LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 12

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Transformer Design Considerations
Transformer specifi cation and design is perhaps the most
critical part of applying the LTC3805-5 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
Turns Ratios
Due to the use of the external feedback resistor divider
ratio to set output voltage, the user has relative freedom
in selecting transformer turns ratio to suit a given ap-
plication. Simple ratios of small integers, e.g., 1:1, 2:1,
3:2, etc. can be employed which yield more freedom in
setting total turns and transformer inductance. Simple
integer turns ratios also facilitate the use of “off-the-shelf”
confi gurable transformers. Turns ratio can be chosen on
the basis of desired duty cycle. However, remember that
the input supply voltage plus the secondary-to-primary
referred version of the fl yback pulse (including leakage
spike) must not exceed the allowed external MOSFET
breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
turn off of MOSFET (Q1) in Figure 8. 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
LTC3805-5
12
technique is a good way to minimize troublesome leak-
age 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
LTC3805-5 sources 5μA out of the RUN pin when operation
of LTC3805-5 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
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
Note that for some applications the RUN pin can be con-
nected to V
and V
V
V
IN RUN FALLING
IN RUN RISING
TURNOFF
(
(
,
,
CC
RUNON
, control operation.
in which case the V
IN
as shown in Figure 3. The voltage thresh-
)
– V
)
=
=
V
RUNOFF
V
RUNON
RUNON
RUNOFF
IN
= 35mV of built-in voltage
rising and V
•
R
is simply
•
CC
1
R
R
+
1
2
thresholds, V
R
R
+
2
2
R
2
−
RUNOFF
IN
R
1 5μ
•
TURNON
falling.
A
38055fa
IN