AN2839 datasheet(9/40 Pages) STMICROELECTRONICS

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AN2839

Designing an application with L6228

Doc ID 15104 Rev 1

9/40

Figure 6.

Two situations that must be avoided

1.5

Sensing resistors

Each motor winding current flows through the corresponding sensing resistor, causing a

voltage drop that is used, by the logic, to control the peak value of the load current. Two

important points 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 the reference voltage (on the Vref

pin) by the internal comparator. The lower the RSENSE value, the higher the peak

current error due to noise on the Vref pin and to the input offset of the current sense

comparator: RSENSE values that are too small must be avoided.

A good compromise is to calculate 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.

The sensing resistor must be of a non-inductive type in order to avoid dangerous negative

spikes on the SENSE pins. Wire-wound resistors cannot be used here, while metallic film

resistors are recommended 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 5 on page 8) must be as short as possible (also see Section 1.4).

The average power dissipated by the sensing resistor is:

●

Fast decay recirculation: PR ≈ Irms

2 · R

SENSE

●

Slow decay recirculation: PR ≈ Irms

2 · R

SENSE · D

where D is the duty-cycle of the PWM current control and Irms is the RMS value of the load

current. Nevertheless, the peak value of the dissipated power should be taken into account

when choosing the power rating of the sensing resistor.

Equation 8

SENSE

⋅

≈

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