LTC4223-1/LTC4223-2
18
422312f
The fi rst step is to select the appropriate value of R
SENSE
for the 12V supply. Calculating R
SENSE
value is based on
the maximum load current and the lower limit for the
circuit breaker threshold, ΔV
SENSE(CB)(MIN)
.
R
V
I
mV
SENSE
SENSE CB MIN
LOAD MAX
=
Δ
=
()( )
()
.
.
47 5
74
AA
m= 6 Ω
If a 1% tolerance is assumed for the 6mΩ sense resistor,
the minimum and maximum circuit breaker trip current is
calculated as follows:
422312 F11
C2
330nF
V
CC
AUXIN
R2
51Ω
Figure 11. RC Network for V
CC
Filtering
APPLICATIONS INFORMATION
PCB Layout Considerations
For proper operation of the LTC4223’s circuit breaker,
Kelvin-connection to the sense resistor is strongly rec-
ommended. The PCB layout should be balanced and
symmetrical to minimize wiring errors. In addition, the
PCB layout for the sense resistor and the power MOSFET
should include good thermal management techniques for
optimal device power dissipation. A recommended PCB
layout for the 12V sense resistor and the power MOSFET
is illustrated in Figure 12.
In applications where load current exceeds 10A, wide
PCB traces are recommended to minimize resistance
and temperature rise. The suggested trace width for 1
oz copper foil is 0.03” for each ampere of DC current to
keep PCB trace resistance, voltage drop and temperature
rise to a minimum. Note that the sheet resistance of 1 oz
copper foil is approximately 0.5mΩ/square, and voltage
drops due to trace resistance add up quickly in high cur-
rent applications.
In most applications, it will be necessary to use plated-
through via to make circuit connections from component
layers to power and ground layers internal to the PCB. For
1 oz copper foil plating, a general rule is 1A of DC current
per via. Consult your PCB fabrication facility for design
rules pertaining to other plating thicknesses.
It is important to place the V
CC
bypass capacitor C2 as
close as possible between V
CC
and GND. The transient
voltage suppressors Z1 and Z2 are also placed between
the supply inputs and ground using short wide traces.
Design Example
As a design example, consider the AMC Hot Swap ap-
plication shown earlier in Figure 1 with the power supply
requirements given in Table 2.
CURRENT FLOW
TO LOAD
TRACK WIDTH W:
0.03" PER AMPERE
ON 1OZ Cu FOIL
SENSE
RESISTOR
POWER PAK
SO-8
12V
OUT
12V
IN
GND
422312 F12
GND
C
G
12V
GATE
VIA TO
GND PLANE
161
2
15
LTC4223CGN*
•
•
CURRENT FLOW
TO LOAD
W
CURRENT FLOW
TO SOURCE
*ADDITIONAL DETAILS OMITTED FOR CLARITY, DRAWING NOT TO SCALE!
VIA TO
GND PLANE
W
W
R
G
R3
R1Z1
C1
8
Figure 12. Recommended Layout for Power MOSFET, Sense
Resistor and GATE Components on 12V Rail
Table 2. AMC Power Supply Requirements
SUPPLY VOLTAGE MAXIMUM LOAD
CURRENT
MAXIMUM LOAD
CAPACITANCE
12V 7.4A 800μF
3.3V
AUX
150mA 150μF
Table 3. MicroTCA Power Supply Requirements
SUPPLY VOLTAGE MAXIMUM LOAD
CURRENT
MAXIMUM LOAD
CAPACITANCE
12V 7.6A 1600μF
3.3V
AUX
150mA 150μF
