Datasheet

LTC4267-1
23
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Resistor R
START
should be selected to yield a worst-case
minimum charging current greater that the maximum rated
LTC4267-1 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 LTC4267-1
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
LTC4267-1 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 LTC4267-1 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.
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267-1 switching regulator circuit. An external series
preregulator consists of a series pass transistor Q1, zener
diode D1, and a bias resistor R
B
. The preregulator holds
P
VCC
at 7.6V nominal, well above the maximum rated P
VCC
turn-off threshold of 6.8V. Resistor R
START
momentarily
charges the P
VCC
node up to the P
VCC
turn-on threshold,
enabling the switching regulator. The voltage on C
PVCC
begins to decline as the switching regulator draws its
normal supply current, which exceeds the delivery of
R
START
. After some time, the output voltage approaches
the desired value. By this time, the pass transistor Q1
catches the declining voltage on the P
VCC
pin, and provides
virtually all the supply current required by the LTC4267-1
switching regulator. C
PVCC
should be sized sufficiently to
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
Figure 12. Powering the LTC4267-1 Switching
Regulator via the Shunt Regulator
Figure 13. Powering the LTC4267-1 Switching
Regulator with an External Preregulator
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267-1
–48
FROM
PSE
R
START
C
PVCC
+
–
PGND
42671 F14
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267-1
–48
FROM
PSE
R
START
C
PVCC
+
–
PGND
PGND
PGND
Q1
D1
8.2V
R
B
42671 F15