Datasheet

LTC3723-1/LTC3723-2
13
372312f
The 50% duty-cycle limit is overcome with the circuit
shown in Figure 7. Operation is similar to external syn-
chronization, except DRVA output is used to terminate its
own clock cycle early. Switching period is now equal to the
oscillator period plus programmable driver dead time.
Maximum on time is equal to oscillator period minus
driver dead time.
Although near 100% duty cycle operation may be of
benefit with non-isolated converters, it is often desirable
to limit the duty cycle of single-ended isolated converters.
Instead of immediately ending the unused clock’s output,
Figure 8 uses a transistor to switch in additional timing
capacitor charge current. This allows one to preset the
maximum duty.
Voltage Mode with LTC3723-2
Figure 9 shows how basic connections differ between
current mode LTC3723-1 and voltage mode LTC3723-2.
Oscillator may be used as the ramp input or the LTC3723-
2 includes an internal 10mA ramp discharge useful when
implementing voltage feedforward. Open loop control in
which the duty cycle varies inversely proportional to input
voltage is shown in Figure 10.
OPERATIO
U
LTC3723-1
DRVA
C
T
68pF
C
T
372312 F07
DRVB
390Ω
BAT54
f
VC
A
T
DPRG
Df
VC
A
T
DPRG
SW
T
MAX SW
T
≅
µ
+
⎛
⎝
⎜
⎞
⎠
⎟
≅
µ
⎛
⎝
⎜
⎞
⎠
⎟
1
256
210
256
210
.•
.•
–
Figure 7. LTC3723-1 > 50% Duty Cycle
LTC3723-1
DRVB
C
T
C
T
TO SYNCHRONOUS
SECONDARY MOSFET
372312 F08
•
SDRBDRVA
V
REF
–V
IN
V
IN
50k
R
MMBT2369
f
VC
AAR
Df
VC
A
T
DPRG
SW
T
MAX SW
T
≅
µ
+
µ+
()
⎛
⎝
⎜
⎞
⎠
⎟
≅
µ
⎛
⎝
⎜
⎞
⎠
⎟
1
256
1
210
1
210 3
256
210
.•
/
.•
–
Figure 8. LTC3723-1 One-Switch Forward or Flyback Converter
(Maximum Duty Cycle 50% to 100%)
Figure 9. LTC3723-1 Current Mode and LTC3723-2 Voltage
Mode Connections
Figure 10. LTC3723-2 Open Loop Control (Duty Cycle is
Inversely Proportional to Input Voltage)
9811
13
372312 F10
C
T
TO INPUT
VOLTAGE
LTC3723-2
RAMPC
T
FB
COMP
R
DPRG
98121
372312 F09
C
T
TO INPUT
VOLTAGE
LTC3723-2
RAMPC
T
DPRGV
REF
R
DPRG
12819
C
T
R
LEB
LTC3723-1
RLEBC
T
V
REF
DPRG
R
DPRG
89121
C
T
LTC3723-2
C
T
RAMP DPRGV
REF