SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

DCR Current Sensing

Either precision sense resistor or inductor DCR can be

used as the inductor current sensing element.

VIN

SW1

V

Voltage on C1 --- VC1(t)

VC1.PEAK

VC1.VALLEY

R1IL1.PEAK

R1IL1.VALLEY

R1IO

L1

R1

R2

SW2

+

CS

-

SC2441A

+VC1-

C1

ICS+ R3

ICS-

Voltage Drop on R1--- IL1(t)R1

0

t

COUT

RLOAD

Figure 13. Voltage Waveform C1 and R1

In Figure 12, R2 and R3 resistive divider attenuates the

sensed signals when I

. The time

constant resulting from L1 and its DCR R1 is:

Figure 12. Current Sensing Circuit.

Define REQU:

In Figure 12 SW1 and SW2 represent the MOSFET

switches. CS is the current sense amplifier. ICS+ and ICS-

are the input bias currents of the CS. L1 is the output

inductor. R1 is the DC resistance of L1. R2, R3 and C1

constitute the DCR current sensing network.

The time constant of the DCR sensing network is:

If τC1 = τL1 , then the peak and valley voltages across

C1will be:

Assuming that CS input bias currents are zero and that

R3 is not used, if the time constant L1/R1 is made equal

to the time constant R2C1, then the voltage across the

inductor DCR, R1, will be replicated across C1 in the steady

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

IL1 .PEAK = IO + DIL1 /2

IL1 .VALLEY = IO - DIL1 /2

VC1 (t) = IL1 (t) × R1

where, IO is the output current and DIL1 is the peak-to-

peak L1 current ripple. The inductor current can therefore

be sensed by monitoring C1 voltage. L should be selected

so that the

is between 25% to 33% of the IO.

VC1×VALLEY

=

ICS+

× REQU

+

R3 ×R1

(R2 + R3

)

×

çèæIO

-

DIL1

2

÷ö

ø

ICS+ therefore introduces an offset error to the sensed

voltage. To reduce this error, REQU must be minimized.

Suppose VIN=5V; VOUT=2.5V; D=50%; IOUT=20A;

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

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

L1 and R1 is

tL1

=

L1

R1

= 0.25ms

=

R2 × R3

R2 + R3

× C1

However CS input bias currents are not zero. ICS+ and ICS-

are typically 0.4µA and 40µA respectively (see electrical

characteristics) and can not be ignored.

DIL1 = 5A

VC1×PEAK

= ICS+

× REQU

+

R3 ×R1

(R2 + R3 )

× çæ 28A

è

+

DIL1

2

÷ö

ø

=

25mV

 2006 Semtech Corp.

24

www.semtech.com

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