FireWire Current Limiter and Low-Drop

ORing Switch Controller

16

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Optimizing for Short-Circuit Conditions

Select a sense resistor that causes the circuit-breaker

voltage drop at a current-limit/circuit-breaker level

above the maximum normal operating current.

Typically, set the overload current at 1.2 to 1.5 times

the full load current.

Choose the sense-resistor power rating to accommodate

an overcurrent condition:

during a current-limit/circuit-breaker fault.

Under short-circuit conditions, it is imperative that the

appropriate sense resistor is utilized. Operating the

MAX5943B–MAX5943E at high input voltages can

cause very large currents during the circuit-breaker

timeout period. The peak current will be limited by the

saturation current of Q2 or the series resistance in the

Using a 30mΩ on-resistance MOSFET at GATE1 and

GATE2 and a 30mΩ sense resistor results in a short-cir-

cuit current approximately equal to:

where:

= 30mΩ + 2 x (30mΩ) = 90mΩ

For example, an input voltage of 20V produces a current

in the power path for the circuit-breaker timeout period.

dissipation during a short-circuit event.

MOSFET Selection

Select external MOSFETs according to the application

should be chosen low enough to have minimum voltage

drop at full load to limit the MOSFET power dissipation.

a pulsating load. Determine the device power rating to

accommodate startup, a short-circuit condition, and

when the device is in autoretry mode.

During normal operation, the external MOSFETs dissi-

pate little power. The power dissipated in normal oper-

ation is:

The most power dissipation occurs during a short-circuit

event, resulting in high power dissipated in Q2 (Figure

11) during the timeout period for the MAX5943A, where

the power dissipated across Q2 is:

For the MAX5943B–MAX5943E, a short-circuit event

results in high power dissipated in both Q1 and Q2 dur-

ing the timeout period (Figure 12) where the total power

dissipated in either MOSFET is:

where:

and

The programmable timeout of the MAX5943 allows the

use of MOSFETs with low power ratings. A MOSFET

typically withstands single-shot pulses with higher dis-

sipation than the specified package rating.

FireWire Power Management

The MAX5943 serves to regulate and protect FireWire

power over a system interface. The MAX5943 program-

mable features make it suitable for both power provider

and power receiver applications. Figure 13 shows a

high-end two-port FireWire power management system

using two MAX5943As and a dual-channel MAX5944

FireWire current-limiting IC.

SENSE GATE1

GATE2

OUT

IN

+-

+-

+-

MAX5943A

Figure 11. Power Dissipated Across MOSFETs During a Short-

Circuit Fault for MAX5943A