AN2836 datasheet(25/57 Pages) STMICROELECTRONICS

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AN2836

Designing an application with L6225, L6226, L6227

Doc ID 15084 Rev 1

25/57

Figure 22.

L6226 overcurrent detection simplified circuitry

This sensed current is compared to an internally-generated reference (adjustable through

the external resistors RCLA and RCLB for L6226) to detect an overcurrent 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 (2.8 A typical value for L6225 and L6227; adjustable

through the external resistors RCLA and RCLB for L6226). In L6225 and L6227 the open

drain is internally connected to the EN pins. With L6226 the OCD pins should be connected

to the EN pins to allow the protection to function. To ensure an overcurrent protection,

connect these pins to an external RC network (see Figure 21 and 22).

Figure 23 and 24 show the device operating in overcurrent condition (short to ground).

When an overcurrent is detected, the internal open-drain MOSFET pulls the EN pin to GND,

switching off all 4 power DMOS of the bridge and allowing the current to decay. Under a

persistent overcurrent condition, like a short to ground or a short between two output pins,

the external RC network on the EN pin (see Figure 21 and 22) reduces the RMS value of the

output current by imposing a fixed disable time after each overcurrent 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 (2.8 A typ. for L6225 and L6227, adjustable for L6226), 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 the EN pin

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

the current restarts. Even if the maximum output current is very high, the external RC

network provides a disable time (tDISABLE) to ensure a safe RMS value (see Figure 23 and

24).

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