SC1486AEVB datasheet(12/30 Pages) SEMTECH

12 / 30 page

 2006 Semtech Corp.

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SC1486A

POWER MANAGEMENT

addition of extra resistance between REFOUT and the

DIMMs to avoid spurious OVP at startup, which can occur

if REFOUT rises really slowly and VTT overshoots it. The

extra resistance allows REFOUT to rise faster, avoiding

this issue.

REFIN should also be filtered so that VDDQ ripple does

not appear at the REFIN pin. If a resistor divider is used

to create REFIN from VDDQ, then a 0.1µF capacitor from

REFIN to VSSA2 will provide adequate filtering.

Dropout Performance

The output voltage adjust range for continuous-

conduction operation is limited by the fixed 550ns

(maximum) minimum off-time one-shot. For best dropout

performance, use the slowest on-time setting of 200kHz.

When working with low input voltages, the duty-factor

limit must be calculated using worst-case values for on

and off times. The IC duty-factor limitation is given by:

)

MAX

(

OFF

)

MIN

(

)

MIN

(

DUTY

Be sure to include inductor resistance and MOSFET on-

state voltage drops when performing worst-case dropout

duty-factor calculations.

SC1486A System DC Accuracy (VTT Controller)

Two IC parameters effect system DC accuracy, the error

comparator offset voltage, and the switching frequency

variation with line and load. The SC1486A regulates to

the REFOUT voltage not the REFIN voltage. Since DDR

specifications are written with respect to REFOUT, the

offset of the reference buffer does not create a regulation

error.

The error comparator offset does not drift significantly

with supply and temperature. Thus, the error comparator

contributes 1% or less to DC system inaccuracy.

The on pulse in the SC1486A is calculated to give a

pseudo fixed frequency. Nevertheless, some frequency

variation with line and load can be expected. This variation

changes the output ripple voltage. Because constant on

regulators regulate to the valley of the output ripple, ½

of the output ripple appears as a DC regulation error.

For example, if REFOUT=1.25V, then the valley of the

output ripple will be 1.25V. If the ripple is 20mV with VIN

= 6V, then the DC output voltage will be 1.26V. If the

ripple is 40mV with VIN = 25V, then the DC output voltage

will be 1.27V.

1486 System DC Accuracy (VDDQ Controller)

Two IC parameters affect system DC accuracy, the error

comparator threshold voltage variation and the switching

frequency variation with line and load.

The error comparator threshold does not drift significantly

with supply and temperature. Thus, the error comparator

contributes 1% or less to DC system inaccuracy.

Board components and layout also influence DC

accuracy. The use of 1% feedback resistors contribute

1%. If tighter DC accuracy is required use 0.1% feedback

resistors.

The on pulse in the SC1486A is calculated to give a

pseudo fixed frequency. Nevertheless, some frequency

variation with line and load can be expected. This variation

changes the output ripple voltage. Because constant on

regulators regulate to the valley of the output ripple, ½

of the output ripple appears as a DC regulation error.

For example, if the feedback resistors are chosen to

divide down the output by a factor of five, the valley of

the output ripple will be 2.5V. If the ripple is 50mV with

VIN = 6V, then the measured DC output will be 2.525V.

If the ripple increases to 80mV with VIN = 25V, then the

measured DC output will be 2.540V.

The output inductor value may change with current. This

will change the output ripple and thus the DC output

voltage. It will not change the frequency.

Switching frequency variation with load can be minimized

by choosing MOSFETs with lower R

DS(ON). High RDS(ON)

MOSFETs will cause the switching frequency to increase

as the load current increases. This will reduce the ripple

and thus the DC output voltage.

DDR Supply Selection

The SC1486A can be configured so that VTT and VDDQ

are generated directly from the battery. Alternatively, the

VTT supply can be generated from the VDDQ supply. Since

the battery configuration generally yields better efficiency

and performance, the evaluation board is configured to

generate both supplies from the battery.

Application Information (Cont.)

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