LTC3803-5_15 Datasheet, PDF (10/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
APPLICATIONS INFORMATION
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 prominent
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_SENSE in Figure 2)
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.
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 accord-
ingly. 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 ap-
parent 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_SL in
Figure 2). This current ramp starts at zero right 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_SL)
connecting the SENSE pin to the current sense resistor
(R_SENSE) thus develops a ramping voltage drop. From
the perspective of the SENSE pin, this ramping voltage
adds to the voltage across the sense resistor, effectively
reducing the current comparator threshold in proportion
38035fd
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