SP6120 datasheet(19/22 Pages) SIPEX

19 / 22 page

Date: 1/21/05

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

In some applications, it may be desirable to

extend the current sense capability of a given

RSEN element (usually the inductor ESR) be-

yond the limit set by the 43mV threshold.

A straight forward way to do this would be to

add a resistor RS2 in parallel with CS, creating a

voltage divider with RS. This changes the rela-

tionship with RS and CS to be

S = 2

ESR R

S //RS2

Using a voltage divider across the inductor, the

new relationship becomes:

LIM =

43mV R

S + RS2

ESR

To calculate RS2, the formula becomes

S2 = RS

LIMESR – 1

)

43mV

VOUT

SWN

RS2

ESR

ISP

ISN

Figure 20: Current Sensing

Figures 21(A), a voltage divider connected to the V

FB pin programs the output voltageand 21(B), a simple circuit using

one external voltage reference programs the output voltages to less than 1.25V.

VREF

VFB

VOUT < 1.25V

VOUT > 1.25V

SP6120

Output Voltage Programming

As shown in Figure 21(A), the voltage divider

connecting to the VFB pin programs the output

voltage according to

OUT = 1.25( 1 +

1 )

where 1.25V is the internal reference voltage.

Select R2 in the range of 10k to 100k, and R1 can

be calculated using

1 =

2(VOUT – 1.25)

1.25

For output voltage less than 1.25V, a simple

circuit shown in Figure 21(B) can be used in

which VREF is an external voltage reference

greater than 1.25V. For simplicity, use the same

resistor value for R1 and R2, then R3 is deter-

mined as follows,

3 =

REF – 1.25)R1

2.5 – V

OUT

APPLICATIONS INFORMATION: Continued

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