ltc3803es6-5 Linear Technology Corporation, ltc3803es6-5 Datasheet - Page 11

• Manufacturer Part Number: ltc3803es6-5
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly promi-
nent at higher load currents, where more stored energy
must be dissipated. In some cases a “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.
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.
U
U
W
SENSE
in Figure 2)
U
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803-5 has only a single sense line
to this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803-5 injects a ramping current through its
SENSE pin into an external slope compensation resistor
(R
after the NGATE pin has been high for the LTC3803-5’s
minimum duty cycle of 6.5%. The current rises linearly
towards a peak of 5μA at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resistor
(R
resistor (R
From the perspective of the SENSE pin, this ramping
voltage adds to the voltage across the sense resistor,
SL
SL
) connecting the SENSE pin to the current sense
in Figure 2). This current ramp starts at zero right
SENSE
) thus develops a ramping voltage drop.
LTC3803-5
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