LTC3805 Linear Technology, LTC3805 Datasheet - Page 13

• Manufacturer Part Number: LTC3805
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

www.datasheet4u.com
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 bifi lar 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 bifi lar winding is not
always practical.
Setting Undervoltage and Hysteresis on V
The RUN pin is connected to a resistive voltage divider
connected to V
old for the RUN pin is V
Note that V
hysteresis that helps eliminate false trips.
To introduce further user-programmable hysteresis, the
LTC3805 sources 5μA 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 V
is therefore
V
IN(RUN,FALLING)
RUNON
IN
as shown in Figure 4. The voltage thresh-
– V
= V
RUNOFF
RUNON
RUNOFF
= 35mV of built-in voltage
rising and V
•
Figure 4. Setting RUN Pin Voltage and Run/Stop Control
R1+ R2
R2
RUNOFF
− R1• 5µA
IN
LTC3805
RUN
GND
falling.
IN
V
IN
R1
R2
where R1(5μA) is the additional hysteresis introduced
by the 5μA current sourced by the RUN pin. When in
shutdown, the RUN pin does not source the 5μA current
and the rising threshold for V
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 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 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.
3805 F04
R3 =
V
IN(RUN,RISING)
RUN/STOP
CONTROL
(OPTIONAL)
V
OUT
OUT
0.8V
− 0.8V
is in regulation the input current to the V
OUT
= V
(R3 and R4 in Figure 2). The ratio
R4
RUNON
OUT
IN
, but just small enough so
•
is simply
R1+ R2
R2
LTC3805
OUT
13
can be
3805fb
FB