LTC4267-3
23
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applicaTions inForMaTion
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
LTC4267-3 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
LTC4267-3 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.
Resistor R
START
should be selected to yield a worst-case
minimum charging current greater that the maximum rated
LTC4267-3 start-up current to ensure there is enough cur-
rent to charge C
PVCC
to the P
VCC
turn-on threshold. R
START
should also be selected large enough to yield a worst-case
maximum charging current less than the minimum-rated
P
VCC
supply current, so that in operation, most of the
P
VCC
current is delivered through the third winding. This
results in the highest possible efficiency.
Capacitor C
PVCC
should then be made large enough to avoid
the relaxation oscillation behavior described previously.
This is difficult 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
VCC
turn-off
threshold. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated P
VCC
clamp voltage to avoid running up against the LTC4267-3
shunt regulator, needlessly wasting power.
P
VCC
Shunt Regulator
In applications including a third transformer winding,
the internal P
VCC
shunt regulator serves to protect the
LTC4267-3 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 LTC4267-3 switching
regulator to be powered through a single dropping resistor
from V
PORTP
as shown in Figure 12. This simplicity comes
at the expense of reduced efficiency due to static power
dissipation in the R
START
dropping resistor.
The shunt regulator can sink up to 5mA through the P
VCC
pin to PGND. The values of R
START
and C
PVCC
must be
selected for the application to withstand the worst-case
load conditions and drop on P
VCC
, ensuring that the P
VCC
turn-off threshold is not reached. C
PVCC
should be sized
sufficiently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
Figure 12. Powering the LTC4267-3 Switching
Regulator via the Shunt Regulator
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267-3
–48
FROM
PSE
R
START
C
PVCC
+
–
PGND
42673 F14
