L6235Q datasheet(12/33 Pages) STMICROELECTRONICS

12 / 33 page

Circuit description

L6235Q

12/33

Doc ID 018997 Rev 2

Figure 10.

Output current regulation waveforms

Figure 11 shows the magnitude of the OFF time tOFF versus COFF and ROFF values. It can

be approximately calculated from the equations:

RCFALL = 0.6 · R

OFF · 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 is only an issue if the capacitor is not completely charged

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

depends on motors and supply parameters, must be bigger than tRCRISE to allow 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

Slow Decay

μs tBLANK

tRCRISE

SYNCHRONOUS RECTIFICATION

μs tBLANK

VRC

VSENSE

VREF

IOUT

VREF

RSENSE

D02IN1351

tOFF

μs tDT

tRCFALL

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