Datasheet

LTC4267-3
21
42673fa
Output Transformer Design Considerations
Since the external feedback resistor divider sets the
output voltage, the PD designer has relative freedom in
selecting the transformer turns ratio. The PD designer
can use simple ratios of small integers (i.e. 1:1, 2:1, 3:2)
which yields more freedom in setting the total turns and
mutual inductance and may allow the use of an off the
shelf transformer.
Transformer leakage inductance on either the primary or
secondary causes a voltage spike to occur after the output
switch (Q1 in Figure 11) turns off. The input supply volt-
age plus the secondary-to-primary referred voltage of the
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating. This spike
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 overvolt-
age breakdown at the MOSFET’s drain node. Application
Note 19 is a good reference for snubber design.
Current Sense Resistor Consideration
The external current sense resistor (R
SENSE
in Figure 11)
allows the designer to optimize the current limit behavior
for a particular application. As the current sense resistor
is varied from several ohms down to tens of milliohms,
peak swing current goes from a fraction of an ampere to
several amperes. Care must be taken to ensure proper
circuit operation, especially for small current sense resis-
tor values.
Choose R
SENSE
such that the switching current exercises
the entire range of the I
TH
/RUN voltage. The nominal voltage
range is 0.7V to 1.9V and R
SENSE
can be determined by
experiment. The main loop can be temporarily stabilized
by connecting a large capacitor on the power supply. Apply
the maximum load current allowable at the power sup-
ply output based on the class of the PD. Choose R
SENSE
such that I
TH
/RUN approaches 1.9V. Finally, exercise the
output load current over the entire operating range and
ensure that I
TH
/RUN voltage remains within the 0.7V to
1.9V range. Layout is critical around the R
SENSE
resistor.
For example, a 0.020Ω sense resistor, with one milliohm
(0.001Ω) of parasitic resistance will cause a 5% reduction
in peak switch current. The resistance of printed circuit
copper traces cannot necessarily be ignored and good
layout techniques are mandatory.
Programmable Slope Compensation
The LTC4267-3 switching regulator injects a ramping
current through its SENSE pin into an external slope
compensation resistor (R
SL
in Figure 11). This current
ramp starts at zero after the NGATE pin has been high for
the LTC4267-3’s minimum duty cycle of 6%. 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
) develops a ramping volt-
age drop. From the perspective of the LTC4267-3 SENSE
pin, this ramping voltage adds to the voltage across the
sense resistor, effectively reducing the current comparator
threshold in proportion to duty cycle. This stabilizes the
control loop against subharmonic oscillation. The amount
of reduction in the current comparator threshold (∆V
SENSE
)
can be calculated using the following equation:
ΔV
SENSE
=5µA•R
SL
•[(DutyCycle–6%)/74%]
Note: The LTC4267-3 enforces 6% < Duty Cycle < 80%.
Designs not needing slope compensation may replace R
SL
with a short-circuit.
Applications Employing a Third Transformer Winding
A standard operating topology may employ a third wind-
ing on the transformer’s primary side that provides power
to the LTC4267-3 switching regulator via its P
VCC
pin
(Figure 11). However, this arrangement is not inherently
self-starting. Start-up is usually implemented by the use of
an external “trickle-charge” resistor (R
START
) in conjunc-
tion with the internal wide hysteresis undervoltage lockout
circuit that monitors the P
VCC
pin voltage.
APPLICATIONS INFORMATION