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AN1451 APPLICATION NOTE

DESIGNING AN APPLICATION WITH L6208

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. L6208 Out pins

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

sponding to a total (for the whole IC) 5.6A rms (11.2A peak). These values are meant to avoid damaging metal

structures, including the metallization on the die and bond wires. In practical applications, though, maximum al-

lowable 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 provides protection against short circuits between

the outputs and between an output and ground (see

Over Current Protection section).

2.2 Voltage Ratings and Operating Range

The L6208 requires a single supply voltage (VS), for the motor supply. Internal voltage regulators provide the

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

undesirable low supply voltage an

Under Voltage Lock Out (UVLO) circuit shuts down the device when supply

voltage falls below 6V; to resume normal operating conditions, VS must then exceed 7V. 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 since 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 60V. However VS

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

supply. In particular, when using the

fast decay mode, at the beginning of the off-time (when all the DMOS are

off during dead-time) 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.

One of the two

OUT pins of the bridge sees a similar behavior, but with a slightly larger voltage due to the for-

ward recovery time of the integrated freewheeling diode and the forward voltage drop across it (see Figure 3).

Typical duration of this spike is 30ns. At the same time, the other

OUT pin of the same bridge sees a voltage

VS [V]

VBOOT - VS

[V]

6.4

6.8

7.2

7.6

8 .5

9 .5

10.5

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