L6207 datasheet(9/53 Pages) STMICROELECTRONICS

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AN1762 APPLICATION NOTE

Figure 8. Two situations that must be avoided.

3.5 Sensing Resistors

Each motor winding current is flowing through the corresponding sensing resistor, causing a voltage drop that

can be used, by the logic (integrated in the L6207; an external logic can be used with L6205 and L6206), to

control the peak value of the load current. Two issues must be taken into account when choosing the RSENSE

value:

– The sensing resistor dissipates energy and provides dangerous negative voltages on the

SENSE pin

during the current recirculation. For this reason the resistance of this component should be kept low.

– The voltage drop across RSENSE is compared with a reference voltage (on Vref pin) by the internal com-

parator (L6207 only). The lower is the RSENSE value, the higher is the peak current error due to noise

on Vref pin and to the input offset of the current sense comparator: too small values of RSENSE must be

avoided.

A good compromise is calculating the sensing resistor value so that the voltage drop, corresponding to the peak

current in the load (Ipeak), is about 0.5 V: RSENSE = 0.5 V / Ipeak.

It should be clear that sensing resistor must absolutely be non-inductive type in order to avoid dangerous neg-

ative spikes on

SENSE pins. Wire-wounded resistors cannot be used here, while Metallic film resistors are rec-

ommended for their high peak current capability and low inductance. For the same reason the connections

between the

SENSE pins, C6, C7, VSA, VSB and GND pins (see Figure 7) must be taken as short as possible

(see also the

Layout Considerations section).

The average power dissipated by the sensing resistor is:

Fast Decay Recirculation: PR ≈ Irms

2 · RSENSE

Slow Decay Recirculation: PR ≈ Irms

2 · RSENSE · D,

D is the duty-cycle of the PWM current control, Irms is the r.m.s. value of the load current.

GND

SENSEA

SENSEB

VSA

VSB

L6205, L6206, L6207

DON’T connect the Logic GND here

Voltage drop due to current in sense

path can disturb logic GND.

DON’T put a diode here!

Recirculating current cannot flow into the

bulk capacitor and causes a high voltage

spike that can destroy the IC.

VS = 8 ÷ 52 V

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