QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 720
HIGH VOLTAGE SYNCHRONOUS STEP-DOWN CONTROLLER
1
LT3800
DESCRIPTION
Demonstration Circuit 720 is a 200kHz high voltage,
synchronous current-mode DC/DC step-down con-
verter featuring the LT3800.
The board operates from a Vin range of 20V – 55V
and outputs 12V @ 6.25A (75W). A soft-start feature
controls the output voltage slew rate at start-up, re-
ducing current surges and voltage overshoots. The
modes of operation, Burst Mode, Discontinuous Cur-
rent Mode and Continuous Current Mode, are jumper
selectable. Both Burst Mode and Discontinuous Cur-
rent Mode increase the efficiency at light loads. Con-
tinuous Current Mode will maintain a constant
switching frequency regardless of the load current
required.
An Optional Boost Bias circuit is provided on the bot-
tom side of the board for back-driving the LT3800
internal regulator from the output voltage. Customers
might want to use this optional circuit with modified
applications that have relatively high input voltages
and low (~ 5V) output voltages. In such applications,
the optional circuit can increase the overall efficiency
by reducing the power lost in the LT3800. The dem-
onstration board has also been layed out with the op-
tion of doubling the switching MOSFETs to facilitate
higher output currents. The circuit design can be
modified for a Boost converter configuration.
This board is suitable for a wide range of industrial
control systems and particularly suitable for 12V/42V
automotive applications and 48V Telecom power sup-
plies.
Design files for this circuit board are available. Call
the LTC factory.
LTC is a trademark of Linear Technology Corporation
Table 1.
Performance Summary (T
A
= 25°C)
PARAMETER CONDITION VALUE
Vin Input Voltage Range 20V 55V
Efficiency
20Vin, 6.25Aout
48Vin, 6.25Aout
55Vin, 6.25Aout
95.6%
93.7%
93.2%
Switching Frequency 200kHz fixed.
Output Voltage 0 – 6.25Aout 12V
Vout_ripple_pp ~ 50mV
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 720
HIGH VOLTAGE SYNCHRONOUS STEP-DOWN CONTROLLER
2
QUICK START PROCEDURE
DC720 is easy to set up to evaluate the performance
of the LT3800. Refer to Figure 1 for proper meas-
urement equipment setup and follow the procedure
below:
1.
Apply a 0 – 6.25A Load across the Vout/Gnd tur-
rets.
2.
Connect Voltmeters and Ammeters as shown in
Fig. 1.
3.
Connect a 0 - 60V Power Supply (or better) across
the Vin/Gnd turrets. Typically it’s best to set the
Current Limit of this Power Supply to its maximum
setting. The power supply must be capable of pro-
viding the required start-up power. If using a bench
power supply, a 60V/15A rated (or better) supply is
recommended. Set Vin to 20V – 55Vin.
4.
Make sure the SHDN/RUN jumper is in the RUN
position.
Figure 1. Proper Measurement Equipment Setup
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 720
HIGH VOLTAGE SYNCHRONOUS STEP-DOWN CONTROLLER
3
