LTC1421 datasheet(18/24 Pages) LINER

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LTC4223-1/LTC4223-2

422312f

The ﬁrst step is to select the appropriate value of RSENSE

for the 12V supply. Calculating RSENSE value is based on

the maximum load current and the lower limit for the

circuit breaker threshold, ΔVSENSE(CB)(MIN).

SENSE

SENSE CB MIN

LOAD MAX

()(

)

()

47 5

74A

= 6 Ω

If a 1% tolerance is assumed for the 6mΩ sense resistor,

the minimum and maximum circuit breaker trip current is

calculated as follows:

422312 F11

330nF

VCC

AUXIN

Figure 11. RC Network for VCC Filtering

APPLICATIONS INFORMATION

PCB Layout Considerations

For proper operation of the LTC4223’s circuit breaker,

Kelvin-connection to the sense resistor is strongly rec-

ommended. 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 management techniques for

optimal device power dissipation. A recommended PCB

layout for the 12V sense resistor and the power MOSFET

is illustrated in Figure 12.

In applications where load current exceeds 10A, wide

PCB traces are recommended to minimize resistance

and temperature rise. The suggested trace width for 1

oz 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 1 oz

copper foil is approximately 0.5mΩ/square, and voltage

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

rent applications.

In most applications, it will be necessary to use plated-

through via to make circuit connections from component

layers to power and ground layers internal to the PCB. For

1 oz copper foil plating, a general rule is 1A of DC current

per via. Consult your PCB fabrication facility for design

rules pertaining to other plating thicknesses.

It is important to place the VCC bypass capacitor C2 as

close as possible between VCC and GND. The transient

voltage suppressors Z1 and Z2 are also placed between

the supply inputs and ground using short wide traces.

Design Example

As a design example, consider the AMC Hot Swap ap-

plication shown earlier in Figure 1 with the power supply

requirements given in Table 2.

CURRENT FLOW

TO LOAD

TRACK WIDTH W:

0.03" PER AMPERE

ON 1OZ Cu FOIL

SENSE

RESISTOR

POWER PAK

SO-8

12VOUT

12VIN

GND

422312 F12

GND

12VGATE

VIA TO

GND PLANE

LTC4223CGN*

•

CURRENT FLOW

TO LOAD

CURRENT FLOW

TO SOURCE

*ADDITIONAL DETAILS OMITTED FOR CLARITY, DRAWING NOT TO SCALE!

VIA TO

GND PLANE

Figure 12. Recommended Layout for Power MOSFET, Sense

Resistor and GATE Components on 12V Rail

Table 2. AMC Power Supply Requirements

SUPPLY VOLTAGE

MAXIMUM LOAD

CURRENT

MAXIMUM LOAD

CAPACITANCE

12V

7.4A

800μF

3.3VAUX

150mA

150μF

Table 3. MicroTCA Power Supply Requirements

SUPPLY VOLTAGE

MAXIMUM LOAD

CURRENT

MAXIMUM LOAD

CAPACITANCE

12V

7.6A

1600μF

3.3VAUX

150mA

150μF

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