AN1625 APPLICATION NOTE

12/39

2.11 Programmable off-time Monostable

The L6235 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 12. 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 mo-

tor 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

the monostable time plus the dead time.

Figure 13 shows the typical operating waveforms of the output current, the voltage drop across the sensing re-

sistor, 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 re-

comparator output so that the current spike cannot prematurely retrigger the monostable.

Figure 12. 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)