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AN1625 APPLICATION NOTE

Figure 6. Two situations that must be avoided.

2.5 Sensing Resistor

Motor winding current flows through the sensing resistor, causing a voltage drop that is used, by the logic, to

value:

s

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

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

s

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

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

(see also the Layout Considerations section).

The average power dissipated by the sensing resistor is:

;

Nevertheless, sensing resistor power rating should be chosen taking into account the peak value of the dissi-

pated power:

,

Using multiple resistors in parallel will help obtaining the required power rating with standard resistors, and re-

GND

GND

GND

GND

R5

C7

C6

L6235

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.

+

-

P

R

I

rms

2

R

SEN SE

D

⋅⋅

≈

P

R

I

pk

2

R

SEN SE

⋅

≈