MCP8026 datasheet(49/64 Pages) MICROCHIP

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DS20005339B-page 49

MCP8025/6

These are approximate calculations. The actual

voltages may vary due to incomplete charging or

discharging of capacitors per cycle due to load

changes. The charge pump calculations assume the

charge pump is able to charge up the external boot

cap within a few cycles.

5.1.2

BOOTSTRAP CAPACITOR

The high-side driver bootstrap capacitor needs to

power the high-side driver and gate for 1/3 of the

motor electrical period for a 3-phase BLDC motor.

Let:

Solve for the smallest capacitance that can supply:

- 130 nC of charge to the MOSFET gate

-1 M

 Gate-Source resistor current

- Driver bias current and switching losses

Sum all of the energy requirements:

Choose a bootstrap capacitor value that is larger than

43.86 nF.

5.1.3

BUCK SWITCHER

5.1.3.1

Calculate the Buck Inductor for

Discontinuous Mode Operation

Let:

Solve for maximum inductance value.

Choose

inductor



3.64 µH

ensure

Discontinuous Conduction mode.

Table 5-1 shows the various maximum inductance

values for a worst case input voltage of 6V and various

output voltages.

MOSFET driver current =300 mA

PWM period = 50µs (20kHz)

Minimum duty cycle =1% (500 ns)

Maximum duty cycle =99% (49.5 µs)

VIN =12V

Minimum gate drive voltage =8V (VGS)

Total gate charge = 130 nC (80A MOSFET)

Allowable VGS drop (VDROP) =3V

Switch RDSON =100 m

Driver internal bias current =20 µA (IBIAS)

QMOSFET = 130 nC

QRESISTOR = [(VGS/R) x TON]

QDRIVER = (IBIAS xTON)

TON = 49.5 µs (99% DC) for worst case

QRESISTOR = (12V/1 M)x 49.5 µs

QRESISTOR = 0.594 nC

QDRIVER = 20 µA x 49.5 µs

QDRIVER = 0.99 nC

C = (QMOSFET +QRESISTOR +QDRIVER)/VDROP

C = (130 nC + 0.594 nC + 0.99 nC)/3V

C = 43.86 nF

VIN = 4.3V (worst case is BUVLO)

VOUT = 3.3V

IOUT = 225 mA

fSW = 468 kHz (TSW = 2.137 µs)

LMAX = VOUT x (1–VOUT/VIN)x TSW/(2 x IOUT)

LMAX = 3.3V x (1 – 3.3V/4.3V) x 2.137 µs/(2 x 225 mA)

LMAX = 3.64 µH

TABLE 5-1:

MAXIMUM INDUCTANCE FOR BUCK DISCONTINUOUS MODE OPERATION

VIN

(worst case)

VOUT

IOUT

Maximum Inductance

4.3V (BUVLO)

250 mA

4.3 µH

4.3V (BUVLO)

3.3V

225 mA

3.6 µH

5.0V

150 mA

5.9 µH

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