1Date 2004

Strictly Confidential

Remote Sense(extracted from a presentation by

Robert Kollman)

2Date 2004

Strictly Confidential

Why Remote Sense?

PowerSupply 3.3V

SystemLoad

100A

3.1V

• Interconnect resistance has many components:– Connector Pins - Etch Resistance– Wiring - Vias

3Date 2004

Strictly Confidential

Calculating ResistanceMaterial -cm -in

Copper 1.70 0.67

Copper (Plated)

6.0 2.36

Gold 2.2 0.87

Lead 22.0 8.66

Silver 1.5 0.59

Silver (Plated) 1.8 0.71

Tin -Lead 15 5.91

Tin (Plated) 11 4.33

Palladium 11 4.3

A

Cur

rent

Flo

w

A

lR

yresistivit

4Date 2004

Strictly Confidential

Counting Squares for Resistance• Copper resistivity is 0.67 mW in. at 25°C and doubles

for 254°C rise

Copper Weight(Oz.)

Thickness(mm/mils)

m per Square

(25oC)

m per Square(100oC)

1/2 0.02/0.7 1.0 1.3

1 0.04/1.4 0.5 0.65

2 0.07/2.8 0.2 0.26

tR

tR

wtR

)(

)(

)(

)(

t

Current Flow

5Date 2004

Strictly Confidential

Don’t forget vias

A

lR

)( 22io rr

lR

mR 4.2)009.001.0(

06.01036.222

6

5 mm(20 mils)

4.5 mm(18 mils)

1.5 mm(60 mils)

CurrentFlow

A

• Rule of thumb limits via current to 1 amp each.

6Date 2004

Strictly Confidential

Differential Amplifier• Used to sense remote

voltage from power supply control ground

• Voltage at op amp inputs must be within common mode range specified in datasheet.

• If R1=R2=RF=RA then VO = V2 - V1. Reduces temperature sensitivity if all resistors drift similarly.

VCMRR

RVVnVp

RA

RFVVVO

21

22

)12(

Set R1 = RA R2 = RF

7Date 2004

Strictly Confidential

Differential Amplifier Errors

• Resistor Tolerance Impact Can Be Calculated Per Previous Chart

• Input Offset is Multiplied by 1 + RF / RA

8Date 2004

Strictly Confidential

Highest Accuracy if R6 is Open

Open R6 Unity Gain Diff Amp(1 + (1 – Ratio) Tol )2 (1 + Tol)2 * (1 + (1 – Ratio) Tol ) 1 + T – T*R 1 + 2T + T2

- TR < 2T + T2

9Date 2004

Strictly Confidential

Remote Sense Faults

Fault Effect Remedy

Open Sense OV RP/RN

Reverse Sense OV Local OV Shutdown

Open Load Lead Vout Across RP/RN RP/RN Power Rating

-+

10Date 2004

Strictly Confidential

Alternate Sense Connections

• Float power supply control with respect to local ground– Requires isolation

between controller and power stage

• Sense only positive output– Assumes

RLINE_P>>RLINE_N

-

+

-

+

11Date 2004

Strictly Confidential

Trimming the Output Voltage of a Module

• Desired output voltage is higher/lower than the setpoint or better accuracy is required

• Circuit provides 0.2% initial accuracy (not including resistor divider tolerance)

• Remote sense is included with this circuit – Control circuit is referenced

to load ground– Requires 3.3V supply for

control circuit