MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 13
to 1/129 under continuous fault conditions. FAULT_
deasserts every time a restart attempt is made.
Applications Information
Startup Considerations
Set the appropriate current-limit threshold for a suc-
cessful MAX5944 startup. A successful startup is
dependent on the MAX5944 current-limit threshold and
timeout period. A large capacitor at OUT will result in a
charging current equivalent to the current-limit thresh-
old and may cause the MAX5944 to exceed its 2ms
timeout period, if the current-limit threshold is set too
low. Use the following formula to compute the minimum
current-limit setting:
where I
LIMIT
is the programmed current limit, C
OUT
is
the capacitor at OUT, V
IN
is the supply voltage, t
ILIM
is
the 2ms current-limit timeout period, and I
LOAD
is the
load current during startup. With V
IN
= 12V, C
OUT
=
220µF, and I
LOAD
= 0, set the MAX5944 current limit
greater than 1.3A. This calculation does not include tol-
erances.
Choosing RSENSE
Select a sense resistor that causes the current-limit
voltage drop at a current-limit level above the maximum
normal operating current. Typically, set the current limit
at 1.2 to 1.5 times the nominal load current.
Choose the sense-resistor power rating to accommo-
date a current-limit condition:
P
SENSE
= (V
TH
)
2
/ R
SENSE
where P
SENSE
is the power dissipated across R
SENSE
during a current-limit fault.
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (R
DS(ON)
)
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High R
DS(ON)
also causes large output ripple if there is
a pulsating load. Determine the device power rating to
accommodate a short-circuit condition on the board, at
startup, and when the device is in autoretry mode.
During normal operation, the external MOSFETs dissipate
little power. The power dissipated in normal operation is:
P = I
LOAD2
x R
DS(ON)
The most power dissipation will occur during a short-
circuit event, resulting in high power dissipated in Q2
(Figure 8) during the timeout period for the MAX5944,
where the power dissipated across Q2 is:
P
Q2
= (V
IN
- V
IS
- V
Q1
) x I
LIMIT
Transient Protection
If IN_ or OUT_ experiences a fast transient rise in volt-
age, the drain-to-gate overlap capacitance of GATE1_
and/or GATE2_ FETs may be sufficient to enhance one
of the transistors, allowing current to flow. If the circuit is
subjected to large transients, connect capacitors from
the gate to source across the appropriate MOSFET to
prevent the overlap capacitance from turning on the
device. This results in the turn-off time of the FETs to
increase due to the additional discharge of the capaci-
tor. Use the minimum capacitor value that will prevent
reverse currents from flowing in hot-plug situations.
FireWire Power Management
The MAX5944 serves to regulate and protect FireWire
power over a system interface. The MAX5944 program-
mable features that make it suitable for both power-
provider and power-receiver applications. Figure 9
shows a high-end two-port FireWire power-manage-
ment system using two MAX5944 dual-channel FireWire
current-limiting ICs.
