LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 13

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LTC3805-5

Manufacturer Part Number
LTC3805-5
Description
Adjustable Frequency Current Mode Flyback DC/DC Controller
Manufacturer
Linear Technology
Datasheet
www.datasheet4u.com
APPLICATIONS INFORMATION
External Run/Stop Control
To implement external run control, place a small N-channel
MOSFET from the RUN pin to GND as shown in Figure 3.
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 V
of R4 to R3 needed to produce a desired V
calculated:
Choose resistance values for R3 and R4 to be as large as
possible in order to minimize any effi ciency loss due to the
static current drawn from V
that when V
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.
R
Figure 3. Setting RUN Pin Voltage and Run/Stop Control
3
=
V
OUT
OUT
0 8
LTC3805-5
.
− .
is in regulation the input current to the V
V
OUT
0 8
RUN
GND
V
(R3 and R4 in Figure 8). The ratio
R
4
V
IN
R1
R2
OUT
, but just small enough so
38055 F03
RUN/STOP
CONTROL
(OPTIONAL)
OUT
can be
FB
Feedback in Isolated Applications
Isolated applications do not use the FB pin and error ampli-
fi er but control the I
driven on the other side of the isolation barrier as shown
in Figure 4. For isolated converters, the FB pin is grounded
which provides pull-up on the I
enough to properly bias the optoisolator which is typically
biased using a resistor to V
sink the optoisolator bias current, a diode is required to
block it from the I
to ensure that the optoisolator is able to pull I
its lower clamp.
Oscillator Synchronization
The oscillator may be synchronized to an external clock
by connecting the synchronization signal to the SYNC pin.
The LTC3805-5 oscillator and turn-on of the switch are
synchronized to the rising edge of the external clock. The
frequency of the external sync signal must be ±33% with
respect to f
value of f
Current Sense Resistor Considerations
The external current sense resistor (R
allows the user to optimize the current limit behavior for
the particular application. As the current sense resistor
is varied from several ohms down to tens of milliohms,
peak switch current goes from a fraction of an ampere
to several amperes. Care must be taken to ensure proper
circuit operation, especially with small current sense
resistor values.
SYNC
OSC
must be between 70kHz and 700kHz.
(as programmed by R
TH
pin. A Schottky diode should be used
TH
pin directly using an optoisolator
CC
. Since the I
TH
pin. This pull-up is not
LTC3805-5
FS
SENSE
). Additionally, the
TH
in Figure 8)
TH
pin cannot
down to
13
38055fa

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