ripple is fed into the FB pin

through a feed-forward capacitor, CFF in this

situation, as shown in Figure 5-3. The typical CFF

value is between 1 nF and 100 nF.

EQUATION 5-11:

With the feed-forward capacitor, the feedback

voltage ripple is very close to the output voltage

ripple.

FIGURE 5-3:

Inadequate Ripple at FB.

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

very low ESR of the output capacitors, such is the

case with ceramic output capacitors.

In this situation, the VFB ripple waveform needs to be

generated by injecting suitable signal. A series RC

network between the SW pin and FB pin, RINJ and

CINJ as shown in Figure 5-4 injects a square-wave

current waveform into the FB pin, which, by means of

integration across the capacitor (CFF), generates an

appropriate sawtooth FB ripple waveform.

FIGURE 5-4:

External Ripple Injection

Circuit at FB.

The injected ripple is calculated via:

EQUATION 5-12:

EQUATION 5-13:

In Equation 5-13 and Equation 5-14, it is assumed that

the time constant associated with CFF must be much

greater than the switching period:

EQUATION 5-14:

If the voltage divider resistors RFB1 and RFB2 are in the

kΩ range, a CFF of 1 nF to 100 nF can easily satisfy the

large time constant requirements.

5.5

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

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.

FB PP





ESR

COUT

LPP







OUT

PGND

MIC45116

FB1

FB2

OUT

ESR

VOUT

PGND

MIC45116

FB1

FB2

OUT

ESR

INJ

FB PP





div



1 D

–









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



Where:

VIN

Power stage input voltage

Duty cycle

fSW

Switching frequency

(RFB1//RFB2//RINJ) x CFF

div

FB1//RFB2

INJ

FB1//RFB2

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

Where:

RINJ

20 kΩ





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



---

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