Rainbow Electronics MAX5943 User Manual
Page 16
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
16
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Optimizing for Short-Circuit Conditions
Choosing R
SENSE
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:
P
RSENSE
= I
2
LIMIT
x R
SENSE
where P
RSENSE
is the power dissipated across R
SENSE
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
power path (R
TOTAL
).
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:
I
SC
= V
IN
/R
TOTAL
where:
R
TOTAL
= R
SENSE
+ 2 x (R
ON
)
= 30mΩ + 2 x (30mΩ) = 90mΩ
For example, an input voltage of 20V produces a current
at approximately 222A (or I
SAT
of Q2, whichever is less)
in the power path for the circuit-breaker timeout period.
Choose an R
SENSE
capable of handling the high power
dissipation during a short-circuit event.
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 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:
P = I
LOAD
2
x R
DS(ON)
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:
P
Q2
= (V
IN
- V
IS
– V
Q1
) x I
LIMIT
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:
P = I
SC
2
x R
DS(ON)
where:
I
SC
= V
IN
/R
EQ
and
R
EQ
= R
SENSE
+R
DS(ON1)
+ R
DS(ON2)
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.
V
Q1
V
Q2
V
IS
SENSE GATE1
GATE2
OUT
IN
+
-
+
-
+
-
V
IN
MAX5943A
Figure 11. Power Dissipated Across MOSFETs During a Short-
Circuit Fault for MAX5943A