SC1406GCTSTR datasheet(12/28 Pages) SEMTECH

12 / 28 page

ã 2000 Semtech Corp.

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POWER MANAGEMENT

SC1406G

Figure 1 - Hysteretic Converter Basics

The major advantages of this approach are simplicity, inherent

stability (there are no reactive elements in the control circuit to

provide the phase shift required for classical stability problems),

and the fastest possible transient response. Any transient

which takes the voltage out of the hysteretic range forces the

converter immediately into the proper response. There are no

error voltages to slew, and no maximum or minimum duty cycle

limits to slow the transient response as in most other control

schemes. A significant benefit of the controller/driver architec-

ture is that low-level analog control functions do not have to

coexist in an environment of thousands of volts and amps per

microsecond, reducing noise problems.

The SC1406G also supports the Intel Mobile Voltage Positioning

(IMVP) functions. In short, IMVP allows notebook designers to

reduce the output voltage with increasing load current. This has

two potential benefits:

IMVP minimizes power by reducing the voltage under

heavy loads; processor power is proportional to V2, so

a 5% reduction in voltage results in a nearly 10%

reduction in power drawn by the processor.

IMVP can reduce the number of capacitors required to

respond to transients by producing a larger allowable

transient; briefly, the no-load voltage is positioned

above nominal, so when a transient occurs, the load

has farther to drop before hitting the regulation limit.

The loaded voltage is allowed to remain below the

nominal so that when the load returns to zero, the

voltage can rise farther without reaching the transient

specification. Since the allowable transient voltages

are larger, less capacitance and higher ESR can

provide the required performance.

The SC1406G provides the precise voltage positioning required

by IMVP because it employs a current-sense resistor, multiplied

by a gain set by external resistors to very accurately set the

voltage as a function of load current.

The SC1406G implements IMVP, previously named DSPS

(dynamic set-point switching) in the following manner: Please

see Figure 2 above, and assume, for simplicity:

·R

OFFSET is open

The current into the CMP and CMPREF pins is zero.

Then, please note:

The SC1406G regulates to the + side of the current

sense resistor, because that is where the CMP pin is

tied, and,

No current flows through R

OH, since the input current of

the comparator is ~0; V(R

OH) = 0.

The difference in voltage between CMP and CMPREF

is ~0V in order for the controller to be in regulation

In these conditions:

At zero load, V

OUT = VDAC and the voltage across the

current sense resistor is also zero;

As the load increases, a voltage is developed across

R23 (RCS)

DAC

(pin 21)

CMP

(pin 25)

CMPREF

(pin 26)

(ROH)

(RCORE)

R22

(ROFFSET)

R21

(RDAC)

VOUT

IOUT

Figure 2IMVP Example Illustration

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