SC2447 datasheet(22/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

Only the voltages in boldface can be precisely set with

standard 1% resistors.

From this table, one may also observe that when the

value

−

and its multiples fall into the standard resistor value

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

resistors to set up the exact and 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 current of the error amplifiers is -

380nA. Assuming the non-inverting input is tied to the

0.5V reference output, the percentage error in the

second output voltage will be –100% · (0.38µA)

/[0.5 · (R

) ]. To keep this error below

0.2%, R

< 2.6k

Ω.

Loop Compensation

SC2447 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 network are 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 SC2447 the

compensation ramp is approximately

ramp=D*0.4V.

The slope of the compensation ramp is then

e=0.4*fs.

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 15.

Figure 15. A Simple Model of Synchronous Buck

Converter with Current Mode Control

The transconductance error amplifier (in the SC2447)

has a gain g

m of 170µA/V. The target of the compensation

design is to select the compensation network consisting

of C

2, C3 and R2, along with the feedback resistors Ro1,

o2 and the current sensing gain, such that the converter

output voltage is regulated with satisfactory dynamic

performance.

Application Information (Cont.)

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