L9959 datasheet(24/49 Pages) STMICROELECTRONICS

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Application information

L9959

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DocID027540 Rev 3

Figure 11. PWM mode output voltage

During PWM mode the high-side (e.g. Figure 10 HS1) output is switched off with a slow slew

rate until it is off and the low-side body-diode has taken over the entire current (passive

freewheeling). Then the associated low-side transistor (e.g. Figure 10 LS1) is turned on with

a fast slope to reduce the voltage across the device and to minimize the power.

The output is pulled to high voltage, by first turning off the low-side driver with a fast slew

rate and, after it is off, the high-side driver is switched on by a slow one (e.g. Figure 10 LS1,

HS1).

This assures, that the voltage and current change over the body diode is done smoothly,

reducing the electromagnetic emission.

4.1.2

DIR-change mode

The first part of the sequence is identical to the PWM-mode (s.a.). After this has been

finished and the associated low-side driver is on (e.g. Figure 12 LS1), in phase 1 the other

low-side driver is turned on (e.g Figure 12 LS2) to enter passive freewheeling phase. Then

in phase 2 the low-side output of OUT2 is switched-off slowly and the current through the

load is taken over by the body-diode of the high-side (e.g. Figure 13 HS2).

Depending on the inductance of the load, the current vanishes more or less quickly. After

the low-side driver is turned off, the high-side is switched on with a slow slew-rate.

This assures, that direction switch occurs while the current over the load has vanished,

which reduces the electromagnetic emission.

Figure 12. DIR-change (current is changing its direction)

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