sc480imltrt Semtech Corporation, sc480imltrt Datasheet - Page 16

• Manufacturer Part Number: sc480imltrt
• Description: Sc480 Complete Ddr1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

© 2006 Semtech Corp.
POWER MANAGEMENT
POWER MANAGEMENT
Application Information (Cont.)
16
where ERR
case, I
For our example:
ERR
R
We will select a value of 6mΩ maximum for our design,
which would be achieved by using two 12mΩ output ca-
pacitors in parallel. Now that we know the output ESR we
can calculate the output ripple voltage:
and,
For our example:
V
Note that in order for the device to regulate in a controlled
manner, the ripple content at the feedback pin, V
be approximately 15mV
no smaller than 10mV
FB is smaller than the voltage ripple at the output capaci-
tor, due to the resistor divider. Also, when using internal
feedback (FB pin tied to 5V or GND), the FB resistor di-
vider is actually inside the IC. If V
the FB point is less than 15mV
ternal FB is used - the above component values should be
revisited in order to improve this. For our example, since
the internal divider reduces the ripple signal by a factor of
(1.5V/1.8V), the internal FB ripple values are then 17mV
and 22mV, which is above the 15mV minimum.
When using external feedback, and with VDDQ greater
than 1.5V, a small capacitor, C
with the top feedback resistor, R
ripple at VFB is large enough. C
than 100pF. The value of C
lows, where R
calculating the value of Z
RIPPLE_VBAT(MAX)
ESR_TR(MAX)
TR
V
V
RIPPLE_VBA
RIPPLE_VBA
= 144mV and ERR
TRANS
= 17.6mΩ for a full 5A load transient.
TR
is the load transient of 5A (10A - 5A).
= 20mV
is the transient output tolerance. For this
BOT
( T
( T
is the bottom feedback resistor. Firstly
MIN
MAX
P-P
)
)
P-P
P-P
and V
R
. Note that the voltage ripple at
DC
R
TOP
at minimum V
ESR
ESR
= 18mV, therefore,
required:
TOP
RIPPLE_VBAT(MIN)
TOP
P-P
can be calculated as fol-
TOP
I
I
TOP
RIPPLE_VBA
RIPPLE_VBA
, can be used in parallel
- whether internal or ex-
, in order to ensure that
RIPPLE_VBAT(MIN)
should not be greater
BAT
= 26mV
, and worst case
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( T
( T
SC480
MIN
MAX
as seen at
FB
)
)
P-P
P V
, should
P V
P
P