Linear Technology LTC3773EG Manual de usuario

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

LTC3773EG

DESCRIPTION

Demonstration circuit 908A is a Polyphase 3-

Output Step-Down Supply with Tracking featuring

the LTC3773EG. The output voltages are 3.3V,

2.5V and 1.8V and the maximum load current for

each is 15A. The input voltage range is 7V to 24V.

Typical applications include s

ervers, communica-

tions, computers and industrial power supplies.

The board can be programmed for either continu-

ous-conduction mode (CCM), pulse skip mode

(DCM) or Burst Mode (BM). Other features of the

board include an SDB pin which can be used for

rail sequencing, a phase mode pin, and a dynamic

load circuit for each rail.

An on-board LT1933 (5V / 500mA buck converter)

provides current for the LTC3773EG’s internal

logic and gate drive circuitry. The 5V bias can also

be supplied externally. If an external bias voltage

is applied it needs to be between 4.5V and 7.0V.

Design files for this circuit board are available. Call

the LTC factory.

Table 1. Performance Summary (T

= 25°C)

PARAMETER CONDITION VALUE

Minimum Input Voltage 7V

Maximum Input Voltage 24V

OUT1

= 7V to 24V, I

OUT

= 0A to 15A 3.3V ±3%

OUT2

= 7V to 24V, I

OUT

= 0A to 15A 2.5V ±3%

OUT3

= 7V to 24V, I

OUT

= 0A to 15A 1.8V ±3%

Typical output voltage ripple for VOUT1 VIN = 12V, IOUT1 = 15A, 20MHz BW 25mVp-p TYP

Typical output voltage ripple for VOUT2 VIN = 12V, IOUT2 = 15A, 20MHz BW 25mVp-p TYP

Typical output voltage ripple for VOUT3 VIN = 12V, IOUT3 = 15A, 20MHz BW 25mVp-p TYP

Nominal Switching Frequency 220kHz TYP

Efficiency

(see Figure 4 for typical efficiency curves)

= 12V, V

OUT1

= 3.3V, I

OUT1

= 15A

= 12V, V

OUT2

= 2.5V, I

OUT2

= 15A

= 12V, V

OUT3

= 1.8V, I

OUT3

= 15A

92.2% TYP

90.9% TYP

88.9% TYP

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

QUICK START PROCEDURE

Demonstration circuit 908A is easy to set up to evaluate

the performance of the LTC3773EG. Refer to Figure 1 for

proper measurement equipment setup and follow the

procedure below:

NOTE:

When measuring the output voltage ripple, care

must be taken to avoid a long ground lead on the oscil-

loscope probe. Measure the output voltage ripple by

touching the probe tip directly across the Vout and GND

terminals. See Figure 2 for proper scope probe tech-

nique.

1. Place the jumpers as shown below:

Description JP# Position

INT5V JP5 ON

5V SELECT JP3 INT5V

SDB JP1 ON

SYNC JP2 CCM

PHMD JP6 LOW

2. With power off, connect the input power supply to

Vin and GND.

3. Turn on the input voltage and set it to a voltage be-

tween 7V and 24V. The input voltage source should

be able to supply at least 20A.

NOTE:

Make sure the input voltage does not exceed

24V.

4. Check for the proper output voltages:

Vout1 = 3.201V to 3.399V

Vout2 = 2.425V to 2.575V

Vout3 = 1.746V to 1.854V

5. Once the proper output voltages are established,

adjust the loads within the operating range and ob-

serve the output voltage regulation, ripple voltage,

efficiency and other parameters.

LOAD STEP RESPONSE

The load step response for each rail can be tested with

an on-board dynamic load circuit and a pulse generator.

Refer to Figure 3 and follow the steps below.

1. Set the output of the pulse generator for a duty cycle

of less than 5% and an amplitude of 1V or below.

2. Connect the output of pulse generator from Pulse

Gen+ to Pulse Gen-.

3. Connect the LOAD STEP BNC connector to an oscil-

loscope with a coaxial cable to monitor the load step

current waveform. 10mV = 1A.

4. Turn-on the 908A and apply the desired amount of

static load to the output.

5. Increase the amplitude of the pulse generator output

to obtain the desired load step height.

6. Monitor the output voltage across the turrets as

shown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

RAIL TRACKING AND SEQUENCING

The DC908A is setup for coincident rail tracking, where

both Vout2 and Vout3 track Vout1 directly through their

TRACK pins and TRACK dividers. Refer to Figure 5 for

the turn-on waveforms. The ramp-rate for Vout1 is de-

termined by a 10nF capacitor on the TRACK1 pin at C8

on the demo board. If desired, Vout1 can be setup to

track an external signal. To do this, remove C8 and stuff

the appropriate values for the TRACK 1 divider at R40

and R41 and apply the tracking signal to the TRACK1

turret.

If desired, the rails can be configured to be turned on

and off independently. To do this, remove the TRACK2

and TRACK3 dividers at R33, R35, R38 and R39, and

stuff TRACK capacitors at C31 and C32 to set the de-

sired ramp-rate. Rail sequencing can be implemented by

placing a capacitor on the SDB pin. As the voltage on the

SDB pin ramps-up, Vout1 will turn on 1

, Vout2 will turn

on 2

and Vout3 will turn-on last. Refer to the data sheet

for more details on rail tracking and sequencing.

Vin

Iin

Vin

supply

Vout1

load

Iout1

Vout1

Vout2

load

Iout2

Vout2

Vout3

load

Iout3

Vout3

Figure 1. Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

VOUT

GND

Figure 2. Measuring Output Voltage Ripple

Oscilloscope

Vout1

load

Iout1

Pulse

generator

COAX cable

Figure 3. Proper Measurement Equipment Setup for Load Step Testing. Only the setup for the 3.3V rail is shown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

Efficiency at Vin = 12V, Fsw = 220kHz

84%

86%

88%

90%

92%

94%

96%

0 3 6 9 12 15 18

Load current (Amps)

Efficiency

3.3V

1.8V

2.5V

Efficiency at Vin = 24V, Fsw = 220kHz

80%

82%

84%

86%

88%

90%

92%

0 3 6 9 12 15 18

Load current (Amps)

Efficiency

3.3V

1.8V

2.5V

Figure 4. Typical efficiency curves

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

3.3V

2.5V

1.8V

SDB

2V / div

2msec / div

Vo1,2,3

1V / div

Figure 5. Turn-on waveforms at Vin = 12V after SDB pin is released. Demo board is setup for coincident rail tracking.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A

POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING

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DC908A

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