sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 13

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

Design Procedure
Prior to designing an output and making component
selections, it is necessary to determine the input voltage
range and the output voltage specifications. For purposes
of demonstrating the procedure the VDDQ output for the
schematic on page 17 will be designed.
The maximum input voltage (V
highest AC adaptor voltage. The minimum input voltage
(V
accounting for voltage drops due to connectors, fuses
and battery selector switches. For the purposes of this
design example we will use a V
Four parameters are needed for the output:
1) nominal output voltage, V
2) static (or DC) tolerance, TOL
+/-0.1V)
3) transient tolerance, TOL
DDR2 this is undefined, so assume +/-8% for purposes
of this demonstration).
4) maximum output current, I
Switching frequency determines the trade-off between
size and efficiency. Increased frequency increases the
switching losses in the MOSFETs, since losses are a
function of VIN
budget for MOSFET switches usually dictates where the
design ends up. It is recommended that the two outputs
are designed to operate at frequencies approximately
25% apart to avoid any possible interaction. It is also
recommended that the higher frequency output is the
lower output voltage output, since this will tend to have
lower output ripple and tighter specifications. The
default R
as a starting point, but these are not set in stone. The
first thing to do is to calculate the on-time, t
and V
R
and
From these values of t
switching frequency as follows:
POWER MANAGEMENT
Application Information (Cont.)
t
t
ON
ON
tON
IN(MIN)
2006 Semtech Corp.
_
_
. For V
VIN
VIN
(
(
IN(MAX)
MAX
MIN
) is determined by the lowest battery voltage after
)
)
tON
OUT
, since this depends only upon V
3
3
values of 1M
3 .
3 .
< 3.3V:
2
. Knowing the maximum input voltage and
10
10
12
12
R
R
ON
tON
tON
we can calculate the nominal
TR
OUT
37
and 649k
37
OUT
IN(MAX)
and size of transient (for
IN
(for DDR2 this is 1.8V)
10
(we will design for 10A)
10
range of 7.5V to 20.5V.
) is determined by the
3
ST
3
(for DDR2 this is
V
V
V
V
IN
IN
OUT
OUT
(
(
MAX
MIN
are suggested
)
)
ON
IN
, V
, at V
50
50
OUT
10
10
IN(MIN)
and
9
9
s
s
13
and
t
V
used to charge an internal 3.3pF capacitor to V
equations above reflect this along with any internal com-
ponents or delays that influence t
example we select R
t
f
Now that we know t
for the inductor. To do this we select an acceptable
inductor ripple current. The calculations below assume
50% of I
and
For our DDR2 VDDQ example:
L
We will select an inductor value of 2.4µH to reduce the
ripple current, which can be calculated as follows:
and
I
I
L
L
f
f
ON
ON_VIN(MIN)
SW_VIN(MIN)
RIPPLE
RIPPLE
SW
SW
VIN(MIN)
IN
VIN
VIN
is generated by a one-shot comparator that samples
_
_
via R
(
(
VIN
VIN
MIN
MAX
_
_
(
(
VIN
VIN
)
MIN
MAX
= 1µH and L
)
OUT
tON
(
(
MIN
MAX
= 871ns and t
)
= 275kHz and f
)
V
, converting this to a current. This current is
V
which will give us a starting place.
)
IN
)
IN
(
V
MIN
(
V
MAX
IN
IN
V
V
(
)
MIN
(
IN
MAX
)
IN
(
MIN
(
)
MAX
V
)
VIN(MAX)
V
V
ON
OUT
tON
)
V
OUT
OUT
t
)
ON
OUT
t
we can calculate suitable values
ON
= 1M :
V
ON_VIN(MAX)
_
V
_
OUT
VIN
SW_VIN(MAX)
= 1.3µH
OUT
VIN
t
0
t
ON
(
0
MIN
ON
5 .
(
MAX
5 .
_
_
)
VIN
VIN
t
)
ON
I
t
Hz
(
OUT
= 350ns
I
ON
MIN
ON
(
Hz
OUT
MAX
_
= 251kHz
. For our DDR2 VDDQ
VIN
_
L
)
VIN
L
)
H
(
MIN
H
(
SC1486A
MAX
)
www.semtech.com
A
)
A
P
P
P
P
OUT
. The