FAN1655M(2004) 데이터 시트보기 (PDF) - Fairchild Semiconductor

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FAN1655M
(Rev.:2004)

3A DDR Bus Termination Regulator

Fairchild Semiconductor 

FAN1655

PRODUCT SPECIFICATION

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 mΩ, and

∆I is the maximum load current.

For example, to handle a 3A maximum load transient, the

ESR should be no greater than 8mΩ. 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 5mΩ of ESR

in the output and is not stable with all-ceramic output

capacitors.

Power Dissipation and Derating

The maximum output current (sink or source) for a 1.25V

output is:

IOUT(MAX)

=

-P---D----(--M----A----X----)

1.25

(1)

where PD(MAX) is the maximum power dissipation which is:

PD(MAX) = T----J---(-M-----Aθ----XJ--A-)---–-----T----A--

(2)

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

5

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