dsPIC33CK Low-Voltage Motor Control Board User’s Guide
DS50002927A-page 62 2020 Microchip Technology Inc.
The major components of the +12V supply are:
• The capacitors, C13 and C15, are the input supply capacitors of the +12V power
supply stage.
• The EN pin of the MIC28511 has an on-board 100 k pull-up resistor (R8) to V
IN,
which allows the output to be turned on when PV
DD exceeds its UVLO threshold.
• The switching frequency of the converter is set by the resistors, R7 and R11.
When R11 is not populated, the switching frequency will typically be 680 kHz, as
is the case in this Motor Control Board. The resistor R7 is selected as 100 k.
• The output is determined by resistors, R6 and R10, where V
OUT = +12V,
V
FB = 0.8V and R6 = 10k. Then, R10 is calculated as:
• The MIC28511 uses the R
DS-ON
and a resistor connected from ILIM to the SW node
to decide the current limit. The current limit resistor R4 value is calculated as:
• The Power Good (PGOOD) pin is an open-drain output, which is pulled up with a
10 kΩ resistor (R9) to V
DD. This indicates a logic high when the output is
nominally 90% of its steady-state voltage.
• The bootstrap circuit, the diode D1, resistor R2 and capacitor C2. This circuit
supplies energy to the high-side drive circuit. In the Motor Control Board, D1 is
selected as BAT46W, R2 is set as 10Ω and C2 is selected as 0.1 µF to hold a
charge for approximately 1.25 µSec.
• In order to have some amount of voltage ripple at the voltage feedback pin, a
ripple injection method is applied for low output voltage ripple applications. In the
Motor Control Board, components C5 (3300 pF), R3 (100k) and C6 (0.1 µF) are
used for this purpose.
• The output stage of the synchronous buck converter is comprised of an inductor
and capacitor. In this case, inductor L1 and capacitors, C7 and C2, are the output
inductor and capacitor.
- The minimum value of the inductance at maximum input voltage (i.e., 60V),
considering 20% ripple current is as follows:
- The minimum value of the output capacitance can be calculated based on the
selected output inductance L1 (100 µH), which is:
In the Motor Control Board, output capacitors, C7 and C2, are set as 47 µF and
2.2 µF; setting total output capacitor value as greater than the calculated value.
For additional information and recommendations, refer to the “MIC28511 – 60 V
IN,
3A Synchronous Buck Regulator Data Sheet” (DS20005520) and “MIC28511-1YFL
Evaluation Board User’s Guide”.
0.8 R6
V
OUT – VFB
0.8 10k
11.2V
= 714.3
(ICLIM – IL(PP) 0.5) R
DS-ON
+ VCL
ICL
(2A – 0.2 0.5) 28 m + 14 mV
70 µA
= = 960
(VINMAX – VOUT)
IL ILMAX F
SW
= = 88.23 µH
V
OUT
VINMAX
(60V – 12V)
0.2 0.8A kHz
12V
60V
100 µH (0.8A +
(12 + 0.1)
2
– (12)
2
== 32 µF
0.8A 0.2
2
100 µH (0.88)
2
(12.1)
2
– (12)
2
L IPK
2
(VOUT + VOUT)
2
– VOUT
2
)
2
=
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