LM5642EVAL-KIT National Semiconductor, LM5642EVAL-KIT Datasheet

LM5642EVAL-KIT

Manufacturer Part Number

LM5642EVAL-KIT

Description

BOARD EVALUATION LM5642

Manufacturer

National Semiconductor

Series

PowerWise®r

Datasheets

1.LM5642MTCNOPB.pdf (28 pages)

2.LM5642EVAL-KIT.pdf (11 pages)

Specifications of LM5642EVAL-KIT

Main Purpose

DC/DC, Step Down

Outputs And Type

2, Non-Isolated

Voltage - Output

1.8V, 3.3V

Current - Output

7A, 4A

Voltage - Input

4.5 ~ 36V

Regulator Topology

Buck

Frequency - Switching

200kHz

Board Type

Fully Populated

Utilized Ic / Part

LM5642

Lead Free Status / RoHS Status

Not applicable / Not applicable

Power - Output

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LM5642 Evaluation Board

Introduction

The LM5642 IC is a dual channel, current-mode, synchro-

nous buck converter controller. It can handle input voltages

of up to 36V and delivers two independent output voltages

from 1.23V up to 90% of the input voltage. Current sensing

can be done using a dedicated resistor or using the R

of the high-side FET. This application note describes the

dedicated evaluation PCBs that are available for both meth-

ods.

Resistor Sense PCB

The first, more common method of sensing current in

current-mode controllers is with an external sense resistor,

placed in series with the high-side FET of each channel.

Sense resistors provide an accurate voltage as the load

current passes through them, and have stable, linear resis-

tance change with temperature. As shipped the Resistor

Sense PCB is designed to deliver 1.8V on Channel 1 at a

maximum current of 7A, and 3.3V on Channel 2 at a maxi-

mum current of 4A. The input voltage can vary anywhere

from 5.5 to 36V. The board has been designed to be flexible

and allow many other circuit configurations by replacing the

original components with user selected ones. Figure 1

shows the circuit diagram representing the standard BOM

that comes with the PCB. Table 1 lists all the components

that are used for this standard configuration BOM. Figure 2

shows the complete circuit diagram with all extra footprints.

Two SPST switches S1 and S2 are provided to turn the two

channels of the converter on and off. The standard BOM that

comes with the LM5642 Resistor Sense evaluation board

uses 10mΩ current sense resistors (R7 for Channel 1, R15

for Channel 2) to provide independent feedback signals to

the IC. The board provides additional resistor and capacitor

footprints for noise filtering, ringing control, and to enable

operation at low input voltages.

Current Sense Filters

R-C filters have been added to the current sense amplifier

inputs of the Resistor Sense evaluation board, comprised of

components C3, C4, C14, C15, R2, R6, and R16. These

resistors and capacitors reduce the sensitivity to switching

noise, especially during high currents, load-transients, and

circuits with short on-times.

Parallel Operation

The two channels of the LM5642 Resistor Sense evaluation

board can be paralleled to provide one high current rail. At

the nominal switching frequency of 200kHz the converter will

run 180˚ out-of-phase. Care must be taken when using this

feature combined with the frequency synchronization, as the

two channels of the converter are no longer 180˚ out-of-

phase when the frequency is above or below 200kHz. The

two feedback inputs FB1 and FB2 should be tied together by

soldering a 0Ω resistor in the position marked J1 on the

bottom side of the PCB. The two COMP pins should be tied

together by soldering a 0Ω resistor in the position J3. The

AN200772

DS(ON)

National Semiconductor

Application Note 1292

Chris Richardson

September 2003

ON/SS1 and ON/SS2 pins must also be connected using a

0Ω resistor in the position J2. One of the two SPDT switches

S1 and S2 should be left ‘ON’ and the other used to turn the

converter off and on. Finally, the two outputs VO1 and VO2

must be tied together by the user, external to the PCB. Only

one of the two resistor divider networks (R10/R11 or R19/

R20) and only one of the compensation networks should be

used. (C18/C19/R22/R23) or C20/C21R24/R25.

The LM5642 IC offers a second current sensing mechanism

that uses the R

current. This method reduces the parts count on the BOM,

however the R

a sense resistor, and suffers from non-linear changes in

resistance with temperature. As a result, the IC is more

sensitive to noise in this mode, especially at input voltages

above 30V. The maximum recommended current using VDS

sensing is 5A per channel. The VDS Sense board has been

designed to deliver 1.8V on Channel 1 with a maximum

current of 5A, and 3.3V on Channel 2 with a maximum

current of 4A. Figure 3 shows the circuit diagram represent-

ing the standard BOM that comes with the PCB. Table 2 lists

all the components that are used for this standard configu-

ration BOM. Figure 4 shows the complete circuit diagram

with all extra footprints.

Frequency Synchronization

A connection point labeled ‘SYNC’ is available on both ver-

sions of the LM5642 evaluation boards in order to adjust the

switching frequency of the IC between 150 and 250kHz.

Both CMOS and TTL level square wave signals can be used.

The SYNC input has a minimum low-to-high transition

threshold of 2.0V and a maximum high-to-low threshold of

0.8V. The SYNC pin is grounded by a 220kΩ pull-down

resistor.

Low Input Voltage Operation

When the input voltage is between 4.5V and 5.5 on either

evaluation board, a 4.7Ω resistor should be installed in po-

sition R26. This will ensure than VLIN5 does not fall below

the UVLO threshold of the IC. When R26 is in place the input

voltage must not exceed 5.5V.

Gate Drive Current Limiting

The LM25642 IC includes powerful gate drivers which can

drive small FETs at high speed, often inducing noise or

ringing into the board. Slowing the gate drivers can help

reduce this noise by increasing the drain current transition

time. While slowing the gate drives can help suppress noise,

it also increases switching losses and gate-charge losses in

the top FET. Slowing of the gate drives can be accomplished

with resistors in series with the CBOOT1 and CBOOT2 pins.

(R9,R18) Placing resistors in series with the CBOOT pins will

Sense PCB

DS(ON)

of a FET is not as tightly controlled as

of the high-side FET to sense the load

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Related parts for LM5642EVAL-KIT

LM5642EVAL-KIT Summary of contents

Page 1

... FB1 and FB2 should be tied together by soldering a 0Ω resistor in the position marked J1 on the bottom side of the PCB. The two COMP pins should be tied together by soldering a 0Ω resistor in the position J3. The © 2003 National Semiconductor Corporation National Semiconductor Application Note 1292 Chris Richardson ...

Page 2

Gate Drive Current Limiting (Continued) slow the top FET rise time only. Generally the values for gate drive limiting resistors are between 1 and 5Ω. R9 and R18 are 0Ω by default. Parallel Low-Side Schottky Diode The LM5642 evaluation boards ...

Page 3

Additional Footprints TABLE 1. Standard Resistor Sense Bill Of Materials ID Part Number U1 LM5642 Q1, Q4 Si4850EY Q2, Q5 Si4840DY D3 BAT54A L1 RLF12560T-4R2N100 Inductor L2 RLF12545T-100M5R1 Inductor C1, C29, C31 C3216X7R1H105K C3, C4, C14, C15 VJ1206Y101KXXAT C27 C2012X5R1C105K ...

Page 4

Additional Footprints FIGURE 2. Complete Resistor Sense Evaluation Board Schematic www.national.com (Continued) 4 20077202 ...

Page 5

Additional Footprints Standard V Sense BOM DS ID Part Number U1 LM5642 Q1, Q4 Si4850EY Q2, Q5 Si4840DY D3 BAT54A L1 RLF12560T-4R2N100 Inductor L2 RLF12545T-100M5R1 Inductor C1, C29, C31 C3216X7R1H105K C27 C2012X5R1C105K C6, C16 C5750X5R1H106M C9, C23 6TPD330M C2, C11, ...

Page 6

Additional Footprints Standard V Sense BOM DS ID Part Number C26 C3216X7R1C475K R1 CRCW1206333J R13 CRCW1206223J R10 CRCW12062261F R23 CRCW12068451F R24 CRCW12061372F R11, R20 CRCW12064991F R19 CRCW12068251F R27 CRCW12064R7J R28 CRCW1206224J www.national.com (Continued) (Continued) Type Size Capacitor 1206 Resistor 1206 ...

Page 7

Additional Footprints (Continued) FIGURE 4. Complete V Sense Eval Board Schematic DS 7 20077204 www.national.com ...

Page 8

FIGURE 5. Resistor Sense PCB Top Layer FIGURE 6. Resistor Sense PCB Bottom Layer 8 20077205 20077206 ...

Page 9

FIGURE 7. Resistor Sense PCB Internal Planes FIGURE 8. V Sense PCB Top Layer DS 9 20077207 20077208 www.national.com ...

Page 10

FIGURE 9. V Sense PCB Bottom Layer DS FIGURE 10. V Sense PCB Internal Planes DS 10 20077209 20077210 ...

Page 11

... NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant ...

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