QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776
36V-72VIN, FORWARD CONVERTER
1
LTC3705 and LTC3706
DESCRIPTION
Demonstration circuit 776 is a 36V-72Vin, forward
converter featuring the LTC3705 and LTC3706. This
circuit was designed specifically to attain a high step-
down ratio power supply in one stage in order to ef-
ficiently power 1.5V loads from a typical telecom in-
put voltage range. Isolation voltage is 1500VDC.
This circuit features secondary-side control of the
supply without the need of an optocoupler, self-
starting architecture, input undervoltage lockout, and
output overvoltage protection.
Design files for this circuit board are available.
Call the LTC factory.
,
LTC and LT are registered trademarks of Linear Technology Corporation.
Table 1.
Performance Summary (T
A
= 25°C)
PARAMETER
CONDITION
VALUE
Minimum Input Voltage
36V
Maximum Input Voltage
72V
Output Voltage V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 45A, 400LFM
1.5V ± 1%
Maximum Output Current
400LFM
200LFM
45A
40A
Typical Output Ripple V
OUT
V
IN
= 48V, I
OUT
= 30A (20MHz BW)
30mV
P–P
Line
±0.01%
Output Regulation
Load
±0.18%
Dynamic Response
Peak Deviation
Load Step of 20A to 40A (1A/us min)
Settling Time (to within 15mV of set point)
65mV
50us
Nominal Switching Frequency
215kHz
Efficiency
V
IN
= 48V, I
OUT
= 45A, 400LFM
84.75% Typical
Logic Low Voltage-Off
1V MAX
On/Off Control
Logic High Voltage-On
2.5V MIN
Isolation Voltage
Basic Insulation
1500VDC
OPERATING PRINCIPLES
The LTC3706 controller is used on the secondary and
the LTC3705 driver with self-starting capability is
used on the primary. When an input voltage is ap-
plied, the LTC3705 begins a controlled soft-start of
the output voltage. As this voltage begins to rise, the
LTC3706 secondary controller is quickly powered up
via T1, D3, and Q13. The LTC3706 then assumes
control of the output voltage by sending encoded
