SLLA643 August   2024 MCF8315C , MCF8315C-Q1 , MCF8316C-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Power Pin Design Recommendations
    1. 2.1 VM
    2. 2.2 Charge Pump: CPH, CPL, CP
    3. 2.3 Buck Converter: FB_BK, SW_BK, GND_BK
    4. 2.4 AVDD
    5. 2.5 DVDD
    6. 2.6 PGND, AGND, DGND
    7. 2.7 Thermal Pad
  6. 3MCF831xC Buck Regulator Overview
    1. 3.1 Buck Regulator Mode of Operation
    2. 3.2 Buck Regulator Output Voltage
    3. 3.3 Buck Power Sequencing
    4. 3.4 Buck Inductor Selection
    5. 3.5 MCF831xC Operation Without Buck Regulator
  7. 4MCF831xC IO Pins Design Recommendations
    1. 4.1 SPEED Pin
    2. 4.2 BRAKE, DIR, DRVOFF pins
    3. 4.3 EXT_CLK, EXT_WD
    4. 4.4 ALARM
    5. 4.5 DACOUT1, DACOUT2
    6. 4.6 SDA, SCL
    7. 4.7 nFAULT and FG pin
  8. 5MCF831xC PCB Schematic and Layout Recommendations
    1. 5.1 Single Ground Plane
    2. 5.2 Single Ground with AVDD Shorted to FB_BK
    3. 5.3 Two Grounds
  9. 6Summary
  10. 7References

3.1 Buck Regulator Mode of Operation

MCF831xC has an integrated buck regulator for providing power to low-voltage (≤5V) internal as well as external circuits. Depending on the external load, the buck regulator can be operated either in inductor mode or in resistor mode as shown in Figure 3-1 and Figure 3-2. If external load is > 10mA, buck regulator needs to be operated in inductor mode and if external load ≤ 10mA buck regulator can be operated in resistor mode to reduce BOM cost

Regulator Buck in Inductor Mode Figure 3-1 Regulator Buck in Inductor Mode
Regulator Buck in Resistor Mode Figure 3-2 Regulator Buck in Resistor Mode

In inductor mode, buck regulator operates as a conventional switching regulator providing high efficiency. In the inductor mode, there are two inductor options to optimize between cost and performance. For external load ≤ 20mA, a 22µH inductor can be used for lower BOM cost and for external load > 20mA, a 47µH inductor needs to be used.

In resistor mode, buck regulator operates as a pseudo LDO wherein a majority of the losses are dissipated in the external resistor instead of within MCF831xC thereby allowing higher power delivery to the BLDC motor. Resistor wattage depends on VM voltage. Refer to Table 3-1 for resistor wattage calculation.

Table 3-1 Resistor Wattage Calculations
VM 12 24 35 V
Buck voltage, VBK 5 5 5 V

Buck load from internal circuits,

(BUCK_PS_DIS = 0b, IBK_EXT = 0mA)

 30 30 30 mA
Power rating 0.21 0.57 0.90 W

In both the modes, a 22µF capacitor needs to be connected across the buck regulator output (FB_BK and GND_BK) to maintain peak to peak voltage ripple within 200mV. Refer to Table 3-2 for detailed specifications of buck regulator inductor/resistor and capacitor.

Table 3-2 Buck Regulator Configuration Depending on External Load
External Load on Buck Output (mA) Mode of Operation Buck Current Limit, BUCK_CL (mA)
0 ≤ IBK_EXT ≤ 10 Resistor, 22Ω 150 (BUCK_CL =1b)
10 < IBK_EXT≤ 20 Inductor, 22µH 150 (BUCK_CL =1b)
20 < IBK_EXT≤ 170 Inductor, 47µH 600 (BUCK_CL =0b)