MAX5944ESE+ Maxim Integrated Products, MAX5944ESE+ Datasheet - Page 13

• Manufacturer Part Number: MAX5944ESE+
• Description: IC FW CURR LIMITER DUAL 16-SOIC
• Manufacturer: Maxim Integrated Products

Specifications of MAX5944ESE+

• Applications: FireWire® Apps
• Fet Type: N-Channel
• Number Of Outputs: 2
• Internal Switch(s): No
• Delay Time - Off: 100ns
• Voltage - Supply: 7.5 V ~ 37 V
• Current - Supply: 1.3mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Delay Time - On: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Figure 7. Setting the MAX5944 Turn-On Voltage
to 1/129 under continuous fault conditions. FAULT_
deasserts every time a restart attempt is made.
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
the capacitor at OUT, V
the 2ms current-limit timeout period, and I
load current during startup. With V
220µF, and I
greater than 1.3A. This calculation does not include tol-
erances.
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:
where P
during a current-limit fault.
LIMIT
SENSE
Dual FireWire Current Limiter and Low-Drop
LOAD
is the programmed current limit, C
I
LIMIT
P
is the power dissipated across R
Applications Information
SENSE
R1
R2
______________________________________________________________________________________
= 0, set the MAX5944 current limit
V
>
IN
C
= (V
IN
OUT
IN_
ON_
GND
Startup Considerations
t
ILIM
is the supply voltage, t
TH
MAX5944
×
)
2
V
IN
/ R
Choosing RSENSE
+
SENSE
I
IN
LOAD
= 12V, C
LOAD
OUT
OUT
ILIM
SENSE
is the
is
is
=
ORing Switch Controller
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (R
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High R
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:
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:
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.
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.
Figure 8. Power Dissipated Across MOSFET Q2 During a
Short-Circuit Fault
V
IN
DS(ON)
IN_
+
P
Q2
also causes large output ripple if there is
V
P = I
IS
= (V
FireWire Power Management
-
SENSE_ GATE1_
IN
LOAD2
MAX5944
- V
+
V
IS
Q1
x R
Transient Protection
-
- V
Q1
MOSFET Selection
DS(ON)
Q1
+
GATE2_ OUT_
) x I
V
Q2
-
LIMIT
Q2
I
LIMIT
DS(ON)
13
)