SC480EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC480EVB' PDF : 25 Pages View PDF
SC480
POWER MANAGEMENT
Application Information (Cont.)
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 specifications. To dem-
onstrate the procedure, the output for the schematic in
Figure 7 on page 18 will be designed.
The maximum input voltage (VBAT(MAX)) is determined by the
highest AC adaptor voltage. The minimum input voltage
(V ) is determined by the lowest battery voltage af-
BAT(MIN)
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.
t ON_VBAT(MIN)
ª
«3.3
x
10 12
x
«¬
RtON 37 x 103
x
VOUT
º
» 50
x 10 9 s
VBAT(MIN) »¼
and,
t ON_VBAT(MAX)
ª
«3.3
x
10 12
x
«¬
RtON 37 x 103
x
VOUT
º
» 50
x 10 9 s
VBAT(MAX) »¼
From these values of tON we can calculate the nominal
switching frequency as follows:
f SW_VBAT (MIN)
VOUT
Hz
¨©§ VBAT(MIN) x tON_VBAT(MIN) ¸¹·
and,
f SW_VBAT (MAX)
VOUT
Hz
¨©§ VBAT(MAX) x tON_VBAT(MAX) ¸¹·
tON is generated by a one-shot comparator that samples
VBAT via RtON, converting this to a current. This current is
used to charge an internal 3.3pF capacitor to VOUT. The
equations above reflect this along with any internal com-
ponents or delays that influence tON. For our example we
select RtON = 1MΩ:
Four parameters are needed for the design:
tON_VBAT(MIN) = 820ns and, tON_VBAT(MAX) = 358ns
1. Nominal output voltage, VOUT. We will use 1.8V with
internal feedback resistors (FB pin tied to VCCA).
2. Static (or DC) tolerance, TOLST (we will use +/-2%).
3. Transient tolerance, TOLTR and size of transient (we
will use +/-8% for a 10A to 5A load release for this
demonstration).
4. Maximum output current, IOUT (we will design for 10A).
Switching frequency determines the trade-off between
size and efficiency. Increased frequency increases
the switching losses in the MOSFETs, and losses are a
function of VBAT2. Knowing the maximum input voltage
and budget for MOSFET switches usually dictates where
the design ends up. The default RtON values of 1MΩ and
715kΩ are suggested only as a starting point.
fSW_VBAT(MIN) = 274kHz and fSW_VBAT(MAX) = 251kHz
Now that we know tON we can calculate suitable values for
the inductor. To do this we select an acceptable inductor
ripple current. The calculations below assume 50% of IOUT
which will give us a starting place.
L VBAT(MIN)
tON_VBAT (MIN)
VBAT(MIN) VOUT x ¨©§ 0.5 x IOUT ¸¹· H
and,
L VBAT (MAX)
VBAT(MAX) VOUT
x tON_VBAT(MAX)
¨©§ 0.5 x IOUT ¸¹·
H
The first thing to do is to calculate the on-time, t , at
ON
VBAT(MIN) and V , BAT(MAX) since this depends only upon VBAT, VOUT
and RtON.
For our example:
LVBAT(MIN) = 1.02μH and LVBAT(MAX) = 1.30μH,
© 2006 Semtech Corp.
15
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