AN2836 datasheet(22/57 Pages) STMICROELECTRONICS

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Designing an application with L6225, L6226, L6227

AN2836

22/57

Doc ID 15084 Rev 1

4.12

Off-time selection and minimum on-time (L6227 only)

Figure 17 also shows the lower limit for the on-time tON for having a good PWM current

regulation capacity. It has to be said that t

ON is always longer than t

ON(MIN) because the

device imposes this condition, but it can be shorter than tRCRISE - tDT. In this last case the

device continues to work, but the off-time tOFF is no longer constant.

So, a small COFF value gives more flexibility for the applications (allows shorter on-time and,

therefore, higher switching frequency), but the smaller the value for COFF, the more

influential the noise on the circuit performance.

4.12.1

Slow decay mode (L6227 only)

Figure 18 shows the operation of the bridge in the slow decay mode. At the start of the off

time, the lower power MOSFET is switched off and the current recirculates around the upper

half of the bridge. Since the voltage across the coil is low, the current decays slowly. After

the deadtime the upper power MOSFET is operated in the synchronous rectification mode.

When the monostable times out, the lower power MOSFET is turned on again after a delay

set by the deadtime to prevent cross conduction.

Figure 16.

Typical off-time vs. COFF for several

values of ROFF

Figure 17.

Minimum on-time vs. COFF

AM01636v1

0.1

100

1 . 10

Coff [nF]

R = 20 k

R = 47 k

R = 100 k

AM01637v1

Coff [nF]

0.1

100

Coff [nF]

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