QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 759
48VIN TO 5VOUT, 20A ISOLATED TELECOM DC/DC CONVERTER
1
LT1952
DESCRIPTION
Demonstration circuit 759 is an isolated input to high
current output converter featuring the LT®1952
switching controller. The DC759 converts isolated
36V to 75V input to 5V output and provides over 20A
of output current. The converter operates at 300kHz
with high efficiency of 92%. The DC759 can be easily
modified to generate output voltages in the range
from 1.23V to 28V. The output currents are limited by
total output power of up to 140W.
Demonstration circuit 759 can generate output cur-
rents as high as 30A with some modifications. The
output voltage can also be changed to any value be-
tween 1.23V and 28V by changing the MOSFETs,
power transformer, output inductor and output ca-
pacitors. Please consult LTC factory for details.
The DC759 circuit features soft-start which prevents
output voltage overshoot on startup or when recover-
ing from overload condition.
The DC759 has a precise over-current protection cir-
cuit that allows for continuous operation under short
circuit conditions. The low power dissipation under
short circuit conditions insures high reliability even
during short circuits.
The LT1952 can be synchronized to an external clock
of up to 400kHz. Please refer to the LT1952 data
sheet for design details and applications information.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
Table 1.
Performance Summary
PARAMETER CONDITION VALUE
Minimum Input Voltage 36V
Maximum Input Voltage 75V
V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 20A 5V ±3%
Typical Output Ripple V
OUT
V
IN
= 36V to 72V, I
OUT
= 0A to 20A 50mV
P–P
Nominal Switching Frequency 300kHz
QUICK START PROCEDURE
Demonstration circuit 759 is easy to set up to evaluate
the performance of the LT1952. Refer to Figure 1. for
proper measurement equipment setup and follow the
procedure below:
NOTE:
When measuring the input or output voltage rip-
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.
1.
With power off, connect the input power supply to
+Vin and -Vin. Make sure that the input power sup-
ply has sufficient current rating at minimum input
voltage for the required output load.
2.
Turn on the power at the input.
NOTE:
Make sure that the input voltage does not ex-
ceed 75V.
3.
Check for the proper output voltage.
Vout = 5V.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 759
48VIN TO 5VOUT, 20A ISOLATED TELECOM DC/DC CONVERTER
2
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
4.
Once the proper output voltage is established, ad-
just the load within the operating range and observe
the output voltage regulation, ripple voltage, effi-
ciency and other parameters.
5.
The DC759 is equipped with an output capacitor
CSYS (470uF) that approximates typical system rail
capacitance. If system board already has capaci-
tance of similar value, CSYS can be removed.
CHANGING THE OUTPUT VOLTAGE
1.
To set the output voltage lower than 5V, change the
bottom voltage divider resistor connected to
LTC1698 Vfb pin (R31). For example, to get 3.3V
output, change R31 resistor value to 22.6k.
2.
To get higher than 5V output voltages a transformer
with higher turns ratio will be required. Also, an
output MOSFET with higher voltage ratings may be
required. Please contact LTC factory for details.
OUTPUT CAPACITORS
The DC759 is equipped with an output capacitor CSYS
(470uF) that approximates typical system rail ca-
pacitance
USING MOSFET BUFFERS
The optional MOSFET buffer Q7 can improve the effi-
ciency if a MOSFET with high Qg is used. By default,
the MOSFET buffer is disabled. To test the perform-
ance with the buffer, perform the following steps.
1.
Turn the power off.
2.
Change the base-emitter resistor R33 to 10ohm.
3.
Turn the power on.
4.
Measure the efficiency.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 759
48VIN TO 5VOUT, 20A ISOLATED TELECOM DC/DC CONVERTER
3
Figure 1. Proper Measurement Equipment Setup
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 759
48VIN TO 5VOUT, 20A ISOLATED TELECOM DC/DC CONVERTER
4
