SC480 SEMTECH [Semtech Corporation], SC480 Datasheet - Page 15

• Manufacturer Part Number: SC480
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: SEMTECH [Semtech Corporation]

For stability, place a 10Ω/1μF series combination from
REF to VSSA. If REF load capacitance exceeds 1μF, place
at least 10Ω in series with the load capacitance to prevent
instability. It is possible to use only one 10Ω resistor, by
connecting the load capacitors in parallel with the 1μF,
and connecting the load REF to the capacitor side of the
10Ω resistor. (See the Typical Application Circuit on Page
1.) Note that this resistor creates an error term when REF
has a DC load. In most applications this is not a concern
since the DC load on REF is negligible.
Design Procedure
Prior to designing a switching output and making com-
ponent selections, it is necessary to determine the input
voltage range and output voltage specifi cations. To dem-
onstrate the procedure, the output for the schematic in
Figure 7 on page 18
The maximum input voltage (V
highest AC adaptor voltage. The minimum input voltage
(V
ter accounting for voltage drops due to connectors, fuses
and battery selector switches. For the purposes of this
design example we will use a VBAT range of 8V to 20V to
design VDDQ.
Four parameters are needed for the design:
1.
2.
3.
4.
Switching frequency determines the trade-off between
size and effi ciency.
the switching losses in the MOSFETs, and losses are a
function of VBAT
and budget for MOSFET switches usually dictates where
the design ends up. The default R
715kΩ are suggested only as a starting point.
The fi rst thing to do is to calculate the on-time, t
V
and Rt
POWER MANAGEMENT
Application Information (Cont.)
© 2006 Semtech Corp.
BAT(MIN)
BAT(MIN)
Nominal output voltage, V
internal feedback resistors (FB pin tied to VCCA).
Static (or DC) tolerance, TOL
Transient tolerance, TOL
will use +/-8% for a 10A to 5A load release for this
demonstration).
Maximum output current, I
ON
and V
) is determined by the lowest battery voltage af-
.
BAT(MAX)
2
. Knowing the maximum input voltage
, since this depends only upon V
will be designed.
Increased
TR
BAT(MAX)
OUT
OUT
and size of transient (we
ST
. We will use 1.8V with
(we will design for 10A).
(we will use +/-2%).
tON
frequency
) is determined by the
values of 1MΩ and
increases
BAT
ON
, V
, at
OUT
15
and,
From these values of t
switching frequency as follows:
and,
t
V
used to charge an internal 3.3pF capacitor to V
equations above refl ect this along with any internal com-
ponents or delays that infl uence t
select R
t
f
Now that we know t
the inductor. To do this we select an acceptable inductor
ripple current. The calculations below assume 50% of I
which will give us a starting place.
and,
For our example:
ON
ON_VBAT(MIN)
SW_VBAT(MIN)
BAT
t
t
ON_VBAT(MI
ON_VBAT(MA
is generated by a one-shot comparator that samples
via R
f
L
SW_VBAT
L
f
SW_VBAT
VBAT
VBAT
L
tON
VBAT(MIN)
tON
N)
= 820ns and, t
= 274kHz and f
X)
= 1MΩ:
(MIN)
(MAX)
, converting this to a current. This current is
3.3
(MIN)
3.3
(MAX)
= 1.02μH and L
10
10
V
ON
V
BAT(MIN)
12
12
BAT(MAX)
V
we can calculate suitable values for
ON
BAT(MIN)
V
BAT(MAX)
R
R
we can calculate the nominal
ON_VBAT(MAX)
tON
SW_VBAT(MAX)
tON
V
37
37
OUT
V
V
VBAT(MAX)
OUT
OUT
t
ON_VBAT(MI
10
V
10
OUT
t
ON
= 358ns
3
ON_VBAT(MA
3
= 251kHz
. For our example we
t
ON_VBAT
= 1.30μH,
V
t
V
0.5
BAT
ON_VBAT(MA
BAT
V
V
OUT
0.5
OUT
(
www.semtech.com
(
MIN
MAX
N)
I
OUT
SC480
)
)
X)
(MIN)
I
OUT
Hz
50
50
Hz
OUT
X)
H
10
10
. The
H
9
9
OUT
s
s