current delivered from the first section. An op-amp compares the voltage drop through Rs which is pro-

portional to the current delivered from the U2 section with the voltage drop across Rs proportional to the

current delivered from the U1 section. The Cin and Rin components introduce a pole and a zero in the

current loop which allows integration of the error signal. The current loop regulates I

result the output current delivered to the load is Iout = 2I- = 2I+ for every load condition.

Current Sharing Accuracy

The accuracy of the current sharing between the two sections depends on the op-amp offset voltage,

Voff, and the value of Rs and its accuracy . The offset voltage introduce an error in the sensing voltage ,

Vs=Rs Iout/2 . The relative percentage current error due to the offset is given by :

e%= (

∆ I/I) ⋅ 100 = (Voffset ⋅ 100) / (Rs ⋅ Iout)

This error is minimum at maximum load. The larger the value of Rs, the smaller the error. Rs must be

chosen as a compromise between error minimization and system efficiency.

For example with Iout = 7A choosing Rs = 25m

Ω ,considering a maximum offset voltage of 3mV

(LM358A), the maximum relative percentage error is 1.7% (120mA @ Iout = 7A).

The total error is given by the sum of this error plus the error due to the sensing resistor ( which corre-

sponds to its accuracy of 1% ). So the maximum error is 2.7% (190mA @Iout = 7A)

Layout Hints

The PCB layout requires some care. The power paths of the two sections must be as short and sym-

metrical as possible. The current sensing wires must be parallel and short to avoid induced noises. The

sensing resistor must be non inductive. The ground pins of the two devices must be at the same voltage

and connected to the output ground point.

Syncronization or Multiphase

In a current sharing application the two sections can be synchronized. This permits a reduction of noise

induced from one section to another. In this case a single RC network can be used for both the oscilla-

tors and the two SYNC pins are connected.

In many application, instead of synchronizing the two oscillator, it is useful to introduce a delay between

the two PWM signals in order to achieve a multiphase application. The phase shift between the two

PWM signals can be easily achieved by two methods :

I +

L

OUT

U1

L497x

COMP

GND

COMP

OUT

to the

current

FB

U2

L497x

Cout

Vout

Rs

Rs

L

I -

FB

Vcc

Vcc

GND

GND

Vcc

GND

to the

current

FB

Figure 2. Layout hints.

AN1126 APPLICATION NOTE

2/16