Datasheet

LTC3723-1/LTC3723-2
11
372312f
very low (145µA typ) start-up current that allows the use
of 1/8W to 1/4W trickle charge start-up resistors.
The trickle charge resistor should be selected as follows:
R
START(MAX)
= V
IN(MIN)
– 10.7V/250µA
Adding a small safety margin and choosing standard
values yields:
APPLICATION V
IN
RANGE R
START
DC/DC 36V to 72V 100k
Off-Line 85V to 270V
RMS
430k
PFC Preregulator 390V
DC
1.4M
V
CC
should be bypassed with a 0.1µF to 1µF multilayer
ceramic capacitor to decouple the fast transient currents
demanded by the output drivers and a bulk tantalum or
electrolytic capacitor to hold up the V
CC
supply before the
bootstrap winding, or an auxiliary regulator circuit takes
over.
C
HOLDUP
= (I
CC
+ I
DRIVE
) • t
DELAY
/3.8V
(minimum UVLO hysteresis)
Regulated bias supplies as low as 7V can be utilized to
provide bias to the LTC3723-1/LTC3723-2. Refer to
Figure 2 for various bias supply configurations.
Figure 2. Bias Configurations
372312 F02
12V ±10%
1.5k
V
CC
V
IN
V
CC
C
HOLD
1N5226
3V
1µF
1µF
V
BIAS
< V
UVLO
R
START
1N914
+
Figure 3. System UVLO Setup
ON OFF
R
BOTTOM
R
TOP
UVLO
372312 F03
OPERATIO
U
UVLO. The amount of DC feed hysteresis provided by this
current is: 10µA • R
TOP
, (Figure 3). The system UVLO
threshold is: 5V • {(R
TOP
+ R
BOTTOM
)/R
BOTTOM
}. If the
voltage applied to UVLO is present and greater than 5V
prior to the V
CC
UVLO circuitry activation, then the internal
UVLO logic will prevent output switching until the follow-
ing three conditions are met: (1) V
CC
UVLO is enabled, (2)
V
REF
is in regulation and (3) UVLO pin is greater than 5V.
UVLO can also be used to enable and disable the power
converter. An open drain transistor connected to UVLO as
shown in Figure 3 provides this capability.
Programming Undervoltage Lockout
The LTC3723-1/LTC3723-2 provides undervoltage lock-
out (UVLO) control for the input DC voltage feed to the
power converter in addition to the V
CC
UVLO function
described in the preceding section. Input DC feed UVLO is
provided with the UVLO pin. A comparator on UVLO
compares a divided down input DC feed voltage to the 5V
precision reference. When the 5V level is exceeded on
UVLO, the SS pin is released and output switching com-
mences. At the same time a 10µA current is enabled which
flows out of UVLO into the voltage divider connected to
Off-Line Bias Supply Generation
If a regulated bias supply is not available to provide V
CC
voltage to the LTC3723-1/LTC3723-2 and supporting
circuitry, one must be generated. Since the power require-
ment is small, approximately 1W, and the regulation is not
critical, a simple open-loop method is usually the easiest
and lowest cost approach. One method that works well is
to add a winding to the main power transformer, and post
regulate the resultant square wave with an L-C filter (see
Figure 4a). The advantage of this approach is that it
maintains decent regulation as the supply voltage varies,
and it does not require full safety isolation from the input
winding of the transformer. Some manufacturers include
a primary winding for this purpose in their standard
Figure 4a. Auxiliary Winding Bias Supply
372312 F04a
+
V
CC
V
IN
C
HOLD
1µF
R
START
2k
15V*
*OPTIONAL