LTC4218

14

4218fb

Figure 7. 3.3V, 6A Card Resident Application

0.1

μF

C1

0.1

μF

R5

14.7k

R6

10k

R2

17.4k

ADC

R3

3.16k

R4

10k

3.3V

20k

4218 F07

R7

10k

330

μF

UV

SOURCE

FB

PG

GND

LTC4218GN

OV

TIMER

FLT

R1

10

Ω

2m

Ω

Q1

Si7104DN

3.3V

+

1k

0.01

μF

3.3V

6A

APPLICATIONS INFORMATION

After the 1.2ms timeout the FLT pin needs to pull down on

the UV pin to restart the power-up sequence.

Since the default values for overvoltage, undervoltage and

powergood thresholds for the 12V fixed version match

the requirements, no external components are required

for the UV, OV and FB pins.

The final schematic results in very few external com-

ponents. Resistor R1 (10

Ω) prevents high frequency

fast turn-off. The pull-up resistor, (R2), connects to the

voltage at a ratio:

V

µA

mV

mV

A

k

V

A

667

2

20

0 267

.

••

.

pin.

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 resistors

and the power MOSFETs should include good thermal

management techniques for optimal device power dissipa-

tion. A recommended PCB layout for the sense resistor

and power MOSFET is illustrated in Figure 6.

In Hot Swap applications where load currents can be 6A,

narrow PCB tracks exhibit more resistances than wider

tracks and operate at elevated temperatures. The minimum

trace width for 1oz copper foil is 0.02” per amp to make sure

the trace stays at a reasonable temperature. Using 0.03”

per amp or wider is recommended. Note that 1oz copper

exhibits a sheet resistance of about 0.5m

Ω/square. Small

resistances add up quickly in high current applications

It is also important to put C1, the bypass capacitor for the

GND. Place the 10

Ω resistor as close as possible to Q1.

This will limit the parasitic trace capacitance that leads to

Q1 self-oscillation.

Additional Applications

The LTC4218 has a wide operating range from 2.9V to

26.5V. The UV, OV and PG thresholds are set with a few

resistors. All other functions are independent of supply

voltage.

The last page includes a 24V application with a UV threshold

of 19.8V, an OV threshold of 28.3V and a PG threshold

of 20.75V. Figure 7 shows a 3.3V application with a UV

threshold of 2.87V, an OV threshold of 3.77V and a PG

threshold of 3.05V.

4218 F06

R1

C

Q1

LTC4218

Figure 6. Recommended Layout