參數(shù)資料
型號(hào)： LTC1735C-1
廠商： Linear Technology Corporation
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 抗輻射高效，5安培開(kāi)關(guān)穩(wěn)壓器
文件頁(yè)數(shù)： 16/32頁(yè)
文件大小： 373K
代理商： LTC1735C-1
16
LTC1735
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin
is low. Note that the voltage across C
B
is about a diode
drop below INTV
CC
. When the topside MOSFET is to be
turned on, the driver places the C
B
voltage across the
gate-source of the MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage
SW rises to V
IN
and the BOOST pin rises to V
IN
+ INTV
CC
.
The value of the boost capacitor C
B
needs to be 100 times
greater than the total input capacitance of the topside
MOSFET. In most applications 0.1
μ
F to 0.33
μ
F is ad-
equate. The reverse breakdown on D
B
must be greater
than V
IN(MAX)
.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If you make a change
and the input current decreases, then you improved the
efficiency. If there s no change n nput current, then there
is no change in efficiency.
SENSE
+
/SENSE
–
Pins
The common mode input range of the current comparator
is from 0V to 1.1(INTV
CC
). Continuous linear operation in
step-down applications is guaranteed throughout this
range allowing output voltages anywhere from 0.8V to 7V.
A differential NPN input stage is used and is biased with
internal resistors from an internal 2.4V source as shown
in the Functional Diagram. This causes current to either be
sourced or sunk by the sense pins depending on the
output voltage. If the output voltage is below 2.4V current
will flow out of both sense pins to the main output. This
forces a minimum load current that can be fulfilled by the
APPLICATIU
W
U
U
Figure 4. Setting the LTC1735 Output Voltage
V
OSENSE
SGND
V
OUT
R2
1735 F04
LTC1735
R1
47pF
V
OUT
resistive divider. The maximum current flowing out
of the sense pins is:
I
SENSE+
+ I
SENSE–
= (2.4V – V
OUT
)/24k
Since V
OSENSE
is servoed to the 0.8V reference voltage, we
can choose R1 in Figure 4 to have a maximum value to
absorb this current:
R
k
V
V
V
MAX
(
OUT
1
24
0 8
.
–
2 4
.
)
=
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that at output voltages above
2.4V no maximum value of R1 is necessary to absorb the
sense pin currents; however, R1 is still bounded by the
V
OSENSE
feedback current.
Soft-Start/Run Function
The RUN/SS pin is a multipurpose pin that provides a soft-
start function and a means to shut down the LTC1735.
Soft-start reduces surge currents from V
IN
by gradually
increasing the controller’s current limit I
TH(MAX)
. This pin
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.5V puts the LTC1735 into
a low quiescent current shutdown (I
Q
< 25
μ
A). This pin can
be driven directly from logic as shown in Figure 5. Releas-
ing the RUN/SS pin allows an internal 1.2
μ
A current
source to charge up the external soft-start capacitor C
SS.
If RUN/SS has been pulled all the way to ground there is
a delay before starting of approximately:
t
V
AC
μ
s F C
/
DELAY
SS
SS
=
=
(
1 5
1 2
.
1 25
.
.
When the voltage on RUN/SS reaches 1.5V the LTC1735
begins operating with a current limit at approximately
25mV/R
SENSE
. As the voltage on the RUN/SS pin increases
from 1.5V to 3.0V, the internal current limit is increased
from 25mV/R
SENSE
to 75mV/R
SENSE
. The output current
limit ramps up slowly, taking an additional 1.25s/
μ
F to
reach full current. The output current thus ramps up
slowly, reducing the starting surge current required from
the input power supply.
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