SC2616 Semtech, SC2616 Datasheet - Page 12

• Manufacturer Part Number: SC2616
• Description: Complete DDR Power Solution
• Manufacturer: Semtech

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.
POWER MANAGEMENT
Applications Information (Cont.)
2003 Semtech Corp.
12
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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SC2616