LTC4267CGNPBF LINEAR TECH, LTC4267CGNPBF Datasheet - Page 22

• Manufacturer Part Number: LTC4267CGNPBF
• Manufacturer: LINEAR TECH

Programmable Slope Compensation
The LTC4267 switching regulator injects a ramping current
through its SENSE pin into an external slope compensation
resistor (R
zero after the NGATE pin has been high for the LTC4267’s
minimum duty cycle of 6%. The current rises linearly to-
wards a peak of 5µA at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resis-
tor (R
resistor (R
the perspective of the LTC4267 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
calculated using the following equation:
Note: The LTC4267 enforces 6% < Duty Cycle < 80%.
Designs not needing slope compensation may replace R
with a short-circuit.
Applications Employing a Third Transformer Winding
A standard operating topology may employ a third
winding on the transformer’s primary side that provides
power to the LTC4267 switching regulator via its P
(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
tion with the internal wide hysteresis undervoltage lockout
circuit that monitors the P
R
typically 100µA, to charge C
voltage on C
LTC4267 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
switching regulator draws its normal supply current, which
exceeds the delivery from R
tens of milliseconds, the output voltage approaches the
desired value. By this time, the third transformer winding
LTC4267
22
APPLICATIO S I FOR ATIO
START
∆V
SENSE
SL
is connected to V
) connecting the SENSE pin to the current sense
SENSE
= 5µA • R
SL
PVCC
in Figure 11). This current ramp starts at
) develops a ramping voltage drop. From
reaches the P
U
SL
• [(Duty Cycle – 6%)/74%]
PVCC
PORTP
U
VCC
START
PVCC
pin voltage.
VCC
begins to decline as the
. After some time, typically
and supplies a current,
. After some time, the
turn-on threshold. The
W
START
SENSE
) in conjunc-
U
) can be
VCC
pin
SL
is providing virtually all the supply current required by the
LTC4267 switching regulator.
One potential design pitfall is under-sizing the value of
capacitor C
drawn through P
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
Resistor R
minimum charging current greater that the maximum rated
LTC4267 start-up current to ensure there is enough current
to charge C
should also be selected large enough to yield a worst-case
maximum charging current less than the minimum-rated
P
P
results in the highest possible effi ciency.
Capacitor C
the relaxation oscillation behavior described previously.
This is diffi cult to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The third transformer winding should be designed so
that its output voltage, after accounting for the forward
diode voltage drop, exceeds the maximum P
threshold. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated P
clamp voltage to avoid running up against the LTC4267
shunt regulator, needlessly wasting power.
P
In applications including a third transformer winding,
the internal P
LTC4267 switching regulator from overvoltage transients
as the third winding is powering up.
If a third transformer winding is undesirable or unavail-
able, the shunt regulator allows the LTC4267 switching
regulator to be powered through a single dropping resistor
from V
at the expense of reduced effi ciency due to static power
dissipation in the R
VCC
VCC
VCC
current is delivered through the third winding. This
Shunt Regulator
supply current, so that in operation, most of the
PORTP
START
PVCC
PVCC
PVCC
as shown in Figure 12. This simplicity comes
VCC
should then be made large enough to avoid
. In this case, the normal supply current
VCC
should be selected to yield a worst-case
to the P
shunt regulator serves to protect the
START
will discharge C
dropping resistor.
VCC
VCC
turn-on threshold. R
node.
PVCC
rapidly before the
VCC
turn-off
START
4267fc
VCC