L6228Q datasheet(14/32 Pages) STMICROELECTRONICS

14 / 32 page

Circuit description

L6228Q

14/32

Doc ID 14321 Rev 4

Figure 13.

Output current regulation waveforms

Figure 14 shows the magnitude of the Off Time tOFF versus COFF and ROFF values. It can be

approximately calculated from the equations:

tRCFALL = 0.6 · ROFF · COFF

tOFF = tRCFALL + tDT = 0.6 · ROFF · COFF + tDT

where ROFF and COFF are the external component values and tDT is the internally generated

Dead Time with:

20 k

Ω ≤ R

OFF ≤ 100 kΩ

0.47 nF

≤ C

OFF ≤ 100 nF

tDT = 1 µs (typical value)

Therefore:

tOFF(MIN) = 6.6 µs

tOFF(MAX) = 6 ms

These values allow a sufficient range of tOFF to implement the drive circuit for most motors.

The capacitor value chosen for COFF also affects the Rise Time tRCRISE of the voltage at the

pin RCOFF. The Rise Time tRCRISE will only be an issue if the capacitor is not completely

charged before the next time the monostable is triggered. Therefore, the on time tON, which

depends by motors and supply parameters, has to be bigger than tRCRISE for allowing a

good current regulation by the PWM stage. Furthermore, the on time tON can not be smaller

than the minimum on time tON(MIN).

OFF

tON

tOFF

2.5V

Fast Deca

Slow Decay

μs tBLANK

tRCRISE

μs tDT

tRCRISE

tRCFALL

SYNCHRONOUS OR QUASI

SYNCHRONOUS RECTIFICATION

μs tBLANK

VRC

VSENSE

VREF

IOUT

VREF

RSENSE

D01IN1334

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