Datasheet

9
LTC4210-1/LTC4210-2
421012f
SENSE RESISTOR
CURRENT FLOW
TO LOAD
TRACK WIDTH W:
0.03" PER AMP
ON 1 OZ COPPER
W
TO
V
CC
TO
SENSE
4210 F01
CURRENT FLOW
TO LOAD
Figure 1. Making PCB Connections to the Sense Resistor
APPLICATIO S I FOR ATIO
WUUU
Current Limit Circuit Breaker Function
The LTC4210 features a current limiting circuit breaker
instead of a traditional comparator circuit breaker. When
there is a sudden load current surge, such as a low
impedance fault, the bus supply voltage can drop signifi-
cantly to a point where the power to an adjacent card is
affected, causing system malfunctions. The LTC4210 fast
response error amplifier (EA) instantly limits current by
reducing the external MOSFET GATE pin voltage. This
minimizes the bus supply voltage drop and permits power
budgeting and fault isolation without affecting neighbor-
ing cards. A compensation circuit should be connected to
the GATE pin for current limit loop stability.
Sense Resistor Consideration
The nominal fault current limit is determined by a sense
resistor connected between V
CC
and the SENSE pin as
given by Equation 1.
I
V
R
mV
R
LIMIT NOM
CB NOM
SENSE NOM SENSE NOM
()
()
() ()
==
50
(1)
The power rating of the sense resistor should be rated at
the fault current level. Table 2 in the Appendix lists some
common sense resistors.
For proper circuit breaker operation, Kelvin-sense PCB
connections between the sense resistor and the LTC4210
V
CC
and SENSE pins are strongly recommended. The
drawing in Figure 1 illustrates the connections between
the LTC4210 and the sense resistor. PCB layout should be
balanced and symmetrical to minimize wiring errors. In
addition, the PCB layout for the sense resistor should
include good thermal management techniques for optimal
sense resistor power dissipation.
Calculating Current Limit
For a selected R
SENSE
, the nominal load current is given by
Equation 1. The minimum load current is given by
Equation 2:
I
V
R
mV
R
LIMIT MIN
CB MIN
SENSE MAX SENSE MAX
()
()
() ()
==
44
(2)
where
RR
R
SENSE MAX SENSE
TOL
()
•=+






1
100
The maximum load current is given by Equation 3:
I
V
R
mV
R
LIMIT MAX
CB MAX
SENSE MIN SENSE MIN
()
()
() ()
==
56
(3)
where
RR
R
SENSE MIN SENSE
TOL
()
•–=






1
100