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參數(shù)資料
| 型號： | LTC1705 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO |
| 中文描述： | 雙550kHz的同步開關，5位VID和150mA LDO穩(wěn)壓器控制器 |
| 文件頁數(shù)： | 21/28頁 |
| 文件大?。?/td> | 326K |
| 代理商： | LTC1705 |
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LTC1705
21
APPLICATIU
Finally, choose a convenient resistor value for R1 (10k is
usually a good value). Now calculate the remaining values:
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in
absolute gain)
W
U
U
TYPE 2 Loop:
K
TanBOOST
1
2
2
C
fGKR
(
C
C K
R
K
fC
V
REF
R
R
V
V
B
OUT
REF
=
+
°
=
=
)
=
=
2
45
2
1
1
1
2
2
1
1
2
π
π
( )
TYPE 3 Loop:
K
Tan
BOOST
4
C
fGR
(
C
C K
2
R
K
fC
1
R
R
K
C
f KR
V
REF
R
R
V
V
B
OUT
REF
=
+
°
=
=
)
=
=
=
=
2
45
2
1
2
1
1
1
2
2
1
3
1
1
3
2
3
1
π
π
π
( )
CURRENT LIMIT PROGRAMMING
Programming current limit on the LTC1705 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 LTC1705 current limit circuit inverts
the negative voltage across QB before comparing it with
the voltage at I
MAX
, 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:
V
PROG
= (I
LIMIT
) (R
DS(ON)
)
I
LIMIT
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
LIMIT
to 150% of the
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. Note that the ringing on the switch node
can cause error for the current limit threshold. This factor
will change depending on the layout and the components
used. V
PROG
is then programmed at the I
MAX
pin using the
internal 10
μ
A pull-up and an external resistor:
R
IMAX
= V
PROG
/10
μ
A
The resulting value of R
IMAX
should be checked in an
actual circuit to ensure that the current 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
(<10k)
should be checked carefully, since small changes in R
IMAX
can cause large I
LIMIT
changes when the switch node
ringing makes up a large percentage of the total V
PROG
value. If V
PROG
is set too low, the LTC1705 may fail to start
up.
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