L6229Q

Circuit description

Doc ID 15209 Rev 3

13/28

5.3

PWM current control

The L6229Q includes a constant off time PWM current controller. The current control circuit

senses the bridge current by sensing the voltage drop across an external sense resistor

connected between the source of the three lower power MOS transistors and ground, as

shown in Figure 9. As the current in the motor increases the voltage across the sense

resistor increases proportionally. When the voltage drop across the sense resistor becomes

greater than the voltage at the reference input pin VREF the sense comparator triggers the

monostable switching the bridge off. The power MOS remain off for the time set by the

monostable and the motor current recirculates around the upper half of the bridge in slow

decay mode as described in the next section. When the monostable times out, the bridge

will again turn on. Since the internal dead time, used to prevent cross conduction in the

monostable time plus the dead time.

Figure 10 shows the typical operating waveforms of the output current, the voltage drop

across the sensing resistor, the pin RC voltage and the status of the bridge. More details

regarding the synchronous rectification and the output stage configuration are included in

the next section.

Immediately after the power MOS turn on, a high peak current flows through the sense

resistor due to the reverse recovery of the freewheeling diodes. The L6229Q provides a 1 µs

prematurely re trigger the monostable.

Figure 9.

PWM current controller simplified schematic

DRIVERS

+

DEAD TIME

S

Q

R

DRIVERS

+

DEAD TIME

DRIVERS

+

DEAD TIME

OUT3

OUT2

SENSEB

SENSEA

RSENSE

D02IN1380

RCOFF

VREF

OUT1

+

+

-

-

1

μs

5mA

BLANKER

SENSE

COMPARATOR

MONOSTABLE

SET

2.5V

5V

FROM THE

LOW-SIDE

GATE DRIVERS

BLANKING TIME

MONOSTABLE

VS

TO GATE

LOGIC

(0)

(1)