sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 14

• Manufacturer Part Number: sc1486aitstr
• Description: Complete Ddr1/2/3 Power Supply Controller
• Manufacturer: Semtech Corporation

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% feed-
back resistors.
For our DDR2 VDDQ example:
ERR
R
Where ERR
this calculation assumes that the worst case load tran-
sient is full load. For half of full load, divide the I
by 2.
For our DDR2 VDDQ example:
I
R
R
POWER MANAGEMENT
Application Information (Cont.)
RIPPLE_VIN(MIN)
INDUCTOR(MIN)
INDUCTOR
ESR_ST(MAX)
ESR
ESR
2006 Semtech Corp.
ESR_TR(MAX)
ST
_
_
ST
TR
= 100mV and ERR
(
(
(
MIN
MAX
MAX
)
= 47m
):
)
= 11.4A
)
TR
ST
= 2.07A
I
is the transient output tolerance. Note that
is the static output tolerance and ERR
OUT
ERR
I
OUT
ERR
(
MAX
I
RIPPLE
(MIN)
P-P
ST
)
I
TR
RIPPLE
and I
I
_
RIPPLE
ERR
VIN
DC
ERR
ESR_ST(MAX)
_
(
2
MAX
VIN
RIPPLE_VIN(MAX)
= 36mV, therefore
_
2
DC
VIN
(
)
MAX
DC
(
MAX
)
2
) and transient ESR
)
Ohms
Ohms
A
(
MIN
= 2.73A
)
P-P
OUT
term
DC
is
14
ERR
R
We will select a value of 12.5m
design, which would be achieved by using two 25m
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. Quite
often a small capacitor, C
the top feedback resistor, R
V
100pF. The value of C
where R
calculating the value of Z
Secondly calculating the value of C
this:
R
V
V
Z
RIPPLE_VIN(MAX)
RIPPLE_VIN(MIN)
ESR_TR(MAX)
IN
FB
RIPPLE
RIPPLE
TOP
ESR
, and worst case no smaller than 10mV
is large enough. C
TR
(
MIN
FB
= 144mV and ERR
_
_
, should be approximately 15mV
)
VIN
VIN
. 0
R
BOT
015
(
(
BOT
= 9.5m
MAX
MIN
2
is less than 15mV
= 34mV
)
is the bottom feedback resistor. Firstly
)
R
R
V
C
ESR
RIPPLE
ESR
3
OUT
for a full 10A load transient
P-P
I
_
I
RIPPLE
TOP
RIPPLE
and V
VIN
TOP
f
SW
DC
TOP
(
TOP
MIN
can be calculated as follows,
should not be greater than
_
= 36mV, therefore
_
, is required in parallel with
VIN
)
required:
TOP
VIN
RIPPLE_VIN(MIN)
(
(
MIN
P-P
, in order to ensure that
. 0
MAX
015
)
the above component
V
TOP
)
V
P
P
P
required to achieve
maximum for our
P
SC1486A
Ohms
= 26mV
P-P
www.semtech.com
at minimum
P-P
P-P
. If