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SC1470ITSTRT датащи(PDF) 10 Page - Semtech Corporation

номер детали SC1470ITSTRT
подробное описание детали  Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
PDF  25 Pages
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производитель  SEMTECH [Semtech Corporation]
домашняя страница  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1470ITSTRT датащи(HTML) 10 Page - Semtech Corporation

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10
 2005 Semtech Corp.
www.semtech.com
SC1470
POWER MANAGEMENT
The IC duty-factor limitation is given by:
)
MAX
(
OFF
t
)
MIN
(
ON
t
)
MIN
(
ON
t
DUTY
+
=
Be sure to include inductor resistance and MOSFET on-
state voltage drops when performing worst-case dropout
duty-factor calculations.
SC1470 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 SC1470 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
DS(ON). High RDS(ON)
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 4 on Page 15 will be designed.
The maximum input voltage (V
BAT(MAX)) is determined by
the highest AC adaptor voltage. The minimum input
voltage (V
BAT(MIN)) is determined by the lowest battery
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
BAT range
of 8V to 20V.
Four parameters are needed for the output:
1) nominal output voltage, V
OUT (we will use 1.2V)
2) static (or DC) tolerance, TOL
ST (we will use +/-4%)
3) transient tolerance, TOL
TR and size of transient (we will
use +/-8% and 6A for purposes of this demonstration).
4) maximum output current, I
OUT (we will design for 6A)
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 VIN2. Knowing the maximum input voltage and
budget for MOSFET switches usually dictates where the
design ends up. A default R
tON value of 1MΩ is suggested
as a starting point, but this is not set in stone. The first
thing to do is to calculate the on-time, t
ON, at VBAT(MIN) and
V
BAT(MAX), since this depends only upon VBAT, VOUT and RtON.
For V
OUT < 3.3V:
()
s
10
50
V
V
10
37
R
10
3
.
3
t
9
)
MIN
(
BAT
OUT
3
tON
12
)
MIN
(
VBAT
_
ON
−
−
•
+
•
•
+
•
•
=
and
()
s
10
50
V
V
10
37
R
10
3
.
3
t
9
)
MAX
(
BAT
OUT
3
tON
12
)
MAX
(
VBAT
_
ON
−
−
•
+
•
•
+
•
•
=
From these values of t
ON we can calculate the nominal
switching frequency as follows:
()
Hz
t
V
V
f
)
MIN
(
VBAT
_
ON
)
MIN
(
BAT
OUT
)
MIN
(
VBAT
_
SW
•
=
and
()
Hz
t
V
V
f
)
MAX
(
VBAT
_
ON
)
MAX
(
BAT
OUT
)
MAX
(
VBAT
_
SW
•
=
t
ON is generated by a one-shot comparator that samples



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