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參數(shù)資料
| 型號： | LTC1703 |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 通用總線功能 |
| 英文描述： | Synchronous 8-Bit Up/Down Binary Counters 24-PDIP 0 to 70 |
| 中文描述： | 雙550kHz的同步二相開關(guān)穩(wěn)壓控制器5位VID |
| 文件頁數(shù)： | 24/36頁 |
| 文件大?。?/td> | 343K |
| 代理商： | LTC1703 |
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24
LTC1703
CURRENT LIMIT PROGRAMMING
Programming the current limit on the LTC1703 is straight-
forward. The I
MAX
pin sets the current limit by setting the
maximum allowable voltage drop across QB (the bottom
MOSFET) before the current limit circuit engages. The
voltage across QB is set by its on-resistance and the
current flowing in the inductor, which is the same as the
output current. The LTC1703 current limit circuit inverts
the voltage at I
MAX
before comparing it with the negative
voltage across QB, allowing the current limit to be set with
a positive voltage.
To set the current limit, calculate the expected voltage
drop across QB at the maximum desired current:
)
(
I
LIM
should be chosen to be quite a bit higher than the
expected operating current, to allow for MOSFET R
DS(ON)
changes with temperature. Setting I
LIM
to 150% of the
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. The 100mV term is an approximate
factor that corrects for errors caused by ringing on the
switch node (illustrated in Figure 6). This factor will
change depending on the layout and the components
used, but 100mV is usually a good starting point. V
DROP
is
then programmed at the I
MAX
pin using the internal 10
μ
A
pull-up and an external resistor:
R
ILIM
= V
PROG
/10
μ
A
The resulting value of R
ILIM
should be checked in an actual
circuit to ensure that the I
LIM
circuit kicks in as expected.
MOSFET R
DS(ON)
specs are like horsepower ratings in
automobiles, and should be taken with a grain of salt.
Circuits that use very low values for R
IMAX
(<20k) should
be checked carefully, since small changes in R
IMAX
can
cause large I
LIM
changes when the 100mV correction
factor makes up a large percentage of the total V
PROG
value. If V
PROG
is set too low, the LTC1703 may fail to
start up.
V
I
R
mV
PROG
ILIM
DS ON
(
=
(
)
+
)
100
Now calculate the remaining values:
(K is a constant used in the calculations)
= chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in absolute
gain)
Type 2 Loop:
K
TanBOOST
1
2
2
C
GKR
2
–
C
C K
R
K
C
R
V
R
( )
–
V
V
B
REF
OUT
REF
=
+
°
=
=
(
π
)
=
π
=
2
45
2
1
1
1
2
2
1
1
Type 3 Loop:
K
Tan
BOOST
4
C
GR
–
C
C K
2
R
K
C
1
–
R
R
K
C
KR
R
( )
–
R
V
V
V
B
REF
OUT
REF
=
+
°
1
=
=
(
π
)
=
π
=
(
)
=
π
=
2
45
2
2
1
1
1
2
2
1
3
1
3
1
2
3
1
APPLICATIOU
W
U
U
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