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FAN1655MTF-2004 View Datasheet(PDF) - Fairchild Semiconductor

Part NameDescriptionManufacturer
FAN1655MTF(2004) 3A DDR Bus Termination Regulator Fairchild
Fairchild Semiconductor Fairchild
FAN1655MTF Datasheet PDF : 9 Pages
1 2 3 4 5 6 7 8 9
Applications Information
Output Capacitor selection
The JEDEC specification for DDR termination requires that
VTT stay within ±40mV of VREF, which must track
VDDQ/2 within 1%. During the initial load transient, the
output capacitor keeps the output within spec. To stay within
the 40mV window, the “load step” due to the load transient
current dropping across the output capacitor’s ESR should be
kept to around 25mV: where ESR < -2---5-I is given in m, and
I is the maximum load current.
For example, to handle a 3A maximum load transient, the
ESR should be no greater than 8m. Furthermore, the output
capacitor must be able to hold the load in spec while the
regulator recovers (about 15µS). A minimum value of 470µF
is recommended.
These requirements can be achieved by a combination of
capacitors. FAN1655 requires a minimum of 5mof ESR
in the output and is not stable with all-ceramic output
Power Dissipation and Derating
The maximum output current (sink or source) for a 1.25V
output is:
where PD(MAX) is the maximum power dissipation which is:
PD(MAX) = T----J---(-M-----Aθ----XJ--A-)---–-----T----A--
where TJ(MAX) is the maximum die temperature of the IC
and TA is the operating ambient temperature.
FAN1655 has an internal thermal limit at 150°C, which
defines TJ(MAX). For the SOIC-14 package, θJA is given at
88°C/W. Using equation 2, the maximum dissipation at
TA = 25°C is 1.4W, which is its rated maximum dissipation.
The e-TSSOP or MLP package, however, use the PCB
copper to cool the IC through the thermal pad on the package
bottom. For maximum dissipation, this pad should be
soldered to the PCB copper, with as much copper area as
possible surrounding it to cool the package. Thermal vias
should be placed as close to the thermal pad as possible to
transfer heat to other layers of copper on the PCB. With large
areas of PCB copper for heat sinking, a θJA of under 40°C/W
can easily be achieved.
REV. 1.1.4 3/24/04
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