22
LTC4212
4212f
Table 5 lists some current sense resistors that can be used
with the circuit breaker. Table 6 lists some power MOSFETs
that are available. Table 7 lists the web sites of several
manufacturers. Since this information is subject to change,
please verify the part numbers with the manufacturer.
OPERATIO
U
where
P
D
= (I
LOAD
)
2
• R
DS(ON)
θ
JA
= junction-to-ambient thermal resistance
T
A(MAX)
= maximum ambient temperature
If a short circuit happens during start-up, the external
MOSFET can experience a big single pulse energy. This is
especially true if the applications only employed a small
gate capacitor or no gate capacitor at all. Consult the safe
operating area (SOA) curve of the selected MOSFET to
ensure that the T
J(MAX)
is not exceeded during start-up.
Using Staggered Pin Connectors
The LTC4212 can be used on either a printed circuit board
or on the backplane side of the connector. Printed circuit
board edge connectors with staggered pins are recom-
mended as the insertion and removal of circuit boards do
sequence the pin connections. Supply voltage and ground
connections on the printed circuit board should be wired
to the edge connector’s long pins or blades. Control and
status signals (like FAULT and ON) passing through the
card’s edge connector should be wired to short length pins
or blades.
PCB Connection Sense
There are a number of ways to use the LTC4212’s ON pin
to detect whether the printed circuit board has been fully
seated in the backplane before the LTC4212 commences
a start-up cycle.
An example is shown in the schematic on the front page
of this data sheet. In this case, the LTC4212 is mounted
on the PCB and a 20k/10k resistive divider is connected to
the ON pin. On the edge connector, R1 is wired to a short
pin. Until the connectors are fully mated, the ON pin is held
low, keeping the LTC4212 in an off state. Once the
connectors are mated, the resistive divider is con
nected
to V
CC
, V
ON
> 1.316V and the LTC4212 begins a start-up
cycle.
PCB Layout Considerations
For proper operation of the LTC4212’s circuit breaker
function, a 4-wire Kelvin connection to the sense resistors
is highly recommended. In Hot Swap applications where
load currents can reach 10A or more, narrow PCB tracks
exhibit more resistance than wider tracks and operate at
more elevated temperatures. Since the sheet resistance of
1 ounce copper foil is approximately 0.54mΩ/square,
track resistances add up quickly in high current applica-
tions. Thus, to keep PCB track resistance and temperature
rise to a minimum, PCB track width must be appropriately
sized. Consult Appendix A of LTC Application Note 69 for
details on sizing and calculating trace resistances as a
function of copper thickness.
In the majority of applications, it will be necessary to use
plated-through vias to make circuit connections from
component layers to power and ground layers internal to
the PC board. For 1 ounce copper foil plating, a good
starting point is 1A of DC current per via, making sure the
via is properly dimensioned so that solder completely fills
any void. For other plating thicknesses, check with your
PCB fabrication facility.
APPE DIX
U
Table 5. Sense Resistor Selection Guide
CURRENT LIMIT VALUE PART NUMBER DESCRIPTION MANUFACTURER
1A LR120601R050 0.05Ω 0.5W 1% Resistor IRC-TT
2A LR120601R025 0.025Ω 0.5W 1% Resistor IRC-TT
2.5A LR120601R020 0.02Ω 0.5W 1% Resistor IRC-TT
3.3A WSL2512R015F 0.015Ω 1W 1% Resistor Vishay-Dale
5A LR251201R010F 0.01Ω 1.5W 1% Resistor IRC-TT
10A WSR2R005F 0.005Ω 2W 1% Resistor Vishay-Dale
