Linear Technology 968A-B Manual de usuario

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B

ISOLATED DC/DC POWER CONVERTER

LT1952

DESCRIPTION

Demonstration circuit 968A-B is isolated input to high

current output 1/8th Brick footprint converter featur-

ing the LT®1952 switching controller. The DC968

converts isolated 18V to 36V input to 5V output and

provides over 20A of output current. The converter

operates at 300kHz with efficiency greater than 93%.

The DC968 can be easily modified to generate output

voltages in the range from 0.6V to 28V. The output

currents are limited by total output power of up to

200W. The other available versions of DC968A are:

DC968A-A 36-75Vin to 3.3V@30A

DC968A-C 36-75Vin to 12V@12A

DC968A-D 36-75Vin to 28V@4A

Also, the DC968 can be modified for other input volt-

ages like 8V-36V, 18V-75V, 44V-52V, and so on. The

wider input voltage range will decrease the converter

efficiency. Therefore, narrow input voltage range will

be more desirable.

The DC968 circuit features soft-start which prevents

output voltage overshoot on startup or when recover-

ing from overload condition.

The DC968 has precise over-current protection circuit

that allows for continuous operation under short cir-

cuit 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 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 IOUT = 0A to 20A 18V

Maximum Input Voltage IOUT = 0A to 20A 36V

VOUT VIN = 18V to 36V, IOUT = 0A to 20A 5V ±3%

Typical Output Ripple VOUT VIN = 18V to 36V, IOUT = 0A to 20A 50mVP–P

Nominal Switching Frequency 300kHz

QUICK START PROCEDURE

Demonstration circuit 968 is easy to set up to evalu-

ate the performance of LT1952 circuit. Refer to Fig-

ure 1 for proper measurement equipment setup and

follow the procedure below:

NOTE:

When measuring the input or output voltage

ripple, care must be taken to avoid a long ground lead

on the oscilloscope probe. Measure the input or out-

put voltage ripple by touching the probe tip directly

across the Vin or Vout and GND terminals. See Figure

2. for proper scope probe technique.

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 968A-B

ISOLATED DC/DC POWER CONVERTER

NOTE:

Make sure that the input voltage does not

exceed 40V.

Check for the proper output voltage.

Vout = 5V.

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, ad-

just the load within the operating range and ob-

serve the output voltage regulation, ripple voltage,

efficiency and other parameters.

The DC968 is equipped with an output capacitor

CSYS (470uF) that approximates typical system

rail capacitance. If system board already has ca-

pacitance of similar value CSYS can be removed.

Figure 1. Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B

ISOLATED DC/DC POWER CONVERTER

Figure 2. Scope Probe Placement for Measuring Input or Output Ripple

CHANGING THE OUTPUT VOLTAGE

To set the output voltage lower than 5V, change the

bottom voltage divider resistor connected to

LT4430 FB pin (see the simplified schematic on

page 5). For example, to get 2.5V output, change

R38 resistor value to 43.2k. However, keep in mind

that changing the transformer as well may increase

the efficiency.

To get higher than 5V output voltages transformer

with higher turns ratio may be required. Also, out-

put MOSFETs with higher voltage ratings may be

required. Please contact LTC factory for details.

PRIMARY MOSFET DRIVER LTC4440

The DC968 has an LTC4440 MOSFET driver U2. The

LTC4440 can be disabled if the efficiency is not im-

portant. If U2 is disabled, the DELAY pin of LT1952

should be readjusted to optimize the efficiency.

Please contact LT factory for assistance.

OPTIONAL RESET CIRCUIT

The DC968 is equipped with an optional active reset

circuit that is disabled on DC968A-B. In most appli-

cations the circuit is not required because there is

no benefit from active reset. However, if the active

reset is required, adding few resistors can enable

the circuit. Please consult LT factory for assistance.

LT1952 DC968A-B 24V in to 5V out

88%

89%

90%

91%

92%

93%

94%

0 5 10 15 20 25

Iout [A]

Efficiency

Figure 3. High efficiency of DC968A-B allows the

board to be used in thermally critical applications

OUTPUT LOAD STEP RESPONSE

The load step response of DC968A-B is very fast

even though relatively small amount of output ca-

pacitance is present (200uF ceramic and 470uF

electrolytic). This is thanks to fast error amplifier of

LT4430, optimal amount of current slope compen-

sation of LT1952, fast opto coupler and fast error

amplifier of LT1952. If higher load steps need to be

handled more output capacitance can be added in

order to keep the voltage transients at the desired

level. The load step transients are shown in Figure

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B

ISOLATED DC/DC POWER CONVERTER

Figure 4. Fast transient response of DC968 is su-

perior to many competing power modules without

the additional output capacitors.

SOFT START FUNCTION

The DC968 features LT4430 opto coupler driver that

has soft start function which produces monotonic

startup ramp shown if Figure 5. The rise time of

output voltage is controlled by capacitor C19 that is

connected to OC (overshoot control) pin of LT4430.

Figure 5. The LT4430 opto coupler driver pro-

duces monotonic output voltage rise at startup

without output voltage overshoot.

DEBUGGING AND TESTING

The DC968 can easily be tested and debugged by

powering the bias circuit separately from the main

power circuit. To place DC968 into debug mode re-

move the resistor R1 and connect 48V, 100mA

power source to +Vb node (right side of R1). By

doing this, the primary PWM controller LT1952 can

be activated without the main primary power being

applied to +Vin.

To activate the secondary side MOSFET driver

(LTC3900) diode OR a 10V, 100mA power source

into collector of Q9.

Once the primary and secondary controllers are

running the MOSFET gate timing can be checked.

If the MOSFET gate timing is correct the main

power input can be applied to +Vin. The delay from

turn on of FG and gate of Q1 should be in the range

from -100ns to +100ns. By slowly increasing the

+Vin from 0V to 48V output voltage and input cur-

rent can be monitored. The input current should not

exceed 100mA without the output load. If one of the

MOSFETs is damaged, the input current will exceed

100mA.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B

ISOLATED DC/DC POWER CONVERTER

1.5n, 200V

Rcs1

0.008R

Cin1

6.8uF, 50V

+Vin

47R

R29

22.1K

Cu1

4.7u

R23 910

R24

82k

Vu1

R15 115k

R20

66.5k

C10

0.1u

Vu1

D10

BAT760

C15

2.2nF, 2kV

C11

10n

R28 196k

R22

13.3k

HAT2244

PA0872.004

GND

MaxDC

FB=1.23V

Comp

Rosc

Vr=2.5V

Blank 9

Delay 12

OC 11

Sync

Isense 10

PGND 13

Out 14

Vin 15

Sout 16

LT1952-1

Sout

R33

158k

C13

.47u

R26

220R

R30

1.2k

R14

33K

C23

1uF

Vfb

R13

22K

R42 0R

Co1

100uF, X7R

+Vo1

R17

560R T2A

PE-68386

Vaux

C14

C12 470p

R19

38.3K

CS-

CS+

Vcc

4FG 5

GND 6

Timer 7

Sync 8

LTC3900

R21

10k

R27

270R

R38

18.2K

R35

137k

R34

4.42k

C19

1uF, X7R

C16

.22u, 10V

C17

4.7n

+Vo1

Vaux R31

10R

Vcc

FB 4

Opto 6

GND

2Comp 5

LT4430

C18

10p

R18

10k

+Vin

Cin2 D5

PDZ9.1B

Vu1

BAS516

R11

33k

-Vin

+Vin

Input

2.2R

R32 332k

CDEP147NP-1R5MC-95

HAT2165

R16

10k

0.22u

B0540W

BCX55

R10

PDZ7.5B

2.2R

10R

Q12

BC857BF

BAS516

Q10

PZTA42

+Vb

1.5mH

PS2801-1

Vu1

D11

BAS516

GND

+Vout

220p

Sout

100p

Csys

470uF, 6.3V

C24

47p

100p

+Vr2

2, 3

4, 5

Inp

Vcc

TS 4

Boost 6

GND

2TG 5

LTC4440

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B

ISOLATED DC/DC POWER CONVERTER

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DC968A-B

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