Datasheet
LTC4267-3
24
42673fa
External Preregulator
The circuit in Figure 13 shows a third way to power the
LTC4267-3 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 LTC4267-3
switching regulator. C
PVCC
should be sized sufficiently to
handle the switching current needed to drive NGATE while
maintaining minimum switching voltage.
Figure 14. Main Loop Compensation for an Isolated Design
R1
R2
C
C
R
Z
TO OPTO-
ISOLATOR
42673 F14
V
OUT
Figure 15. Main Loop Compensation for a Nonisolated Design
LTC4267-3
C
C
R
Z
I
TH
/RUN
PGND
42673 F15
APPLICATIONS INFORMATION
Figure 13. Powering the LTC4267-3 Switching
Regulator with an External Preregulator
V
PORTP
P
VCC
PGND
P
OUT
V
PORTN
LTC4267-3
–48
FROM
PSE
R
START
C
PVCC
+
–
PGND
PGND
PGND
Q1
D1
8.2V
R
B
42673 F15
The external preregulator has improved efficiency over
the simple resistor-shunt regulator method mentioned
previously. R
B
can be selected so that it provides a small
current necessary to maintain the zener diode voltage and
the maximum possible base current Q1 will encounter. The
actual current needed to power the LTC4267-3 switching
regulator goes through Q1 and P
VCC
sources current on
an “as-needed” basis. The static current is then limited
only to the current through R
B
and D1.
Compensating the Main Loop
In an isolated topology, the compensation point is typically
chosen by the components configured around the external
error amplifier. Shown in Figure 14, a series RC network
is connected from the compare voltage of the error am-
plifier to the error amplifier output. In PD designs where
transient load response is not critical, replace R
Z
with a
short. The product of R2 and C
C
should be sufficiently large
to ensure stability. When fast settling transient response
is critical, introduce a zero set by R
Z
C
C
. The PD designer
must ensure that the faster settling response of the output
voltage does not compromise loop stability.
In a nonisolated design, the LTC4267-3 incorporates an
internal error amplifier where the I
TH
/RUN pin serves as
a compensation point. In a similar manner, a series RC
network can be connected from I
TH
/RUN to PGND as
shown in Figure 15. C
C
and R
Z
are chosen for optimum
load and line transient response.