Linear Technology LT1952-1 Manuale utente

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

LT1952-1

DESCRIPTION

Demonstration circuit 1317A-F is isolated input to

high current output 1/8th Brick footprint converter

featuring the LT®1952 switching controller with Ac-

tive Reset circuit. The Active Reset circuit can im-

prove the efficiency in wide input voltage applica-

tions. Also, the Active Reset allows the implementa-

tion of self-driven synchronous secondary rectifiers

in some applications.

The DC1317A-F converts isolated 9V to 36V input to

3.3V output and provides over 22A of output current

depending on cooling. When determining the cooling

requirements the actual input voltage range and con-

tinuous maximum output current must be taken into

account. The converter operates at 200kHz with the

peak efficiency greater than 92%. The DC1317 can be

easily modified to generate output voltages in the

range from 0.6V to 48V. The output currents are lim-

ited by total output power of up to 150W.

The available versions of DC1317A are:

DC1317A-A, 34-75Vin to 3.3V, 35A

DC1317A-B, 18-72Vin to 5V, 25A

DC1317A-C, 18-72Vin to 12V, 8A-12A

DC1317A-D, 18-72Vin to 24V, 5A

DC1317A-E, 36-72Vin to 5V, 12A

DC1317A-F, 9-36Vin to 3.3V, 20A

DC1317A-G, 9-36Vin to 12V, 5A

DC1317A-H, 9-36Vin to 48V, 1.5A

The DC1317 circuit features soft-start which prevents

output voltage overshoot on startup or when recover-

ing from overload condition.

The DC1317 has 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 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 22A 9V

Maximum Input Voltage IOUT = 0A to 22A 36V

VOUT VIN = 9V to 36V, IOUT = 0A to 22A

(27Amax)

3.3V ±3%

Typical Output Ripple VOUT VIN = 9V to 36V, IOUT = 0A to 22A 50mVP–P

Nominal Switching Frequency 200kHz

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

QUICK START PROCEDURE

Demonstration circuit 1317 is easy to set up to evalu-

ate the performance of LT1952-1 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.

NOTE:

Make sure that the input voltage does not

exceed 36V.

Check for the proper output voltage.

Vout = 3.3V.

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 DC1317 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 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

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

ACTIVE RESET CIRCUIT

The Active Reset circuit on DC1317A-F demo board

consists of a small P-Channel MOSFET Q13 and

reset capacitor C25. The MOSFET Q13 is used to

connect the reset capacitor across the transformer

T1 primary winding during the reset period when

Q1 MOSFET is off. The voltage across capacitor

C25 automatically adjusts with the duty cycle to

provide complete transformer reset under all oper-

ating conditions.

Also the active reset circuit shapes the reset voltage

into a square waveform that results in lower drain

voltages for Q1 and Q2 MOSFETs. The lower

MOSFET drain voltages allow lower voltage and

lower Rdson MOSFETs to be used. The MOSFETs

must be avalanche rated for the peak reset voltage.

If non-avalanche rated MOSFETs are used a proper

drain voltage derating should be used.

The main benefit of active reset circuit in the case of

DC1317A-F demo board is high efficiency (shown

in Figure 3), wide input range, high power density

and small size. To achieve such high efficiency all of

the power components were carefully selected.

Please consult LT factory for assistance if any

changes to the circuit are required.

DC1317A-F (LT1952) 3.3V Output Efficiency

72%

74%

76%

78%

80%

82%

84%

86%

88%

90%

92%

94%

0 5 10 15 20 25

Iout [A]

Efficiency

12V input

24V input

Figure 3. High efficiency of DC1317A-F allows the

board to be used in thermally critical applications

OUTPUT LOAD STEP RESPONSE

The load step response of DC1317A-F 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 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

Figure 4. Fast transient response of DC1317A-F is

superior to many competing power modules with-

out the additional output capacitors.

SOFT START FUNCTION

The DC1317 features LT4430 opto coupler driver

that has soft start function which produces mono-

tonic 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. Also, the soft-start function will prevent

input current surges even with full load at the out-

put.

Figure 5. The LT4430 opto coupler driver pro-

duces monotonic output voltage rise at startup

without output voltage overshoot.

DEBUGGING AND TESTING

The DC1317 can easily be tested and debugged by

powering the bias circuit separately from the main

power circuit. To place DC1317 into debug mode

remove the resistor R1 and connect 12V, 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 control circuit

LT4430 diode OR a 5V, 100mA power source into

pin 1 of LT4430 controller.

Once the primary and secondary controllers are

running the main power (+Vin) can be applied

slowly while observing the switching waveforms

and output voltage.

The input current supplying the power transformer

T1 should not exceeded 200mA without the output

load. If one of the MOSFETs is damaged, the input

current will exceed 200mA.

PCB LAYOUT

The PCB layout should be carefully planned to avoid

potential noise problems. The PCB layout for DC1317A

can be used as a guide. Since demo board DC1317A

has 8 versions the PCB layout has optional compo-

nents that can be removed. Also, the PCB layout has a

common schematic that is used just for the layout.

The actual circuit schematic shows component values.

The PCB layout schematic does not show the compo-

nent values.

In some cases, a different component like a diode is

used in a place holder for a capacitor such as in the

case of C6. Please modify the reference designators in

your schematic to reflect the actual component used.

The following simple PCB layout rules should be help-

ful.

If possible use solid ground planes on layers 2 and

n-1. The ground planes will prevent the switching

noise from coupling into sensitive lines.

Place sensitive lines on the inner layers that will be

shielded by grounds on layers 2 and n-1.

Keep the loop formed by Q1, RCS1, Cin and T1 tight.

Keep the loop formed by Q2, Q3 and T1 tight.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

Keep noise sensitive nodes like SD/VSEC, ROSC, FB,

COMP, ISENSE, BLANK and DELAY as small as posible

by placing the associated components close to the

LT1952 and LT4430 chips.

Use local vias for all components that connect to

ground planes.

Do not place any traces on the layers 2 and n-1 to

avoid ground planes from being compromised.

If the PCB layout has to be done on 2 or 4-layer PCB

try to stay close to the guidelines outlined above. Also,

maximize the ground connections between compo-

nents by placing the components tight together.

Please contact LT factory for additional assistance.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

Rcs1

0.005R

+Vin

47R

R54

2.2R

Cu1

4.7u

R23 910R

R24

22k

Vu1

R15 178k

R20

100k

C10

0.1u

Si7138

D10

BAT760

249R

C15

2.2nF, 2kV

C11

4.7n

R28

115k

R22

13.3k

G45R2-0302.007

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

R33

158k

C13

.47u

R26

220R

R30

1.2k

R14

33K

C23

1uF

Vfb

R13

22K

Co1

100uF, X7R

+Vo1

R17

560R

Co2

T2A

PE-68386

C14

C12

470p

R19

56K

CS-

CS+

Vcc

4FG 5

GND 6

Timer 7

Sync 8

LTC3900

R21

10k

R27

470R

R38

18.2K

R35

82.5k

R34

8.66k

C19

1uF, X5R

C16

.22u, 10V

C17

15n

+Vo1

R31

10R

Vcc

FB 4

Opto 6

GND

2Comp 5

LT4430

R18

10k

Cin

3x6.8uF

+Vin

PDZ10B

Vu1

BAS516

R11

10k

-Vin

+Vin

9V-36V Input

2.2R

R32

196k

PA1671.112

HAT2165

R16

33k

0.22u

B0540

BCX55

R10

PDZ7.5B

2.2R

Q12

BC856T

Q10

PBSS8110

+Vb

1.5mH

PS2801-1

Inp

Vcc

TS 4

Boost 6

GND

2TG 5

LTC4440-5

C21

470p

BAS516

Vu1

Out

D11

BAS516

GND

+Vout

220p

Out

Csys

470u, 6.3V, 4m

C24

39p

100p

C25

33n

Q13

Si7309

D14

BAS516

R45

10k

+Vr2

D17

BAS516

7, 8

10, 11

C33

0.1u

* R29

PMEG3002

C34

0.33uF

10uH

R53

200R

560k

100p

* Please change the ref designator to reflect the symbol.

Demo Board DC1317A-F schematic

NOTE:

This schematic shows only the components required for operation of -F version

of DC1317A demo board.

All optional components of DC1317A demo board have been removed.

Also, all zero-ohm resistors have been replaced with wires.

Please consult the full DC1317A-F schematic to decide if any of the optional

components should be included in your design.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F

ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER

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