LTC4255

14

4255f

Schottky diodes should be added between the drains of

the FETs and AGND using a layout that minimizes the

trace lengths between the parts.

Address Selection

The lower 5 bits of the serial interface address can be set

by the address selection pins AD0 to AD4 with AD0 being

the least significant bit. The upper two bits are preset to

AD5 = high and AD6 = low. To force an address bit to low,

the address selection pin should be connected to DGND.

To force an address bit to high, the address selection pin

can be left floating and an internal pull-up resistor will pull

the pin up to the internal digital supply voltage (typically

5V). If more noise immunity is needed, the pin can be

connected directly to a 5V or 3.3V external supply.

If the autoretry enable bit (B1) is set, then the channel will

automatically restart after the retry delay timer expires

(Figure 12, point E). If the channel successfully powers up

after an autoretry, the Power_OK bit (CO) will be set.

However, FAULT will continue to pull down and the short-

circuit bit (DO) will remain high until the fault is cleared by

setting the FAULT clear bit (BO).

In the case of a short-circuit occuring after a normal

power-up, high rates of change of current (high di/dt)

through the inductance of the cable or even the traces on

the PCB can cause the voltages at OUT1-4 to overshoot.

A 100k

Ω resistor should be placed between the drains of

the FETs and the OUT1-4 pins to limit the energy absorbed

by the LTC4255 if the drains of the FETs exceed OUT1-4’s

absolute maximum voltage rating. Also, if the FETs are

not capable of safely absorbing this energy, high-speed

APPLICATIO S I FOR ATIO

Figure 12. Power-Up Sequence with Shorted Load on Channel 1 with Autoretry Enabled

4255 F12

AO

ACK

STOP BIT

SCL

SDA

SHORT-CIRCUIT STATUS

(D0)

OPEN-CIRCUIT STATUS

(D4)

POWER_OK

(C0)

FAULT

RELAY1

GATE1

OUT1

AB

C

D

E

F

GH

FET

TURNS ON

CL TIMER

EXPIRES

CL TIMER

EXPIRES

FAULT CLEARED

FET

CURRENT

BO

ACK

STOP BIT

AUTO-RESTART

INITIATED