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參數(shù)資料
| 型號(hào)： | LTC3805IDD#TRPBF |
| 廠(chǎng)商： | LINEAR TECHNOLOGY CORP |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件頁(yè)數(shù)： | 5/20頁(yè) |
| 文件大?。?/td> | 260K |
| 代理商： | LTC3805IDD#TRPBF |
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LTC3805
3805fe
Figure 4. Setting RUN Pin Voltage and Run/Stop Control
LTC3805
RUN
RUN/STOP
CONTROL
(OPTIONAL)
R1
R2
GND
3805 F04
VIN
applications inForMation
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
MOSFET (Q1) turn-off. This is increasingly prominent at
higher load currents, where more stored energy must be
dissipated. In some cases an RC “snubber” circuit will be
required to avoid overvoltage breakdown at the MOSFET’s
drain node. Application Note 19 is a good reference on
snubber design. A bifilar or similar winding technique is a
good way to minimize troublesome leakage inductances.
However, remember that this will limit the primary-to-
secondary breakdown voltage, so bifilar winding is not
always practical.
Setting Undervoltage and Hysteresis on VIN
The RUN pin is connected to a resistive voltage divider
connected to VIN as shown in Figure 4. The voltage thresh-
old for the RUN pin is VRUNON rising and VRUNOFF falling.
Note that VRUNON – VRUNOFF = 35mV of built-in voltage
hysteresis that helps eliminate false trips.
To introduce further user-programmable hysteresis, the
LTC3805 sources 5A out of the RUN pin when operation
of LTC3805 is enabled. As a result, the falling threshold
for the RUN pin also depends on the value of R1 and can
be programmed by the user. The falling threshold for VIN
is therefore
VIN(RUN,FALLING) = VRUNOFF
R1
+ R2
R2
R1 5A
where R1(5A) is the additional hysteresis introduced
by the 5A current sourced by the RUN pin. When in
shutdown, the RUN pin does not source the 5A current
and the rising threshold for VIN is simply
VIN(RUN,RISING) = VRUNON
R1
+ R2
R2
External Run/Stop Control
Toimplementexternalruncontrol,placeasmallN-channel
MOSFET from the RUN pin to GND as shown in Figure 4.
Drive the gate of this MOSFET high to pull the RUN pin
to ground and prevent converter operation.
Selecting Feedback Resistor Divider Values
The regulated output voltage is determined by the resistor
divider across VOUT (R3 and R4 in Figure 2). The ratio
of R4 to R3 needed to produce a desired VOUT can be
calculated:
R3
=
VOUT 0.8V
0.8V
R4
Choose resistance values for R3 and R4 to be as large as
possible in order to minimize any efficiency loss due to the
static current drawn from VOUT, but just small enough so
that when VOUT is in regulation the input current to the VFB
pin is less than 1% of the current through R3 and R4. A
good rule of thumb is to choose R4 to be less than 80k.
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