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参数资料
| 型号： | LTC3805IMSE-5#TRMPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 20/20页 |
| 文件大小： | 203K |
| 代理商： | LTC3805IMSE-5#TRMPBF |
LTC3805-5
9
38055fb
Figure 1. Start-Up/Shutdown State Diagram
LTC3805-5
SHUTDOWN
LTC3805-5
FAULT
TIMEOUT
38055 F01
VRUN > VRUNON
AND VCC > VTURNON
VRUN < VRUNOFF
VCC < VTURNOFF
VSSFLT < 0.7V
VOC > 100mV
LTC3805-5
ENABLED
OPERATION
Start-Up/Shutdown
The LTC3805-5 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the VCC
supply pin voltage, and a precision-threshold RUN pin. The
voltage on both pins must exceed the appropriate threshold
before operation is enabled. The LTC3805-5 transitions
into and out of shutdown according to the state diagram
shown in Figure 1. Operation in fault timeout is discussed
in a subsequent section. During shutdown the LTC3805-5
draws only a small 40μA current.
The undervoltage lockout (UVLO) mechanism prevents
the LTC3805-5 from trying to drive the external MOSFET
gate with insufcient voltage on the VCC pin. The voltage
at the VCC pin must initially exceed VTURNON = 4.5V to en-
able LTC3805-5 operation. After operation is enabled, the
voltage on the VCC pin may fall as low as VTURNOFF = 4V
before undervoltage lockout disables the LTC3805-5. See
the Applications Information section for more detail.
The RUN pin is connected to the input voltage using a
voltage divider. Converter operation is enabled when the
voltage on the RUN pin exceeds VRUNON = 1.207V and
disabled when the voltage falls below VRUNOFF = 1.170V.
Additional hysteresis is added by a 5μA current source
acting on the voltage divider’s Thevenin resistance. Setting
the input voltage range and hysteresis is further discussed
in the Applications Information section.
Setting the Oscillator Frequency
Connect a frequency set resistor RFS from the FS pin to
ground to set the oscillator frequency over a range from
70kHz to 700kHz. The oscillator frequency is calculated
from:
f
R
OSC
FS
=
24 10
1500
9
The oscillator may be synchronized to an external clock
using the SYNC input. The rising edge of the external clock
on the SYNC pin triggers the beginning of a switching pe-
riod, i.e., the GATE pin going high. The pulse width of the
external clock is quite exible. The clock must stay high
only for about 200ns to trigger the start of a new switching
period. Conversely, the pulse width can be increased to a
duty cycle not greater than 55%.
Overcurrent Protection
With the OC pin connected to the external MOSFET’s current
sense resistor, the converter is protected in the event of
an overload or short-circuit on the output. During normal
operation the peak value of current in the external MOSFET,
as measured by the current sense resistor (plus any adjust-
ment for slope compensation), is set by the voltage on the
ITH pin operating through the current comparator. As the
output current increases, so does the voltage on the ITH
pin and so does the peak MOSFET current.
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