Linear Technology DEMONSTRATION CIRCUIT 518 Manuel utilisateur

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518

SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER

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.

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.

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.

Turn on the power at the input.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518

SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER

NOTE:

Make sure that the input voltage does not

exceed 15V.

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.

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

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

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

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.

Install 0

Ω

resistor R7.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 518

SLIC POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER

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

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