LTC4267-1
21
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
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 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
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 LTC4267-1 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 LTC4267-1’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 LTC4267-1 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 (ΔV
SENSE
)
can be calculated using the following equation:
∆V
SENSE
= 5µA • R
SL
• [(Duty Cycle – 6%)/74%]
Note: The LTC4267-1 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 LTC4267-1 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.
R
START
is connected to V
PORTP
and supplies a current,
typically 100µA, to charge C
PVCC
. After some time, the
voltage on C
PVCC
reaches the P
VCC
turn-on threshold. The
LTC4267-1 switching regulator then turns on abruptly and
draws its normal supply current. The NGATE pin begins
switching and the external MOSFET (Q1) begins to deliver
power. The voltage on C
PVCC
begins to decline as the
switching regulator draws its normal supply current,
which
exceeds the delivery from R
START
. After some time, typically
tens of milliseconds, the output voltage approaches the
desired value. By this time, the third transformer winding
is providing virtually all the supply current required by the
LTC4267-1 switching regulator.
One potential design pitfall is under-sizing the value of
capacitor C
PVCC
. In this case, the normal supply current
drawn through P
VCC
will discharge C
PVCC
rapidly before the
third winding drive becomes effective. Depending on the
particular situation, this may result in either several off-on
cycles before proper operation is reached or permanent
relaxation oscillation at the P
VCC
node.