L6207 datasheet(19/53 Pages) STMICROELECTRONICS

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AN1762 APPLICATION NOTE

This sensed current is compared to an internally generated reference (adjustable through the external resistors

RCLA and RCLB for L6206) to detect an over current condition. An internal open drain mosfet turns on when the

sum of the currents in the bridges 1A and 2A or 1B and 2B reaches the threshold (5.6A typical value for L6205

and L6207; adjustable through the external resistors

RCLA and RCLB for L6206); in L6205 and L6207 the open

drain are internally connected to the

EN pins; with L6206 OCD pins should be connected to EN pins to allow the

protection working. To ensure an over current protection, connect these pins to an external RC network (see

Figure 19).

Figure 20 shows the device operating in overcurrent condition (short to ground). When an over current is de-

tected the internal open drain mosfet pull the

EN pin to GND switching off all 4 power DMOS of the interested

bridge and allowing the current to decay. Under a persistent over current condition, like a short to ground or a

short between two output pins, the external RC network on the

EN pin (see Figure 19) reduces the r.m.s. value

of the output current by imposing a fixed disable-time after each over current occurrence. The values of

REN

and

CEN are selected to ensure proper operation of the device under a short circuit condition. When the current

flowing through the high side DMOS reaches the OCD threshold (5.6 A typ. for L6205 and L6207, adjustable for

L6206), after an internal propagation delay (tOCD(ON)) the open drain starts discharging CEN. When the EN pin

voltage falls below the turn-off threshold (VTH(OFF)) all the Power DMOS turn off after the internal propagation

delay (tD(OFF)EN). The current begins to decay as it circulates through the freewheeling diodes. Since the DMOS

are

off, there is no current flowing through them and no current to sense so the OCD circuit, after a short delay

(tOCD(OFF)), switches the internal open drain device off, and REN can charge CEN. When the voltage at EN pin

reaches the turn-on threshold (VTH(ON)), after the tD(ON)EN delay, the DMOS turn on and the current restarts.

Even if the maximum output current can be very high, the external RC network provides a disable time (tDISABLE)

to ensure a safe r.m.s. value (see Figure 20).

Figure 20. Over Current Operation.

The maximum value reached by the current depends on its slew-rate, so on the short circuit nature and supply

voltage, and on the total intervention delay (tDELAY). It can be noticed that after the first current peak, the max-

imum value reached by the output current becomes lower, because the capacitor on

EN pins is discharged start-

ing from a lower voltage, resulting in a shorter tDELAY.

The following approximate relations estimate the disable time and the first OCD intervention delay after the short

circuit (worst case).

Output Current

VTH(OFF)

tD(OFF)EN

ISOVER

tOCD(ON)

tDISABLE

tOCD(OFF)

tDELAY

tEN(FALL)

tEN(RISE)

VEN(LOW)

Output Current

tDISABLE

VTH(ON)

tD(ON)EN

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