Linear Technology 868 Manuale utente

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 868

LOW QUIESCENT CURRENT DUAL SYNCHRONOUS BUCK CONVERTER

LTC3827EG-1

DESCRIPTION

Demonstration circuit 868 is a dual-output stepdown

power supply. The input voltage is from 4.5V to 32V.

The supply has two outputs: 3.3V/5A

MAX

and

8.5V/3.5A

MAX

. When the input voltage is close to the

output voltage, the supply is in dropout mode. The de-

sign features the LTC3827-1, the low quiescent current,

2-phase, dual-output synchronous buck regulator.

Operating the two high side MOSFETs 180 degrees out

of phase significantly reduces peak input ripple current;

thereby reducing input capacitance and EMI noise. Both

controllers have internal current foldback for overload

condition. Current foldback limits MOSFET heat dissipa-

tion during short-circuit conditions. The LTC3827-1 fea-

tures a precision 0.8V reference and a power good out-

put indicator. The PLLIN/MODE pin selects among Burst

Mode operation, pulse skipping mode, or continuous

inductor current mode at light loads. The controller can

operate at up to 99% duty cycle for very low dropout

capability.

Design files for this circuit board are available. Call

the LTC factory.

,LTC and LT are registered trademarks of Linear Technology Corporation.

Table 1.

Performance Summary

PARAMETER CONDITION VALUE

Input Voltage Range If the input voltage must exceed 32V, use higher voltage rating input capacitors. The IC

works up to 36V. Minimum of 9V input is needed for 8.5V output regulation at 3A load.

4.5V to 32V

Outputs V

OUT1

= 3.3V, 0A to 5A 3.3V

OUT2

= 8.5V, 0A to 3.5A, Vin>9V 8.5V

Typical Output Ripple (V

OUT1

) 20MHz BW, 5A load, V

= 32V, frequency = 400kHz 35mV

P-P

Typical Output Ripple (V

OUT2

) 20MHz BW, 3.5A load, V

= 32V, frequency = 400kHz 30mV

P-P

Typical Operating Frequency PLLLPF floating (position JP1 to 400Khz) 400kHz

Typical input current@no load CH1 on, no load, CH2 off, frequency=400kHz, Vin=16V, Burst Mode 85uA

Efficiency for Vout1

Efficiency for Vout2

VIN=12V, Vout1=3.3V at 3.5A, frequency =400KHz

VIN=12V, Vout2=8.5V at 2.5A, frequency =400kHz

94%

98%

Demonstration circuit 868 is easy to set up to evaluate

the performance of the LTC3827-1. Refer to Figure 1 for

proper measurement equipment setup and follow the

procedure below:

Place RUN1 and RUN2 in OFF position, the FREQ

jumper at the selected frequency and the

PLLIN/MODE jumper JP2 in desired Mode (Burst or

CCM Mode) position. Table 2 shows the default set-

ting for DC868.

JP1

FREQ

JP2

PLL/MODE

JP3

RUN1

JP4

RUN2

JP5

TRACK1

JP6

TRACK2

400kHz BURST ON ON SS1 SS2

Table 2. Default Jumper Settings

Connect the desired loads between 3.3V and 8.5V

terminals and their closest PGND terminals on the

board.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 868

LOW QUIESCENT CURRENT DUAL SYNCHRONOUS BUCK CONVERTER

NOTE:

The maximum load at 3.3V is 5A, and the

maximum at 8.5V is 3.5A.

Connect the input power supply to the VIN

and GND

terminals on the right, center of the board.

NOTE:

Do not increase V

over 32V.

Switch on the desired channel(s) by moving the

RUN1 and/or RUN2 jumper to “on” position.

Measure the output voltages. They should be 3.3V

0.066V and 8.5V

0.17V respectively, within each

specified maximum load current.

For better efficiency at high input voltage, tie EXTVCC

to external source of 5V-9V or mount the optional re-

sistor R1(0 ohm).

If external synchronization is desired, remove JP2 and

tie PLLIN/MODE to external clock signal. Please refer

to datasheet for more details.

The following table shows the different output tracking options for Vout1 and Vout2

JP5 Set Function R25 R26 R27

SS1* SS w/o tracking DNP DNP DNP

TRACK Track Track1 35.7k 11.5k DNP

TRACK Track Vout2 35.7k 11.5k zero

Table 3. Tracking Jumper Settings

*With default JP5 and JP6 setting, Vout1 and Vout2 will start independently. Please refer to datasheet for TBD

value calculation.

JP6 Set

Function

R28

R29

SS2* SS w/o tracking DNP DNP

TRACK Track Track2 TBD* TBD*

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 868

LOW QUIESCENT CURRENT DUAL SYNCHRONOUS BUCK CONVERTER

Load

++++

Power

supply

++++

Power

supply

++++

Power

supply

++++ ++++

Figure 1.

Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 868

LOW QUIESCENT CURRENT DUAL SYNCHRONOUS BUCK CONVERTER

Mouser Electronics

Authorized Distributor

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Analog Devices Inc.:

DC868A

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