SC1406GCTSTR datasheet(13/28 Pages) SEMTECH

13 / 28 page

ã 2000 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1406G

CS; V(RCS) = IOUT * RCS.

In order to keep the voltage between CMP and

CMPREF = 0V, V(R

CS) appears across RCORE.

·V(R

CS) + V(RCORE) = 0, so V(RCORE) = -V(RCS); therefore, a

current flows through R

CORE from VOUT to CMPREF.

I(R

CORE) = IOUT * RCS / RCORE

An equal, but opposite current must flow in R

DAC, so the

voltage at CMPREF is reduced by I(R

CORE) l RDAC, so

CMPREF = VDAC - I(RCORE) * RDAC

Substituting the equations from above: V

CMPREF = VDAC

OUT * RCS * RDAC / RCORE

Since V

OUT = VCMPREF V(RCS), with a little algebra:

·V

OUT = VDAC - IOUT * RCS (1 + RDAC / RCORE)

So, with the SC1406G, you get an accurate, linear droop greater

than the drop across the current sense resistor, without the

efficiency penalty of a large R

CS, with a gain that is set by 1%

resistors! Adding R

OFFSET shifts the position at zero current, as

described later.

For further information on IMVP, consult the Intel yellow-cover

document Intel® Mobile Voltage Positioning Voltage Regulation

Controller Application Note, Reference Number OR-2101.

The SC1405, a smart MOSFET driver, provides industry-leading

performance, driving a 3000pF load in under 15nS, typically.

Not only does the SC1405 offer built-in shoot-through protec-

tion, but also has externally programmable dead time. In

addition, it provides other protection and performance features,

such as under-voltage lock-out (UVLO), programmable adaptive

over-voltage protection (OVP), and the SMOD pin, which may be

used to force the low-side gate drive LO during very light load

conditions to prevent negative circulating current in the inductor.

It comes in the small TSSOP-14 package.

DESIGN PROCEDURE:

Requirements:

The first step in designing any converter is in defining the

requirements, which can come from many sources. For the

SC1406G, you need to determine the minimum and maximum

input voltages, which are determined by the battery and AC

adapter characteristics, unless you plan to use an existing

regulated voltage, such as +5V. The processor determines other

requirements; they are:

Maximum output voltage

Minimum output voltage

Maximum output current

Minimum output current

Maximum transient current

Transient voltage requirements

One decision you need to make is whether a positive offset

voltage at zero current is desirable. Providing this offset results

in having a larger transient response band to work with, thereby

providing a solution with the fewest output capacitors. On the

other hand, this also means that at low currents, the processor

will burn more power than without the offset, since CV2F still

applies, so battery life will be reduced.

The numbers in the sample calculations are taken from the Intel

Mobile Pentium III Processor in

BGA2 and Micro-PGA2 Packages Datasheet, Revision 1.0,

Document Number: 245302-002. Please consult the data for

your specific processor.

Hysteretic Converter Design Equations:

The Typical Application Schematic on Page 12 is a schematic of

the sample converter. Note that several of the resistors have

annotations along with reference designators and values.

These resistors set the basic functions and IMVP.

Referring to Figure 1, the basic equations for a hysteretic

converter are:

where,

IIn equation 2), d is commonly referred to as the duty cycle.

The output voltage of the SC1406G is set digitally by the VID

(0:4) inputs, producing a voltage at the DAC output (pin 21)

accurate to better than 0.85%. The DAC voltage requirements

are given in the referenced Intel document and the SC1406G

datasheet. A similarly accurate fixed voltage internal bandgap

reference, Vref, has a nominal value of 1.70V, and is used for

setting voltage hysteresis and current limiting levels. In addition,

these references are used to provide active voltage positioning

adjusting the output voltage as a function of load current

scaled by external resistors.

The output voltage of an IMVP converter has three

components:

The programmed DAC voltage, V

DAC

The load dependent droop V

IMVP

An optional positive offset, V

OFFSET

In equation form,

The full equations for the IMVP converter are:

VHYS

VIN

VOUT

−

() ESR R

()

⋅

FS L

⋅

TON

TOFF

()

VOUT

VIN

VOUT

VDAC

VIMVP

−

VOFFSET

VOUT

VDAC ROFFSET

ROH

()

⋅

RCORE

⋅

IOUT RCS

⋅

ROFFSET

⋅

RCORE

RDAC

()

⋅

−

RCORE ROFFSET

⋅

RDAC ROH

⋅

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