SC2441AEVB datasheet(32/37 Pages) SEMTECH

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 2006 Semtech Corp.

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POWER MANAGEMENT

SC2441A

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

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.

The input bias current of the error amplifier also causes

an error in the output voltage. The inverting input bias

currents of error amplifiers 1 and 2 are –60nA and

–280nA respectively. Since the non-inverting input is

biased to 0.5V, the percentage error in the second output

voltage will be –100% · (0.28

µA) · R

/[0.5 ·

)]. To keep this error below 0.2%, R

< 4k

Ω.

Loop Compensation in Step-down Section

The SC2441A 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.

Applications Information

Using low gate charge MOSFETs reduces switching loss.

It is possible to trade driver IC losses for MOSFET

switching losses by adjusting the gate resistance. Lower

gate resistance results in higher gate driving current and

faster MOSFET switching. However the driver incurs

higher losses. Conversely higher gate drive resistance

limits the gate drive current, thus lowering the driver

dissipation. MOSFET switching loss is higher.

To prevent shoot-through between the top and the

bottom MOSFETs during commutation, one MOSFET

should be completely turned off before the other is turned

on. The SC2441A uses adaptive non-overlapping timing

to prevent shoot-through.

Optimize MOSFET Driving Voltage

The on-state DC resistance of a MOSFET, R

DS_ON, is

determined by its gate to source voltage. The higher the

GS, the lower the RDS_ON will be. Once the gate-source

voltage exceeds a certain level, the R

DS_ON becomes

relatively constant. There is no benefit except higher

dissipation if you further increase the MOSFET gate drive

voltage. It is recommended to select gate drive voltage

CC pin) of the SC2441A in between 5V to 7V.

Setting the Output Voltage of the Step-down Section

The non-inverting inputs of the error amplifiers are

internally biased to 0.5V voltage reference. A simple

voltage divider (R

o1 at top and Ro2 at bottom) sets the

converter output voltage. R

o2 can be expressed as a

function of the voltage feedback gain h=0.5/V

o and Ro1

caused by the feedback voltage divider ratio. It cannot

be corrected by the feedback loop.

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