LT8584
24
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applicaTions inForMaTion
HOT SWAP™ PROTECTION
Large currents are developed when hot swapping a bat-
tery with
a LT8584 application due to the large input bulk
capacitance
coupled with the low ESR of the batteries. In
most cases, the LT8584 should have no problem handling
the overshoot voltage that follows the large inrush current.
The downstream BSM, however, might encounter damage
that requires additional steps and/or circuitry to protect
against hot swapping. Several solutions use a two-path
method incorporating a pre-charge resistive path and a
shunt path (see Figure 11).
This method has the disadvantage of lower efficiency and
higher cost. Use FETs for M1 in Figure 12 that have low
R
DS,ON
to maximize converter efficiency and have less than
a 1.25V V
GS
threshold. Table 7 lists several recommended
FETs for M1. C1 should be sized such that C1 ≥ C
VIN
/500.
The third active solution protects the flyback output capaci-
tors. All
flyback outputs sum together and flow through
D13.
During a Hot Swap condition, D13 will reverse bias
and prevent a large inrush current into the flyback output
capacitors. The peak repetitive reverse voltage, V
RRM
,
should exceed the maximum module voltage, V
MODULE
.
Several recommended diodes for D13 are given in Table 8.
Mechanical Solution
A
mechanical approach may result in a more cost effective
solution. A 10Ω resistor is used to pre-charge the C
VIN
capacitor to the battery voltage, limiting the inrush cur-
rent. After
the C
VIN
cap is charged, a mechanical short is
connected across the resistor and remains there during
all normal operations. There are three recommended solu
-
tions for the mechanical short: 1.) use a > 3A rated jumper
2.) use a mechanical switch or 3.) use a staggered-pin
battery connector. The staggered pin connection has the
long pins connecting to LT8584 through the 10Ω resistor.
The short pins connect directly to the LT8584, shorting
out the 10Ω resistor. Normal insertion has a delay on the
order of milliseconds between the long pin connecting
and short pin connecting to the circuit, allowing C
VIN
to
charge up through a current limiting resistor before the
mechanical short is made.
Order of Assembly
The order of assembly of the battery stack, the LT8584
balancers, and the BSM can also mitigate hot swapping
issues. Having separate boards for both the LT8584
balancers and the BSM is recommended. This allows the
LT8584 balancers to be built and connected during the
battery
stack assembly. The last step involves mating the
battery stack and LT8584 assembly with the BSM board.
Additional filters on the inputs into the BSM also reduce
possible issues during final assembly, see the OUT Pin
Compensation and Filtering section for more detail.
C
V
BAT
BATTERY
CONNECTION
+
–
LT8584
D
IN
Figure 11. Dual Path Hot Swap Solution
For most applications, use the recommended Hot Swap
Solution shown as Active Solution 1 in Figure 12 and in
the Typical Application Section. Several other mechanical,
active, and order-of-assembly solutions are also given as
alternatives or as supplements.
Active Solution
An active solution has the added advantage of automatic
hot swap protection; no additional steps are needed when
connecting batteries. Tw o input protection solutions are
shown with the first solution using only TVS diodes. D1
is selected to trigger around 6V and to take the brunt of
the connection input pulse. The reverse leakage current
is more significant in low-voltage TVS’s. Table 5 gives
several diodes for D1 that have adequate current and
voltage characteristics while minimizing reverse leakage
current. D2 provides secondary protection for the BSM
inputs. These should be smaller than D1 since the LT8584’s
OUT pin limits current. Table 6 gives several diodes that
are optimal for D2.
The second active solution has additional overvoltage
protection via a fuse, F1, and a pre-charge MOSFET circuit.
