LTC3722-1 LINER [Linear Technology], LTC3722-1 Datasheet - Page 16

• Manufacturer Part Number: LTC3722-1
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC3722-1/LTC3722-2
Programming Undervoltage Lockout
The LTC3722-1/LTC3722-2 provides undervoltage lock-
out (UVLO) control for the input DC voltage feed to the
power converter in addition to the V
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 10 A current is enabled which
flows out of UVLO into the voltage divider connected to
UVLO. The amount of DC feed hysteresis provided by this
current is: 10 A • R
threshold is: 5V • {(R
voltage applied to UVLO is present and greater than 5V
prior to the V
UVLO logic will prevent output switching until the follow-
ing three conditions are met: (1) V
V
UVLO can also be used to enable and disable the power
converter. An open drain transistor connected to UVLO as
shown in Figure 6 provides this capability.
Off-Line Bias Supply Generation
If a regulated bias supply is not available to provide V
voltage to the LTC3722-1/LTC3722-2 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 7a). The advantage of this approach is that it
16
REF
is in regulation and (3) UVLO pin is greater than 5V.
ON OFF
CC
UVLO circuitry activation, then the internal
Figure 6. System UVLO Setup
U
TOP
TOP
, see Figure 6. The system UVLO
+ R
R
R
TOP
BOTTOM
BOTTOM
CC
UVLO is enabled, (2)
UVLO
)/R
CC
3722 F0A
BOTTOM
UVLO function
}. If the
CC
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
product offerings as well. A different approach is to add a
winding to the output inductor and peak detect and filter
the square wave signal (see Figure 7b). The polarity of this
winding is designed so that the positive voltage square
wave is produced while the output inductor is freewheel-
ing. An advantage of this technique over the previous is
that it does not require a separate filter inductor and since
the voltage is derived from the well-regulated output
voltage, it is also well controlled. One disadvantage is that
this winding will require the same safety isolation that is
required for the main transformer. Another disadvantage
is that a much larger V
it does not generate a voltage as the output is first starting
up, or during short-circuit conditions.
Programming the LTC3722-1/LTC3722-2 Oscillator
The high accuracy LTC3722-1/LTC3722-2 oscillator cir-
cuit provides flexibility to program the switching fre-
quency, slope compensation, and synchronization with
minimal external components. The LTC3722-1/LTC3722-2
V
V
IN
CC
Figure 7a. Auxiliary Winding Bias Supply
R
Figure 7b. Output Inductor Bias Supply
START
0.1 F
15V*
ISO BARRIER
*OPTIONAL
CC
C
R
HOLD
filter capacitor is needed, since
START
+
V
L
IN
OUT
C
HOLD
2k
1922 F05a
+
1922 F05b
V
V
CC
OUT
0.1 F
372212i