LTC3723EGN-2 Linear Technology, LTC3723EGN-2 Datasheet - Page 13

• Manufacturer Part Number: LTC3723EGN-2
• Description: IC CTRLR PWM SYNC PUSHPLL 16SSOP
• Manufacturer: Linear Technology

Specifications of LTC3723EGN-2

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 50%
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-SSOP
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Supply: -

OPERATIO
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.
MMBT2369
Figure 8. LTC3723-1 One-Switch Forward or Flyback Converter
(Maximum Duty Cycle 50% to 100%)
50k
68pF
–V
V
390Ω
IN
IN
Figure 7. LTC3723-1 > 50% Duty Cycle
DRVA
C
T
LTC3723-1
DRVA
V
REF
R
C
T
LTC3723-1
U
DRVB
DRVB
C
T
BAT54
C
SDRB
T
D
f
SW
MAX
f
D
SW
MAX
TO SYNCHRONOUS
SECONDARY MOSFET
≅
≅
≅
⎛
⎜
⎝
≅
2 56
2 56
f
SW
.
f
.
210
SW
⎛
⎜
⎝
V C
V C
⎛
⎜
⎝
2 56
µ
•
•
2 56
.
A
210
.
210
T
T
1
V C
⎛
⎜
⎝
+
µ
V C
210
•
µ
A
•
T
A
DPRG
T
1
µ
T
–
A
–
1
T
372312 F07
+
T
⎞
⎟
⎠
DPRG
DPRG
210
µ +
⎞
⎟
⎠
⎞
⎟
⎠
A
1
(
372312 F08
3
/
R
)
⎞
⎟
⎠
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.
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)
C
C
T
T
8
8
C
C
RAMP
DPRG
T
T
LTC3723-1
LTC3723-2
9
9
R
DPRG
LTC3723-1/LTC3723-2
V
V
REF
REF
1
R
C
1
DPRG
T
8
RLEB
DPRG
C
RAMP
T
12
12
R
LTC3723-2
9
LEB
COMP
TO INPUT
VOLTAGE
11
FB
RAMP
372312 F10
TO INPUT
VOLTAGE
13
9
LTC3723-2
C
T
8
C
V
T
REF
1
R
DPRG
DPRG
372312 F09
13
12
372312f