LTC4211 datasheet(19/22 Pages) LINER

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LTC4235

4235f

For more information www.linear.com/LTC4235

applicaTions inForMaTion

PCB Layout Considerations

To achieve accurate current sensing, a Kelvin connection

for the sense resistor is recommended. The PCB layout

should be balanced and symmetrical to minimize wiring

errors. In addition, the PCB layout for the sense resistor

and the power MOSFET should include good thermal

managementtechniquesforoptimaldevicepowerdissipa-

tion. A recommended PCB layout is illustrated in Figure 7.

Connect the IN and OUT pin traces as close as possible to

the MOSFETs’ terminals. Keep the traces to the MOSFETs

wideandshorttominimizeresistivelosses.ThePCBtraces

associated with the power path through the MOSFETs

should have low resistance. The suggested trace width for

1oz copper foil is 0.03" for each ampere of DC current to

keep PCB trace resistance, voltage drop and temperature

rise to a minimum. Note that the sheet resistance of 1oz

copper foil is approximately 0.5mΩ/square, and voltage

drops due to trace resistance add up quickly in high cur-

rent applications.

It is also important to place the bypass capacitor C1 for

the INTVCC pin, as close as possible between INTVCC and

GND. Also place C2 near the CPO1 and IN1 pins, C3 near

the CPO2 and IN2 pins, and C4 near the REG and SENSE+

pins. The transient voltage suppressors Z1 and Z2, when

used, should be mounted close to the LTC4235 using

short lead lengths.

Prioritizing Supplies with D2OFF

Figure 8 shows an application where the IN1 supply

is passed to the output on the basis of priority, rather

than simply allowing the highest voltage to prevail. This

is achieved by connecting a resistive divider from IN1

at the D2OFF pin to suppress the turn-on of the ideal

diode MOSFET MD2 in the IN2 power path. When the IN1

supply voltage falls below 11.4V, it allows the ideal diode

MOSFET MD2, to turn on, causing the diode-OR output

to be switched from the main 12V supply at IN1 to the

auxiliary 12V supply at IN2. This configuration permits the

load to be supplied from a lower IN1 supply as compared

to IN2 until IN1 falls below the MD2 turn-on threshold. The

threshold value used should not allow the IN1 supply to

be operated at more than one diode voltage below IN2.

Otherwise, MD2 conducts through the MOSFET’s body

diode. The resistive divider connected from SENSE+ at

the ON pin provides the undervoltage threshold of 9.7V

for the diode-OR output supply.

BACKPLANE

CONNECTOR

VMAIN

12V

VAUX

12V

CARD

CONNECTOR

CPO1

GND

FAULT

PWRGD

470µF

12V

INTVCC

SMAJ15A

D2OFF

IN1 DGATE1

DGATE2

LTC4235

4235 F08

MD2

SiR818DP

0.004

HGATE

OUT

SENSE+

VSENSE+

SENSE–

CPO2

0.1µF

IN2

13.7k

100k

RHG

CHG

10nF

REG

0.1µF

MD1

SiR818DP

CFT

0.1µF

SMAJ15A

FTMR

IMON

ADC

2.49k

21k

0.1µF

Figure 8. Plug-In Card 12V Prioritized Power Supply at IN1

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