LTC4267CGNPBF LINEAR TECH, LTC4267CGNPBF Datasheet - Page 23

• Manufacturer Part Number: LTC4267CGNPBF
• Manufacturer: LINEAR TECH

APPLICATIO S I FOR ATIO
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267 switching regulator circuit. An external series
preregulator consists of a series pass transistor Q1, zener
diode D1, and a bias resistor R
P
turn-off threshold of 6.8V. Resistor R
charges the P
enabling the switching regulator. The voltage on C
begins to decline as the switching regulator draws its
normal supply current, which exceeds the delivery of
R
the desired value. By this time, the pass transistor Q1
catches the declining voltage on the P
virtually all the supply current required by the LTC4267
switching regulator. C
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
The external preregulator has improved effi ciency over
the simple resistor-shunt regulator method mentioned
previously. R
current necessary to maintain the zener diode voltage and
the maximum possible base current Q1 will encounter. The
The shunt regulator can sink up to 5mA through the P
pin to PGND. The values of R
selected for the application to withstand the worst-case
load conditions and drop on P
turn-off threshold is not reached. C
suffi ciently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
VCC
START
at 7.6V nominal, well above the maximum rated P
. After some time, the output voltage approaches
Figure 12. Powering the LTC4267 Switching
Regulator via the Shunt Regulator
FROM
B
– 48
PSE
VCC
can be selected so that it provides a small
node up to the P
+
–
U
PVCC
U
V
PORTN
should be sized suffi ciently to
LTC4267
VCC
V
START
B
PORTP
PGND
P
P
. The preregulator holds
VCC
OUT
, ensuring that the P
VCC
W
PVCC
VCC
and C
START
turn-on threshold,
pin, and provides
should be sized
R
PGND
PVCC
START
C
momentarily
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PVCC
U
must be
PVCC
VCC
VCC
VCC
actual current needed to power the LTC4267 switching
regulator goes through Q1 and P
an “as-needed” basis. The static current is then limited
only to the current through R
Compensating the Main Loop
In an isolated topology, the compensation point is typically
chosen by the components confi gured around the external
error amplifi er. Shown in Figure 14, a series RC network
is connected from the compare voltage of the error am-
plifi er to the error amplifi er output. In PD designs where
transient load response is not critical, replace R
short. The product of R2 and C
to ensure stability. When fast settling transient response
is critical, introduce a zero set by R
must ensure that the faster settling response of the output
voltage does not compromise loop stability.
In a nonisolated design, the LTC4267 incorporates an
internal error amplifi er where the I
a compensation point. In a similar manner, a series RC
network can be connected from I
shown in Figure 15. C
load and line transient response.
Figure 14. Main Loop Compensation for an Isolated Design
FROM
– 48
PSE
ISOLATOR
Figure 13. Powering the LTC4267 Switching
Regulator with an External Preregulator
TO OPTO-
+
–
V
PORTN
LTC4267
V
PORTP
PGND
P
P
C
OUT
VCC
and R
R
Z
B
C
8.2V
Z
PGND
should be suffi ciently large
C
D1
and D1.
C
R
are chosen for optimum
B
VCC
TH
PGND
Z
TH
C
/RUN pin serves as
sources current on
C
/RUN to PGND as
. The PD designer
V
LTC4267
Q1
OUT
PGND
C
R2
R1
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PVCC
R
START
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Z
23
with a
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