MIC24045 Datasheet, PDF (29/46 Pages) Microchip Technology – I2C Programmable, 4.5V-19V Input, 5A Step-Down Converter

MIC24045
The inductance value is critical to the operation of
MIC24045. Since the MIC24045 is a 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.
TABLE 7-1: MINIMUM RECOMMENDED INDUCTANCE VALUES
Nominal
I_OUT
V_OUT
3A-4A-5A 0.64V-1.28V
3A-4A-5A 1.29V-1.95V
1.27
1.96
0.97
1.51
Minimum Inductance L_MIN (µH)
0.78
0.68
0.58
0.49
1.21
1.06
0.91
0.76
0.39
0.61
0.29
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
2A 0.64V-1.28V 2.52
1.94
1.55
1.36
1.16
0.97
0.78
0.58
2A 1.29V-1.95V 4.07
3.13
2.50
2.18
1.87
1.56
1.25
0.94
2A 1.98V-3.42V 6.53
5.03
4.01
3.52
3.02
2.52
2.01
1.51
2A 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
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:
I_L,PEAK =
I_OUT
+
V_O

UT_
1-----–_{--2---V---O---f-U-s---T----}_{--L-V---I--N--}
As shown in Equation 7-3, the peak inductor current
decreases with the switching frequency and the
inductance. At a given I_OUT 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 C_OUT:
• 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.
 2016 Microchip Technology Inc.
DS20005568A-page 29