sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 11

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

Be sure to include inductor resistance and MOSFET on-
state voltage drops when performing worst-case dropout
duty-factor calculations.
SC483 System DC Accuracy
Two IC parameters affect system DC accuracy, the error
comparator threshold voltage variation and the
switching frequency variation with line and load. The
error comparator threshold does not drift significantly
with supply and temperature. Thus, the error
comparator contributes 1% or less to DC system
inaccuracy.
Board components and layout also influence DC
accuracy. The use of 1% feedback resistors contribute
1%. If tighter DC accuracy is required use 0.1%
feedback resistors.
The on pulse in the SC483 is calculated to give a pseudo
fixed frequency. Nevertheless, some frequency variation
with line and load can be expected. This variation changes
the output ripple voltage. Because constant-on
regulators regulate to the valley of the output ripple, ½
of the output ripple appears as a DC regulation error.
For example, if the feedback resistors are chosen to
divide down the output by a factor of five, the valley of
the output ripple will be VOUT. For example: if VOUT is
2.5V and the ripple is 50mV with VBAT = 6V, then the
measured DC output will be 2.525V. If the ripple increases
to 80mV with VBAT = 25V, then the measured DC output
will be 2.540V.
The output inductor value may change with current. This
will change the output ripple and thus the DC output
voltage. It will not change the frequency.
Switching frequency variation with load can be minimized
by choosing MOSFETs with lower R
MOSFETs will cause the switching frequency to increase
as the load current increases. This will reduce the ripple
and thus the DC output voltage.
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 output for the
schematic in Figure 8 on Page 16 will be designed.
POWER MANAGEMENT
2005 Semtech Corp.
DS(ON)
. High R
DS(ON)
11
The maximum input voltage (V
the highest AC adaptor voltage. The minimum input
voltage (V
voltage after accounting for voltage drops due to
connectors, fuses and battery selector switches. For the
purposes of this design example we will use a V
of 8V to 20V and design OUT2. The design for OUT1
employs the same technique.
Four parameters are needed for the output:
1) nominal output voltage, V
2) static (or DC) tolerance, TOL
3) transient tolerance, TOL
use +/-8% and 6A 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
For V
and
From these values of t
switching frequency as follows:
and
t
t
f
ON
ON
SW
tON
_
_
_
VBAT
VBAT
.
VBAT
OUT
BAT(MAX)
(
(
MAX
MIN
(
MIN
< 3.3V:
)
)
tON
BAT(MIN)
)
, since this depends only upon V
3
values of 1M
3
3 .
3 .
2
V
. Knowing the maximum input voltage and
) is determined by the lowest battery
BAT
10
10
(
MIN
12
12
)
V
R
R
ON
OUT
t
tON
tON
ON
we can calculate the nominal
TR
_
VBAT
and size of transient (we will
OUT
37
37
and 715k
OUT
ST
(we will use 1.2V)
BAT(MAX)
(
MIN
10
10
(we will design for 6A)
(we will use +/-4%)
)
3
3
Hz
) is determined by
V
V
BAT
V
BAT
V
OUT
OUT
www.semtech.com
(
(
are suggested
MAX
MIN
SC483
)
ON
)
BAT
, at V
, V
50
50
BAT
OUT
range
BAT(MIN)
10
10
and
9
9
s
s