QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 610
DUAL OUTPUT ISOLATED POWER SUPPLY
LT3804, LT3781, and LTC1693-1
DESCRIPTION
Demonstration circuit 610 is a dual output high effi-
ciency isolated DC/DC power supply with 36V to 72V
input range. It has two outputs: 3.3V at 15A max and
1.8V at 15A max. The LT3804, a secondary side dual
output controller, is used to tightly regulate both out-
puts. True differential Kelvin sensing is used for each
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
V
OUT1
V
IN
= 36V to 72V, I
OUT1
= 0A to 15A,
I
OUT2
= 0A to 15A
V
OUT2
V
IN
= 36V to 72V, I
OUT1
= 0A to 15A,
I
OUT2
= 0A to 15A
Typical Output Ripple V
OUT1
Typical Output Ripple V
OUT2
Output Regulation
Load
Nominal Switching Frequency
Efficiency
V
IN
= 48V, I
OUT1
= I
OUT2
= 15A
V
IN
= 48V, I
OUT1
= I
OUT2
= 15A. With output current sense
resistor R55=R49=0 ohm
Power Good (high)
On/Off Control
Logic High Voltage-On
Isolation Voltage
Basic Insulation
1V MIN
1500VDC
Normal operation (V
OUT1
= 3.3V and V
OUT2
= 1.8V)
Logic Low Voltage-Off
±0.2%
230kHz
85% Typical
87% Typical
9.3V Typical
0.6V MAX
V
IN
= 48V, I
OUT1
= 15A (20MHz BW)
V
IN
= 48V, I
OUT2
= 15A (20MHz BW)
Line
30mV
P–P
15mV
P–P
±0.1%
1.81V ±2%
CONDITION
VALUE
36V
72V
3.31V ±2%
output feedback amplifier to achieve high regulation ac-
curacy. The complete design is mounted within a stan-
dard half brick PCB area with about half inch height.
Design files for this circuit board are available. Call
the LTC factory.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 610
DUAL OUTPUT ISOLATED POWER SUPPLY
OPERATING PRINCIPLES
Demonstration circuit 610 features the LT3804, LT3781
and LTC1693-1 controllers. The basic power stage to-
pology is a two-switch forward converter with synchro-
nous rectification. The primary side controller uses an
LT3781, a current mode two-switch forward controller
with built-in MOSFET drivers. On the secondary side, the
LT3804 is used to provide the voltage feedback for the
3.3V output (Vo1). The output from the built-in opto-
coupler driver is fed into an opto-coupler (MOC207) and
then transferred to LT3781 on the primary side to com-
plete the 3.3V regulation. An LTC1693-1 high speed dual
N-channel MOSFET driver provides the gate driving for
the synchronous MOSFETs at the 3.3V output stage. The
LTC1693-1 driver’s input signals come from SG and BG
outputs of the LT3781 through two small gate drive
transformers (T2 and T3).
The LT3804 also precisely regulates the 1.8V output
(Vo2) by further reducing and controlling the duty cycle
of the switching waveform from the power transformer
(T1) secondary winding. In fact, the 1.8V circuit is a
special synchronous buck converter whose input is a
pulsed waveform instead of a DC voltage.
True differential remote sensing is provided for both
outputs to achieve high regulation accuracies. Power
good indicator PGOOD, will be high only if both outputs
are within +/-10% of their nominal values.
The LT3804 provides output current limit function for
both 1.8V and 3.3V outputs. The current limits for 3.3V
and 1.8V outputs are estimated to be 50mV/R55 and
50mV/R49, respectively.
A planar transformer PA0191 by Pulse Engineering is
employed as the power transformer in this design. This
transformer is constructed on a PQ20 core with nine
turns of primary windings, two turns of secondary wind-
ings and seven turns of auxiliary windings for the
LT3781 bias supply. Si7892DP MOSFETs are selected for
the secondary side due to their low Rds (on), 30V V
DSS
rating and the compact and thermally enhanced Power-
Pak SO-8 package.
The switching frequency of the circuit is about 230KHz.
1500V input to output isolation is provided. Additional
features of this design include primary side on/off con-
trol, input over voltage protection, under voltage lockout,
soft start and board over temperature shutdown.
Without airflow, the hottest spot on the demo board has
a temperature rise about 75°C at 48Vin and full loads.
Airflow is required at heavy load with elevated tempera-
ture operation.
QUICK START PROCEDURE
Demonstration circuit 610 is easy to set up to evaluate
the performance of the LT3804, LT3781, and LTC1693-1.
Refer to Figure 1 for proper measurement equipment
setup and follow the procedure below:
NOTE:
When measuring the input or output voltage rip-
1.
With power off, connect the input power supply to
+Vin (36V-72V) and –Vin (input return).
2.
The on/off control pin should be left open for normal
operation. (Connecting the on/off pin to the –Vin will
turn off the converter.)
3.
Connect the 3.3V load (Load 1 in Figure 1) between
ple, care must be taken to avoid a long ground lead on
the oscilloscope probe. Measure the input or output
voltage ripple by touching the probe tip directly across
the Vin or Vout and GND terminals. See Figure 2 for
proper scope probe technique.
+Vo1 and Vo RTN; connect the 1.8V load (Load 2 in
Figure 1) between +Vo2 and Vo RTN. (Initial loads: no
load.)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 610
DUAL OUTPUT ISOLATED POWER SUPPLY
4.
Connect remote sense pins to loads: Vo1s+ to load
6.
Turn on the input power supply and check for the
1+; Vo1s- to load 1-; Vo2s+ to load 2+; Vo2s- to load
2- (see Figure 1).
Note: If remote sense function is not used, then con-
nect Vo1s- and Vo2s- to Vo RTN; connect Vo1s+ to
+Vo1; and connect Vo2s+ to +Vo2.
5.
Connect the DVMs to the input and outputs.
proper output voltages. Vo1 should be 3.31V+/-2%.
Vo2 should be 1.81V+/-2%.
7.
Once the proper output voltages are established, ad-
just the loads within the operating range and observe
the output voltage regulation, ripple voltage and other
parameters.
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
VIN
GND
3
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