SC2616 datasheet(12/17 Pages) SEMTECH

12 / 17 page

 2003 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2616

the interface chipset (Southbridge, etc.) and the maximum

voltage applied to these pins do not exceed the chipsets

specifications. A separate lower pullup supply may be

necessary to avoid damage to the chipset.

“Back Feeding” the Input Supply

When in S3 state, VDDQ is supplied by the linear regulator

and current can flow back from the VDDQ supply through

the body diode of the Top switching MOSFET to the 5V

supply of the Silver Box, which is off during the S3 state.

This in turn shorts out the VDDQ supply and is not

desirable.

There are two approaches to avoiding this reverse current

flow. One method is to place a MOSFET in series with

the top switching MOSFET, but with the source and drain

reversed. (see typical application circuit). The MOSFETs

should be connected with sources connected to each

other, to prevent Gate Source (Vgs) break-down in the

even the inductor current flows in the negative direction,

which subsequenstly can give rise to the switching (Phase)

node voltage flying up to voltages higher than

VGS_Breakdown.

The connection of the MOSFETs in this manner places

the body diodes back to back, thus removing a current

path from VDDQ supply back into the input power source.

Another way is to use the Instantly available ACPI

controllers (such as Semtech SC1549). Such controllers

serve to provide a 5V bus to the user, irrespective of the

Status of S3 and S5 signals. Thus the 5V supply and the

5V Standby are multiplexed to provide an “always On”

5V to the VDDQ supply. Since the 5V supply is always

greater than VDDQ, the back to back MOSFET connection

is no longer necessary.

Current Limit

Current limit is implemented by sensing the VDDQ voltage

If it falls to 60% off its nominal voltage, as sensed by the

FB pin, the TG and BG pins are latched off and the

switcher and the linear converters are shut down. To

recover from the current limit condition, either the power

rails, 5VCC or 12VCC have to be recycled, or the SS/EN

pin must be pulled low and released to restart switcher

operation.

Applications Information (Cont.)

Thermal Shutdown

There are three independent Thermal Shutdown

protection circuits in the SC2616: the VDDQ linear

regulator, the VTT source regulator, and the VTT sink

regulator. If any of the three regulators’ temperature

rises above the threshold, that regulator will turn off

independently, until the temperature falls below the

thermal shutdown limit.

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