ltc4267-3 Linear Technology Corporation, ltc4267-3 Datasheet - Page 21

• Manufacturer Part Number: ltc4267-3
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated 300khz Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
collector resistor of the optoisolator can be selected for
an increase in bandwidth at the cost of a reduction in
gain of this stage.
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
fl yback 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 overvoltage
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
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
the entire range of the I
age range is 0.7V to 1.9V and R
SENSE
such that the switching current exercises
U
TH
/RUN voltage. The nominal volt-
U
SENSE
W
SENSE
can be determined
in Figure 11)
U
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
current sense resistor (R
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
can be calculated using the following equation:
Note: The LTC4267-3 enforces 6% < Duty Cycle < 80%.
Designs not needing slope compensation may replace
R
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
such that I
output load current over the entire operating range and
ensure that I
1.9V range. Layout is critical around the R
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
ramp starts at zero after the NGATE pin has been high for
∆V
SL
with a short-circuit.
SENSE
TH
= 5µA • R
TH
/RUN approaches 1.9V. Finally, exercise the
/RUN voltage remains within the 0.7V to
SL
SL
) connecting the SENSE pin to the
• [(Duty Cycle – 6%)/74%]
SENSE
SL
in Figure 11). This current
) develops a ramping volt-
LTC4267-3
SENSE
resistor.
21
SENSE
SENSE
42673f
)