MIC24045 datasheet(29/46 Pages) MICROCHIP

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DS20005568A-page 29

MIC24045

The inductance value is critical to the operation of

MIC24045.

Since

the

MIC24045

valley

current-mode regulator, it needs a slope compensation

for the stable current loop operation where duty cycles

are below 50%. Slope compensation is internally

programmed according to the frequency, output

voltage and nominal load current selection, assuming

there is a minimum inductance value for the given

operating condition.

Table 7-1 lists the assumed minimum inductor values

recommended for stable current-loop operation. Note

that the minimum suggested inductance values should

be met when taking into account the inductor tolerance

and its change with current level.

The slope compensation is also internally adapted to

the input-output voltage differential.

In practical implementations of valley current-mode

control, slope compensation is also added to any duty

cycle larger than 50% as part of improving current loop

stability and noise immunity for all input and output volt-

age ranges. Consequently, the MIC24045 adds internal

slope compensation signal up to 80% duty cycle.

Above this, no slope compensation is added. For this

reason, the PWM modulator gain exhibits an abrupt

change when the duty cycle exceeds 80%, possibly

leading to some increase in jitter and noise susceptibil-

ity. If operation around and above 80% duty cycle is

considered, a more conservative design of the com-

pensation loop might help in reducing jitter and noise

sensitivity.

Inductor current ratings are generally stated as

permissible DC current and saturation current.

Permissible DC current can be rated for a +20°C to

+40°C temperature rise. Saturation current can be

rated for a 10% to 30% loss in inductance. Ensure that

the nominal current of the application is well within the

permissible DC current ratings of the inductor,

depending on the allowed temperature rise. Note that

the inductor permissible DC current rating typically

does not include inductor core losses. These are very

important contributors of total inductor core loss and

temperature increase in high-frequency DC/DC

converters because core losses increase rapidly with

the excitation frequency.

When saturation current is specified, make sure that

there are enough design margins so the peak current

does not cause the inductor to enter deep saturation.

Pay attention to the inductor saturation characteristic in

current limit. The inductor should not heavily saturate,

even in current limit operation. If there is heavy satura-

tion, the current may instantaneously run away and

reach

potentially

destructive

levels.

Typically,

ferrite-core inductors exhibit an abrupt saturation char-

acteristic, while powdered-iron or composite inductors

have a soft-saturation characteristic. Peak current can

be calculated with Equation 7-3.

EQUATION 7-3:

As shown in Equation 7-3, the peak inductor current

decreases with the switching frequency and the

inductance. At a given IOUT load current, the lower the

switching frequency or inductance, the higher the peak

current. As input voltage increases, the peak current

also increases.

7.5

Output Capacitor Selection

Two main requirements determine the size and

characteristics of the output capacitor COUT:

• Steady-state ripple

• Maximum voltage deviation during load transient

For steady-state ripple calculation, both the ESR and

the capacitive ripple contribute to the total ripple

amplitude.

TABLE 7-1:

MINIMUM RECOMMENDED INDUCTANCE VALUES

Nominal

IOUT

VOUT

Minimum Inductance LMIN (µH)

3A-4A-5A

0.64V-1.28V

1.27

0.97

0.78

0.68

0.58

0.49

0.39

0.29

3A-4A-5A

1.29V-1.95V

1.96

1.51

1.21

1.06

0.91

0.76

0.61

0.45

3A-4A-5A

1.98V-3.42V

3.14

2.42

1.94

1.70

1.46

1.21

0.97

0.73

3A-4A-5A

4.57V-5.25V

3.69

2.36

2.27

1.99

1.70

1.42

1.14

0.85

0.64V-1.28V

2.52

1.94

1.55

1.36

1.16

0.97

0.78

0.58

1.29V-1.95V

4.07

3.13

2.50

2.18

1.87

1.56

1.25

0.94

1.98V-3.42V

6.53

5.03

4.01

3.52

3.02

2.52

2.01

1.51

4.57V-5.25V

9.14

6.99

5.60

4.91

4.18

3.49

2.80

2.10

310

400

500

570

660

780

970

1200

L,PEAK

OUT

OUT VIN



–

2fs



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





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