SC2441AEVB datasheet(24/37 Pages) SEMTECH

24 / 37 page

 2006 Semtech Corp.

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POWER MANAGEMENT

SC2441A

DCR Current Sensing

Either precision sense resistor or inductor DCR can be

used as the inductor current sensing element.

Figure 12. Current Sensing Circuit.

In Figure 12 SW1 and SW2 represent the MOSFET

switches. CS is the current sense amplifier. I

CS+ and ICS-

are the input bias currents of the CS. L

1 is the output

inductor. R

1 is the DC resistance of L1. R2, R3 and C1

constitute the DCR current sensing network.

Assuming that CS input bias currents are zero and that

3 is not used, if the time constant L1/R1 is made equal

to the time constant R

2C1, then the voltage across the

inductor DCR, R

1, will be replicated across C1 in the steady

state (see Figure 13). The following equations apply:

PEAK

VALLEY

(t)

where, I

O is the output current and

D is the peak-to-

peak L

1 current ripple. The inductor current can therefore

be sensed by monitoring C

1 voltage. L should be selected

so that the

is between 25% to 33% of the I

However CS input bias currents are not zero. I

CS+ and ICS-

are typically 0.4

µA and 40µA respectively (see electrical

characteristics) and can not be ignored.

Figure 13. Voltage Waveform C

1 and R1

In Figure 12, R

2 and R3 resistive divider attenuates the

sensed signals when I

. The time

constant resulting from L

1 and its DCR R1 is:

Define R

EQU:

The time constant of the DCR sensing network is:

C1 = τL1 , then the peak and valley voltages across

1will be:

= +

EQU

VALLEY

)

(

CS+ therefore introduces an offset error to the sensed

voltage. To reduce this error, R

EQU must be minimized.

Suppose V

IN=5V;

OUT=2.5V;

D=50%; I

OUT=20A;

SW=500KHz; L1=0.5µH; R1=2mΩ; ICS+=1µA. The output

current limit is set at 28A. The time constant formed by

1 and R1 is

0.25ms

25mV

28A

= +

EQU

PEAK

)

(

Voltage Drop on R

1--- IL1(t)R1

Voltage on C

1 --- VC1(t)

C1.PEAK

C1.VALLEY

1IL1.PEAK

1IL1.VALLEY

1IO

Applications Information

SC2441A

SW1

SW2

C1-

OUT

LOAD

CS+

CS-

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