AN1625 datasheet(3/39 Pages) STMICROELECTRONICS

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AN1625 APPLICATION NOTE

DESIGNING AN APPLICATION WITH L6235

2.1 Current Ratings

With MOSFET (DMOS) devices, unlike bipolar transistors, current under short circuit conditions is, at first ap-

proximation, limited by the RDS(ON) of the DMOS themselves and could reach very high values. L6235 Out pins

and the two VSA and VSB pins are rated for a maximum of 2.8 A r.m.s. and 5.6 A peak (typical values). These

values are meant to avoid damaging metal structures, including the metallization on the die and bond wires. In

practical applications, though, maximum allowable current is less than these values, due to power dissipation

limits (see Power Management section).

The device has a built-in Over Current Detection (OCD) that allows protection against short circuits between the

outputs and between an output and ground (see Over Current Detection Section).

2.2 Voltage Ratings and Operating Range

The L6235 requires a single supply voltage (VS), for the motor supply. Internal voltage regulators provide the

5V and 10 V required for the internal circuitry. The operating range for VS is 8 to 52 V. To prevent working into

undesirable low supply voltage an Under Voltage Lock Out (UVLO) circuit shuts down the device when supply

voltage falls below 6 V; to resume normal operating conditions, VS must then exceed 7 V. The hysteresis is pro-

vided to avoid false intervention of the UVLO function during fast VS ringings. It should be noted, however, that

DMOS's RDS(ON) is a function of the VS supply voltage. Actually, when VS is less than 10V, RDS(ON) is adversely

affected, and this is particularly true for the High Side DMOS that are driven from VBOOT supply. This supply is

obtained through a charge pump from the internal 10V supply, which will tend to reduce its output voltage when

VS goes below 10V. Figure 2 shows the supply voltage of the high side gate drivers (VBOOT - VS) versus the

supply voltage (VS).

Figure 2. High side gate drivers supply voltage versus supply voltage.

Note that VS must be connected to both VSA and VSB because the bootstrap voltage (at VBOOT pin) is the same

for the two H-bridges. The integrated DMOS have a rated Drain-Source breakdown voltage of 60 V. However

VS should be kept below 52 V, since in normal working conditions the DMOS see a Vds voltage that will exceed

VS supply. In particular when a high-side DMOS turns off due to a phase change (OUT1 in Figure 3), if one of

the other outputs (OUT2 in Figure 3) is high (during the off-time all active bridges turn their high-side on) the

load current starts flowing in the low-side freewheeling diode and the SENSE pin sees a negative spike due to

a not negligible parasitic inductance of the PCB path from the pin to GND. This spike is followed by a stable

negative voltage due to the drop on RSENSE. The output pin sees a similar behavior, but with a slightly larger

voltage due to the forward recovery time of the integrated freewheeling diode and the forward voltage drop

across it. Typical duration of this spike is 30 ns. At the same time, the OUT2 pin (in the example of Figure 3)

VS [V]

VBOOT - VS

[V]

6.4

6.8

7.2

7.6

8 .5

9 .5

10.5

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