SC486 Semtech Corporation, SC486 Datasheet - Page 16

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

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Design Procedure (Cont.)
For our DDR2 VDDQ example:
I
From this we can calculate the minimum inductor
current rating for normal operation:
For our DDR2 VDDQ example:
I
Next we will calculate the maximum output capacitor
equivalent series resistance (ESR). This is determined by
calculating the remaining static and transient tolerance
allowances. Then the maximum ESR is the smaller of the
calculated static ESR (R
(R
Where ERR
the DC error. The DC error will be 1% plus the tolerance
of the feedback resistors, thus 2% total for 1%
feedback resistors.
For our DDR2 VDDQ example:
ERR
R
Where ERR
this calculation assumes that the worst case load
transient is full load. For half of full load, divide the I
term by 2.
For our DDR2 VDDQ example:
ERR
I
R
R
POWER MANAGEMENT
RIPPLE_VBAT(MIN)
INDUCTOR(MIN)
INDUCTOR
ESR_ST(MAX)
ESR
ESR
2006 Semtech Corp.
ESR_TR(MAX)
ST
TR
_
_
ST
TR
= 72mV and ERR
= 100mV and ERR
(
(
(
MAX
MIN
MAX
)
= 22m
):
)
)
= 11.6A
ST
TR
= 2.62A
is the transient output tolerance. Note that
I
is the static output tolerance and ERR
OUT
ERR
I
OUT
(
ERR
MAX
I
RIPPLE
(MIN)
ST
)
P-P
I
RIPPLE
TR
_
I
and I
RIPPLE
VBAT
ERR
DC
DC
ERR
ESR_ST(MAX)
= 36mV, therefore
_
(
VBAT
2
MAX
= 36mV, therefore
_
RIPPLE_VBAT(MAX)
DC
VBAT
2
)
(
DC
MAX
(
MAX
2
)
) and transient ESR
Ohms
)
Ohms
A
(
MIN
= 3.28A
)
P-P
DC
OUT
is
16
R
We will select a value of 7.5m
design, which would be achieved by using two 15m
output capacitors in parallel.
Note that for constant-on converters there is a minimum
ESR requirement for stability which can be calculated as
follows:
This criteria should be checked once the output
capacitance has been determined.
Now that we know the output ESR we can calculate the
output ripple voltage:
and
For our DDR2 VDDQ example:
V
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, V
V
V
values should be revisited in order to improve this. A small
capacitor, C
feedback resistor, R
enough. C
of C
bottom feedback resistor. Firstly calculating the value of
Z
R
V
V
Z
RIPPLE_VBAT(MAX)
RIPPLE_VBAT(MIN)
TOP
ESR_TR(MAX)
BAT
RIPPLE
RIPPLE
ESR
TOP
, and worst case no smaller than 10mV
TOP
required:
(
MIN
FB
_
_
, should be approximately 15mV
can be calculated as follows, where R
)
VBAT
VBAT
. 0
R
015
TOP
BOT
= 5.5m
2
(
(
MIN
MAX
TOP
should not be greater than 100pF. The value
is less than 15mV
= 25mV
)
, may be required in parallel with the top
)
V
R
C
R
RIPPLE
3
ESR
OUT
ESR
TOP
for a full 10A load transient
P-P
, in order to ensure that V
_
I
VBAT
I
RIPPLE
f
and V
RIPPLE
SW
(
MIN
_
_
VBAT
)
VBAT
RIPPLE_VBAT(MIN)
P-P
. 0
(
(
MIN
MAX
the above component
015
)
V
)
maximum for our
V
P
P
P
Ohms
P
P-P
www.semtech.com
= 20mV
SC486
at minimum
FB
BOT
is large
P-P
is the
P-P
. If