ltc4221 Linear Technology Corporation, ltc4221 Datasheet - Page 19

• Manufacturer Part Number: ltc4221
• Description: Dual Hot Swap Controller/power Sequencer With Dual Speed, Dual Level Fault Protection
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
latch is cleared and the FAULT pin sources a 3.8μA pull-up
current to charge up C
As shown in the timing diagram of Figure 11, the autoretry
circuitry will attempt to restart the LTC4221 with a duty
cycle:
t
tion 3. t
Equation 9 located in the Initial Timing Cycle section.
t
and found in the Start-Up Cycle Without Current Limit
section. Using the capacitor values as shown in Figure 10,
the Autoretry Duty cycle works out to be approximately 6%.
Sense Resistor Consideration
The fault current level at which the LTC4221’s internal
electronic circuit breaker trips is determined by sense
resistors connected between each channel’s V
SENSE pins. For both channels, the slow comparator trip
current and the fast comparator trip current are given by
equations (5) and (6) respectively.
The power rating of the sense resistor should be rated at
the fault current level. Table 1 in the Appendix lists some
common sense resistors.
For proper circuit breaker operation, Kelvin-sense PCB
connections between the sense resistor and each channel’s
V
drawing in Figure 12 illustrates the connections between
the LTC4221 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
FILTER
STARTUP
CC
Duty Cycle =
I
I
t
TRIP SC
TRIP FC
ON
and SENSE pins are strongly recommended. The
is defined in Equation 1 and t
=
(
( )
INITIAL
, the start-up cycle delay, is given in Equation 10
(
0 851
)
.
=
=
V
, the initial timing cycle delay, is given in
V
SENSE FC
R
V
SENSE SC
R
SENSE
SENSE
– .
t
ON
U
0 4
(
( )
(
t
+
ON1
STARTUP
V
t
)
)
INITIAL
U
=
. The typical delay t
•
=
3 8
R
100
C
R
.
25
SENSE
ON
SENSE
μ
+
mV
+
mV
1
A
t
t
FILTER
W
STARTUP
ON
is defined in Equa-
)
•
100
+
ON
t
U
FILTER
%
is :
CC
and
(3)
(4)
(5)
(6)
include good thermal management techniques for optimal
sense resistor power dissipation.
Calculating Current Limit
For a selected R
I
where
The maximum I
where
If a 7mΩ sense resistor with ±1% tolerance is used for
current limiting, the nominal slow comparator trip current
is 3.57A. From Equations 7 and 8, I
I
I
current. For reliability purposes, the operation at the
maximum trip current must be evaluated carefully. If
necessary, two resistors with the same R
connected in parallel to yield a nominal R
fits the circuit requirements.
TRACK WIDTH W:
TRIP(SC)
TRIP(SCMAX)
TRIP(SC)
ON 1oz COPPER
0.03" PER AMP
I
I
R
R
TRIP SCMIN
TRIP SCMAX
SENSE MAX
SENSE MIN
Figure 12. PCB Connections to the Sense Resistor
(
(
. The minimum I
must exceed the circuit maximum operating load
(
(
CURRENT FLOW
= 4.26A. For proper operation, the minimum
TO LOAD
)
)
)
)
=
TRIP(SC)
=
SENSE
=
=
W
R
V
R
R
V
SENSE SCMIN
SENSE
R
SENSE SCMAX
SENSE
SENSE MAX
SENSE MIN
, the load current must not exceed
is given by Equation 8:
TRIP(SC)
(
(
(
•
•
(
⎛
⎜
⎝
SENSE RESISTOR
⎛
⎜
⎝
1
V
1
TO
CC n
–
+
)
)
)
R
R
100
)
SENSE n
is given by Equation 7:
=
100
TOL
TOL
TRIP(SCMIN)
=
TO
R
R
SENSE MAX
⎞
⎟
⎠
20 5
⎞
⎟
⎠
SENSE MIN
29 5
CURRENT FLOW
.
SENSE
LTC4221
.
mV
TO LOAD
(
mV
(
TOL
= 2.9A and
value that
)
)
19
can be
4221 F12
4221fa
(7)
(8)