Datasheet

LTC3723-1/LTC3723-2
12
372312f
OPERATIO
U
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 4b). 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
CC
filter capacitor is needed, since
it does not generate a voltage as the output is first starting
up, or during short-circuit conditions.
Figure 4b. Output Inductor Bias Supply
372312 F04b
V
CC
V
OUT
V
IN
C
HOLD
R
START
+
1µF
L
OUT
ISO BARRIER
Programming the LTC3723-1/LTC3723-2 Oscillator
The high accuracy LTC3723-1/LTC3723-2 oscillator cir-
cuit provides flexibility to program the switching fre-
quency and slope compensation required for current
mode control (LTC3723-1). The oscillator circuit pro-
duces a 2.35V peak-to-peak amplitude ramp waveform on
C
T
. Typical maximum duty cycles of 49% are possible. The
oscillator is capable of operation up to 1MHz by the
following equation:
C
T
= 1/(14.8k • F
OSC
)
Note that this is the frequency seen on C
T
. The output
drivers switch at 1/2 of this frequency. Also note that
higher switching frequency and added driver dead-time
via DPRG will reduce the maximum duty cycle.
The LTC3723-1/LTC3723-2 can be synchronized to an
external frequency source such as another PWM chip. In
Single-Ended Operation
In addition to push-pull and full-bridge topologies, single-
ended topologies such as the forward and flyback con-
verter can benefit from the many advanced features of the
LTC3723. In Figure 6, the LTC3723 is used with the
LTC4440, 100V high side driver to implement a two-
transistor forward converter. DRVB is used which limits
the converter’s maximum duty cycle to 50% (less pro-
grammable driver dead time).
LTC3723
C
T
f
OSC
< f
EXT
< 1.25 • f
OSC
f
SW
= f
OSC
/2 f
OSC
≅
68pF
C
T
372312 F05
390Ω
BAT54
210µA
2.56V •
C
T
EXTERNAL
FREQUENCY
SOURCE
Figure 5. Synchronization from External Source
Figure 6. Two-Transistor Forward Converter (Duty Cycle < 50%)
210µA
2 • 2.56V • C
T
LTC3723
DRVB
C
T
GND
C
T
IN
TO SYNCHRONOUS
SECONDARY MOSFET
TS
LTC4440
372312 F06
GND
TG
•
SDRB
V
IN
f
SW
≅
–V
IN
Figure 5, the leading edge of an external pulse is used to
terminate the natural clock cycle. If the external frequency
is higher than the oscillator frequency, the internal oscil-
lator will synchronize with the external input frequency.