SC488 Semtech Corporation, SC488 Datasheet - Page 14

• Manufacturer Part Number: SC488
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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For stability, place a 10Ω/1μF series combination from REF
to VSSA. If REF load capacitance exceeds 1μF, place at
least 10Ω’s 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 19
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. Increased frequency increases the
switching losses in the MOSFETs, and losses are a func-
tion of VBAT
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
© 2006 Semtech Corp.
POWER MANAGEMENT
Application Information (Cont.)
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-
.
2
BAT(MAX)
. Knowing the maximum input voltage and
, since this depends only upon V
will be designed.
TR
BAT(MAX)
OUT
OUT
and size of transient (we
ST
. We will use 1.8V with
tON
(we will design for 10A).
(we will use +/-2%).
) is determined by the
values of 1MΩ and
BAT
ON
, V
, at
OUT
14
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
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
BAT
t
t
ON_VBAT(MI
ON_VBAT(MA
is generated by a one-shot comparator that samples
via R
f
f
L
L
SW_VBAT
t
f
L
SW_VBAT
tON
ON_VBAT(MIN)
SW_VBAT(MIN)
VBAT
VBAT
VBAT(MIN)
tON
N)
X)
= 1MΩ:
, converting this to a current. This current is
(MIN)
(MAX)
3.3
3.3
(MIN)
= 1.02μH and L
(MAX)
10
10
= 820ns and, t
= 274kHz and f
ON
V
12
12
BAT(MIN)
V
we can calculate suitable values for
BAT(MAX)
ON
V
V
BAT(MIN)
R
R
we can calculate the nominal
BAT(MAX)
tON
tON
37
37
V
OUT
VBAT(MAX)
V
V
OUT
10
ON_VBAT(MAX)
10
OUT
t
V
SW_VBAT(MAX)
ON
ON_VBAT(MI
OUT
t
3
3
ON_VBAT(MA
. For our example we
t
ON_VBAT
= 1.30μH,
V
V
t
BAT
0.5
BAT
ON_VBAT(MA
V
V
OUT
OUT
0.5
(
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(
= 358ns
MIN
MAX
= 251kHz
N)
I
OUT
SC488
)
)
X)
(MIN)
I
OUT
50
Hz
50
Hz
OUT
H
X)
10
10
. The
H
9
9
OUT
s
s