SC486 Semtech Corporation, SC486 Datasheet - Page 14

• Manufacturer Part Number: SC486
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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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:
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 .
The minimum VTTIN for a given VTT and ITT can be
calculated as follows:
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
DUTY
POWER MANAGEMENT
VTTIN
2006 Semtech Corp.
(
MIN
t
ON
(
)
MIN
t
ON
)
VTT
(
MIN
t
OFF
)
(
MAX
ITT
)
R
PULLUP
(
MAX
)
14
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
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).
DS(ON)
. High R
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DS(ON)
SC486
MOSFETs