SC2434SWTR datasheet(8/19 Pages) SEMTECH

8 / 19 page

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PRELIMINARY

POWER MANAGEMENT

SC2434

Applications Information (Cont.)

Program The Controller

Please refer to Fig. 1 and the application schematics in

this data sheet for the discussion. The resistor from pin

10 to ground, R

OSC, programs the switching frequency. The

resistor from pin 11 to ground, R

DAC, sets the DAC current

step size. The resistors, R

FB, ROS , and RDRP set the DAC

step size, the output voltage set point, and the droop,

respectively.

MathCAD programs are available to calculate the required

parameters upon request.

Programming The Switching Frequency

The oscillator frequency can be selected first by setting the

value of R

OSC as given below:

R OSC

28.5 K

Ω

750 KHz

F osc

I DAC_LSB

V bg

R DAC

R FB

VID step

I DAC_LSB

The per phase switching frequency is 1/3 of the oscilla-

tor frequency in three-phase mode. It is recommended

that per phase switching frequency is 200~300KHz for

good trade off of efficiency vs. transient responses.

Programming The DAC Step Size

The SC2434 allows programming of the output voltage

and the DAC step size by selecting external resistors. The

LSB of the DAC current is given by:

where V

bg is the trimmed voltage reference (Vbg = 1.5V)

and R

DAC is the resistor from pin 11 to ground.

For the

given VID step size (25mV for VRM9.0 and VRM9.2 speci-

fications), the feedback resistor can be calculated accord-

ing to the LSB of DAC current:

The above two equations are for choosing R

DAC and RFB

simultaneously. The advantage of this method is that new

VID step size can be accommodated by modifying external

components while maintaining the required precision.

Choose Current Sensing Resistor According To The

Threshold Of OCP

The SC2434 controller has an over current protection (OCP)

threshold of 120mV. The normal practice is to let the

peak voltage across the sensing resistor corresponding to

full-load operation be 75% of the given OCP threshold:

R drp

R FB R sense

G ca

∆V

out

∆I

out

N phase

where I

peak is the peak current of the output inductor. Since

the choice of sensing resistor values are limited, typically 3

mOhm, 4 mOhm, or 5 mOhm, it is recommended to choose

the sensing resistor with a bigger value than that was cal-

culated, and to use a resistive divider to get the equiva-

lent R

sense value.

The two attenuation resistors should

have value of 20 Ohm in parallel. A filter capacitor of

10nF is also needed to be across the OC+ and OC- pins of

the controller IC. Please refer the application circuit sche-

matic.

Programming The Dynamic (Active) Droop

To optimize transient responses, the SC2434 actively regu-

lates output voltage as a function of output current. At

zero current the output is positioned to the upper limit of

the regulation window. As the load increases, the output

“droops” towards the lower limit. This makes optimum

use of the output voltage error band, yielding minimum

output capacitor size and cost.

The droop is adjusted by setting the DC gain of the error

amplifier. This is done by choosing the resistor from the

ERROUT pin to the FB pin (R

DRP) of the controller.

While

the optimum value of R

DRP may be derived experimentally,

the following equation can provide the first order calcula-

tion for given droop slope:

where R

sense is the current sensing resistance after taken

into account of attenuation, and G

ca is the gain of the

current amplifier while N

phase is number of phases being

used.

Any output interconnection impedance not within the feed-

back loop can contribute to additional drooping. This ef-

fect has to be taken into account. Usually, when testing

the regulation at different CPU pins, the results may vary

slightly by same token.

It is important to use surface mount current sensing resis-

tor to minimize the parasitic inductance for accurate cor-

relation between the above equation and the test results.

This is because the inductive contribution, which may also

R sense

75% "120mV

I peak

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