Linear Technology DEMONSTRATION CIRCUIT 518 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518
SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER
1
LTC3704
DESCRIPTION
Demonstration circuit 518 is a positive input to
negative output SLIC (Subscriber Line Interface
Circuit) converter featuring the LTC3704 switch-
ing controller. DC518 converts 4.5V to 15V input
to –24V and –70V output and provides over
200mA of output current from a 5V input. The
converter operates at 200kHz with high efficiency
of up to 92%. The DC518 is set to operate with
current sense resistor RS1 and input voltages up
to 15V. However, DC518 can be modified for No-
RsenseTM operation with a 5V input voltage, pro-
vided that MOSFET drain voltage does not exceed
36V (main output set between 0V and -24V). The
circuit features an optional soft-start circuit that can
be enabled or disabled with a single jumper.
DC518 has a jumper to program either continuous
or Burst-Mode operation and can be synchronized
to an external clock of up to 400kHz. Please refer
to the LTC3704 data sheet for design details and
applications information.
The DC518 can produce up to 200mA at both out-
puts from a 5V input source.
Design files for this circuit board are available.
Call the LTC factory.
Table 1.
Performance Summary
PARAMETER CONDITION VALUE
Minimum Input Voltage 4.5V
Maximum Input Voltage 15V
VOUT1 V
IN = 5V to 15V, IOUT1 = 0A to 0.2A -24V ±3%
VOUT2 V
IN = 5V to 15V, IOUT2 = 0A to 0.2A -70V ±10%
Typical Output Ripple VOUT1 V
IN = 5V to 15V, IOUT1 = 0A to 0.2A 200mVP–P
Typical Output Ripple VOUT2 V
IN = 5V to 15V, IOUT2 = 0A to 0.2A 500mVP–P
Nominal Switching Frequency 200kHz
QUICK START PROCEDURE
Demonstration circuit 518 is easy to set up to
evaluate the performance of the LTC3704. Refer to
Figure 1 for proper measurement equipment setup
and follow the procedure below:
NOTE:
When measuring the input or output volt-
age ripple, 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. Place jumper JP1 in the Vin position. The “Pre
Reg” position is an optional bias source for appli-
cations modified for Vin higher than 36V. A bias
circuit including Q4 and D5 is installed on the
board in order to facilitate modifications for higher
Vin.
2. With power off, connect the input power supply to
Vin and GND. Make sure that the input power sup-
ply has sufficient current rating at minimum input
voltage for the required output load.
3. Turn on the power at the input.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518
SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER
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NOTE:
Make sure that the input voltage does not
exceed 15V.
4. Check for the proper output voltages. V
OUT1
= -24V
and V
OUT2
= -70V
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
5. Once the proper output voltage is established,
adjust the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameter
Figure 1. Proper Measurement Equipment Setup
Figure 2. Scope Probe Placement for
Measuring Input or Output Ripple

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518
SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER
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EXTERNAL CLOCK CONECTION
1. To use an external clock source, turn the
input power source Off and remove the Burst
Mode jumper.
2. Set the clock source pulse amplitude to 5V
and set the frequency between 220kHz and
300kHz.
3. Turn the clock source off.
4. Connect the clock source (pulse generator
with TTL output) to the middle pin of the
Burst Mode jumper.
5. Apply input power and observe the switching
frequency of 200kHz.
6. Turn the clock source ON and observe the
switching frequency go up to the switching
frequency of the external clock.
USING NO-RSENSE MODE OF OPERATION
1. No-Rsense
TM
mode of operation can be
used with an input voltage of 5V. The 5V in-
put voltage combined with the –24V main
output will result in a sense pin voltage of
29V, which is within the maximum rating of
the sense pin of the LTC3704 controller. To
use No -Rsense
TM
Mode of operation, the
current sense resistor RS1 should be
shorted out. Resistor R8 must be removed
and resistor R7 should be installed.
2. Short out current sense resistor RS1.
3. Remove resistor R8.
4. Install 0 Ω resistor R7.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518
SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER
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MONITORING MOSFET CURRENT
1. To monitor the MOSFET current through
resistor RS1, connect a 50 Ω coaxial cable
across RS1 with the shield connected to the
ground connection of RS1.
2. Connect the center conductor of the coaxial
cable to the RS1 terminal that is connected
to the source of MOSFET Q1. Use a 50 Ω
surface mount resistor in series with the
center conductor.
3. Set the scope input to 50 Ω and connect the
coaxial cable to the scope.
4. Set the scope to 5mV/division or less. The
current scale with 15m Ω for RS1 and
5mV/div will be .66A/div (50 Ω resistor at the
input of coaxial cable forms a 2:1 divider
with the 50 Ω input impedance of scope).

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518
SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER
5