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

3.4 Buck Inductor Selection

Table 3-4 Mapping Inductor Spec to MCF831xC Buck Regulator Specs
Description Inductor specification MCF831xC buck regulator specification
Value 47µH (for BUCK_CL = 0b)22µH (for BUCK_CL = 1b) 47µH ± 20% tolerance(for BUCK_CL = 0b)22µH ± 20% tolerance (for BUCK_CL = 1b)
Inductance at Isat Depends on vendor

MCF831xC buck regulator implements a pulse frequency modulation (PFM) scheme with peak current mode control – when inductor current reaches BUCK_CL limit, high-side FET in buck regulator is turned off. Due to internal circuit delay, inductor peak current can go up to 0.5A for BUCK_CL = 1b (Typ: 150mA) and up to 1A for BUCK_CL = 0b (Typ: 600mA).

Tolerance spec at Isat depends on vendor and expected range is 10% to 50%. Below is the minimum required inductance and Isat for a given BUCK_CL setting.

Inductance = 17.6µH at Isat = 0.5A for BUCK_CL = 1b

Inductance = 37.6µH at Isat = 1A for BUCK_CL = 0b
DCR DC resistance Max. DC resistance (across operating conditions) needs to be < 1Ω
IRMS Typical operating RMS current Needs to be greater than or equal to max DC load
Note: Recommendation is to use a package with magnetic shielding for better EMI/EMC performance.