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| 型号： | LTC1735IS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/32页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16-SOIC |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 335kHz |
| 占空比： | 99.4% |
| 电源电压： | 4 V ~ 30 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1333 (CN2011-ZH PDF) |
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LTC1735
APPLICATIO S I FOR ATIO
V OUT
R2
V OUT resistive divider. The maximum current flowing out
of the sense pins is:
V OSENSE
LTC1735
SGND
1735 F04
47pF
R1
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 1 MAX ) = 24 k ? ?
Figure 4. Setting the LTC1735 Output Voltage
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 is no change in input current, then there
is no change in efficiency.
? 0 . 8 V ?
? 2 . 4 V – V OUT ?
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
shutdown. This pin can be driven directly from logic as
shown in Figure 5. The V IN quiescent current is a function
of RUN/SS voltage (refer to Typical Performance Charac-
teristics graphs on page 6). Releasing 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
t DELAY =
SENSE + /SENSE – Pins
The common mode input range of the current comparator
is from 0V to 1.1(INTV CC ). Continuous linear operation in
of approximately:
1 . 5 V
1 . 2 μ A
C SS = ( 1 . 25 s / μ F ) C SS
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
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.
1735fc
17
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