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LT8584

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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

theovershootvoltagethatfollowsthelargeinrushcurrent.

ThedownstreamBSM,however,mightencounterdamage

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

RDS,ONtomaximizeconverterefficiencyandhavelessthan

a 1.25V VGS threshold. Table 7 lists several recommended

FETs for M1. C1 should be sized such that C1 ≥ CVIN/500.

Thethirdactivesolutionprotectstheflybackoutputcapaci-

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, VRRM,

should exceed the maximum module voltage, VMODULE.

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 CVIN

capacitor to the battery voltage, limiting the inrush cur-

rent. After the CVIN 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 CVIN 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.

8584 F11

CVIN

VBAT

BATTERY

CONNECTION

–

10Ω

LT8584

DIN

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. Two 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.TheseshouldbesmallerthanD1sincetheLT8584’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.

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