參數(shù)資料
型號(hào)： LT1432CS8
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 5V High Efficiency Step-Down Switching Regulator Controller
中文描述： SWITCHING CONTROLLER, PDSO8
封裝： PLASTIC, SO-8
文件頁(yè)數(shù)： 7/28頁(yè)
文件大小： 611K
代理商： LT1432CS8
7
LT1432
U
S
A
O
PPLICATI
U
U
D1, L1 and C2 act as the conventional catch diode and
output filter of the buck converter. These components
should be selected carefully to maintain high efficiency
and acceptable output ripple. See other sections of this
data sheet for detailed discussions of these parts.
Current limiting is performed by R2. Sense voltage is only
60mV to maintain high efficiency. This also reduces the
value of the sense resistor enough to utilize a printed
circuit board trace as the sense resistor. The sense voltage
has a positive temperature coefficient of 0.33%/
°
C to
match the temperature coefficient of copper. See Current
Limiting section for details.
The basic regulator has three different operating modes,
defined by the mode pin drive. Normal operation occurs
when the mode pin is grounded. A low quiescent current
“burst” mode can be initiated by floating the mode pin.
Input supply current is typically 1.3mA in this mode, and
output ripple voltage is 100mV
p-p
. Pulling the mode pin
above 2.5V forces the entire regulator into micropower
shutdown where it typically draws less than 20
μ
A. See
Mode Pin Drive for details.
Efficiency
Efficiency in normal mode is maximum at about 500mA
load current, where it exceeds 90%. At lower currents, the
operating supply current of the switching IC dominates
losses. The power loss due to this term is approximately
8mA
×
5V, or 40mW. This is 4% of output power at a load
current of 200mA. At higher load currents, losses in the
switch, diode, and inductor series resistance begin to
increase as the square of current and quickly become the
dominant loss terms.
Loss in inductor series resistance;
P = R
S
(I
OUT
)
2
Loss in switch on resistance;
P
V
R
(
I
V
OUT
SW
OUT
2
IN
=
)
(
)
Loss in switch driver current;
P
I
V
40V
OUT
OUT
2
IN
=
(
)
Diode loss;
P
V V
V
V
I
IN–
OUT
OUT
IN
=
)(
)
(Use V
F
vs I
F
graph on diode data sheet, assuming I
F
=
I
OUT
)
R
S
= Inductor series resistance
R
SW
= Switch resistance of LT1271, etc.
I
F
= Diode current
V
F
= Diode forward voltage at I
F
= I
OUT
Inductor core loss depends on peak-to-peak ripple current
in the inductor, which is independent of load current for
any load current large enough to establish continuous
current in the inductor. Believe it or not, core loss is also
independent of the physical size of the core. It depends
only on core material, inductance value, and switching
frequency for fixed regulator operating conditions. In-
creasing inductance or switching frequency will reduce
core loss, because of the resultant decrease in ripple
current. For high efficiency, low loss cores such as ferrites
or Magnetics Inc. molypermalloy or KoolM
μ
are recom-
mended. The lower cost Type 52 powdered iron from
Phillips is acceptable only if larger inductance is used and
the increased size and slight loss in efficiency is accept-
able. In a typical buck converter using the LT1271 (60kHz)
with a 12V input, and a 50
μ
H inductor, core loss with a
Type 52 powdered iron core is 203mW. A molypermalloy
core reduces this figure to 28mW. With a 1A output, this
translates to 4% and 0.56% core loss respectively – a big
difference in a high efficiency converter. For details on
inductor design and losses, see Application Note 44.
What are the benefits of using an active (synchronous)
switch to replace the catch diode This is the trendy thing
to do, but calculations and actual breadboards show that
the improvement in efficiency is only a few percent at best.
This can be shown with the following simplified formulas:
Diode Loss
V V
V
V
I
IN–
OUT
OUT
IN
=
)(
)
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