SC486 datasheet(14/26 Pages) SEMTECH

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SC486

POWER MANAGEMENT

Dropout Performance

VDDQ: 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.

VTT: the minimum input voltage allowed to be applied to

VTTIN (if a supply other than VDDQ is being used) should

be determined using the required maximum output

current and the maximum VTT pull-up resistance, 0.45

Ω.

The minimum VTTIN for a given VTT and ITT can be

calculated as follows:

)

MAX

(

PULLUP

ITT

VTT

)

MIN

(

VTTIN

•

For example: for VTT = 0.9V out and ITT = 1.25A, VTTIN

can go as low as 1.463V.

SC486 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 SC486 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-time

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

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

If the ripple increases to 80mV with VBAT = 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.

SC486 System DC Accuracy (VTT Sink/Source LDO)

The VTT LDO is designed to track the voltage at REF, with

a guaranteed DC accuracy of REF +/-20mV for -2A to

+2A. Thus the DDR/DDR2 absolute requirement of

+/-40mV including transients is an easy goal to achieve

provided that careful attention is paid during board layout

to reduce parasitic ESR/ESL.

DDR Supply Selection

The SC486 can be configured so that the VTT supply can

be generated from the VDDQ supply, or from an alternate

supply (usually lower to minimize power dissipation). If

the VTT LDO is going to be powered from the VDDQ output,

the electrical design of the VDDQ output needs to be for

IDDQ(MAX) + ITT(MAX).

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