SC2446 datasheet(22/38 Pages) SEMTECH

22 / 38 page

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POWER MANAGEMENT

SC2446

Application Information (Cont.)

Once either R

o1 or Ro2 is chosen, the other can be

calculated for the desired output voltage V

o. Since the

number of standard resistance values is limited, the

calculated resistance may not be available as a standard

value resistor. As a result, there will be a set error in the

converter output voltage. This non-random error is

caused by the feedback voltage divider ratio. It cannot

be corrected by the feedback loop.

The following table lists a few standard resistor

combinations for realizing some commonly used output

voltages.

)

(

)

(

)

(

)

(

Only the voltages in boldface can be precisely set with

standard 1% resistors.

From this table, one may also observe that when the

value

o −

−

and its multiples fall into the standard resistor value

chart (1%, 5% or so), it is possible to use standard value

resistors to exactly set up the required output voltage

value.

The input bias current of the error amplifier also causes

an error in setting the output voltage. The maximum

inverting input bias currents of error amplifiers 1 and 2

is -250nA. Since the non-inverting input is biased to 0.5V,

the percentage error in the second output voltage will be

–100% · (0.25

µA) · R

/[0.5 · (R

) ]. To keep

this error below 0.2%, R

< 4k

Ω.

Loop Compensation

SC2446 uses current-mode control for both step-down

channels. Current-mode control is a dual-loop control

system in which the inductor peak current is loosely

controlled by the inner current-loop. The higher gain outer

loop regulates the output voltage. Since the current loop

makes the inductor appear as a current source, the

complex high-Q poles of the output LC networks is split

into a dominant pole determined by the output capacitor

and the load resistance and a high frequency pole. This

pole-splitting property of current-mode control greatly

simplifies loop compensation.

The inner current-loop is unstable (sub-harmonic

oscillation) unless the inductor current up-slope is steeper

than the inductor current down-slope. For stable

operation above 50% duty-cycle, a compensation ramp

is added to the sensed-current. In the SC2446 the

compensation ramp is made duty-ratio dependent. The

compensation ramp is approximately

ramp

The slope of the compensation ramp is then

)

(

The slope of the internal compensation ramp is well above

the minimal slope requirement for current loop stability

and is sufficient for all the applications.

With the inner current loop stable, the output voltage is

then regulated with the outer voltage feedback loop. A

simplified equivalent circuit model of the synchronous

Buck converter with current mode control is shown in

Figure 17.

Figure 17. A simple model of synchronous buck converter

with current mode control.

SC2446 Datasheet(PDF) 22 Page - Semtech Corporation

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