LTC3722-2_15 Datasheet, PDF (16/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
Regulated bias supplies as low as 7V can be utilized to
provide bias to the LTC3722-1/LTC3722-2. Figure 5 shows
various bias supply configurations.
12V ±10%
1.5k
V_CC
1N5226
3V
0.1µF
V_BIAS < V_UVLO^VIN
1N914 R_START
+
0.1µF
V_CC
C_HOLD
372212 F05
Figure 5. Bias Configurations
Programming Undervoltage Lockout
The LTC3722-1/LTC3722-2 provides undervoltage lockout
(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 commences. At
the same time a 10µA 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:
10µA • R_TOP , see Figure 6. 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 following 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 6, provides this capability.
Off-Line Bias Supply Generation
If a regulated bias supply is not available to provide V_CC
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
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
ON OFF
R_TOP
UVLO
R_BOTTOM
372212 F06
Figure 6. System UVLO Setup
V_IN
R_START
2k
+
15V*
C_HOLD
V_CC
0.1µF
*OPTIONAL
372212 F07a
Figure 7a. Auxiliary Winding Bias Supply
V_IN
R_START
L_OUT
ISO BARRIER
V_OUT
+
0.1µF
C_HOLD
V_CC
372212 F07b
Figure 7b. Output Inductor Bias Supply
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