STEVAL-ISF002V1 STMicroelectronics, STEVAL-ISF002V1 Datasheet - Page 39

• Manufacturer Part Number: STEVAL-ISF002V1
• Description: BOARD EVALUATION
• Manufacturer: STMicroelectronics

Specifications of STEVAL-ISF002V1

• Main Purpose: *
• Embedded: *
• Utilized Ic / Part: *
• Primary Attributes: *
• Secondary Attributes: *
• Product: Power Management Modules
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-10422

UM0877
Figure 30. Digital PFC timing
The frequency of TIM3 is fixed at 80 kHz while the duty cycle varies according to the digital
PFC control. TIM3/CH.4 is used to drive the PFC power MOSFET. TIM3/CH.3 is used to
trigger the ADC. The ADC conversion starts at the end of the TIM3/CH.3 duty cycle. The
TIM3/CH.3 duty cycle is equal to half of the TIM3/CH.4 duty cycle, but never lower than 1 µs
to avoid invalidating the conversion with noise coming from the switching of the power
MOSFET. For the STM32 ADC the total conversion time is calculated as follows.
Equation 41
For design purposes, the following values have been selected.
Equation 42
Equation 43
Thus yielding:
Equation 44
Through experimental measurements, the CPU performs a "PFC routine" for a time of
4.27 µs every control loop, then the CPU’s load for the PFC routine
becomes:
T
ADC
. 4
25
conv
27
μs
μs
=
SamplingTi
≅
17
%
ADC
T
ADC
clock
Doc ID 16854 Rev 1
me
conv
SamplingTi
+
=
12
ADC
SamplingTi
5 .
=
clock
(
me
7
12
5 .
=
=
+
MHz
12
me
12
7
5 .
MHz
5 .
+
cycles
) (
12
=
5 .
. 0
cycles
625
+
. 1
041
Digital PFC firmware
)
μs
=
. 1
666
μs
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