Design Procedure

Fuse Drive Options

The MAX1906 supports two methods for blowing the

external protection fuse: the internal SCR can be directly

connected to the fuse’s heater terminal or an external

MOSFET can be used to drive the heater. The design

procedure for both methods requires matching the drive

capabilities in the SCR or the MOSFET with the dissipa-

tion required to blow the fuse.

The SCR configuration is simple, low cost, and does not

require external components. The circuit in Figure 1 is

appropriate for fuses that require heater currents up to

2A. Since the voltage drop across the SCR can be up to

2V, care must be taken not to exceed the device’s power

able to conduct heat away from the MAX1906, it can dis-

sipate 1.6A at typically 1.7V indefinitely. When smaller

copper planes are used, the time to clear the fuse must

be less than the time for the MAX1906 to exceed its

absolute maximum thermal ratings.

The transient thermal characteristics for the MAX1906

are shown in the Typical Operating Characteristics.

Since the thermal resistance varies inversely with the

area of the copper plane attached to the device, the time

to reach thermal limit also varies with copper area.

External MOSFETs should be used with the MAX1906

when the heater current must be greater than 2.0A.

MOSFETs with the required thermal characteristics are

available from multiple manufacturers (see Table 1).

Figure 2 shows the typical application circuit using an

external MOSFET.

Protection Fuse Selection

Protection fuse characteristics can vary considerably

from manufacturer to manufacturer. Always review the

data sheet carefully when selecting the protection fuse.

Table 2 lists the contact information for manufacturers

of compatible fuses.

There are two methods for opening the protection fuse.

The fuse can be blown through the heater or by too

much dissipation along the high-current path. The fuse

must be selected to accommodate the required operat-

ing current without placing stress on the fuse. Once the

nominal current-handling characteristics of the fuse are

set, determine the amount of drive current and the time

required to blow the fuse through the heater terminal.

These quantities are also listed in the fuse manufactur-

er’s data sheet.

The fuse blows when sufficient power is dissipated in the

heater resistor to melt the fuse’s internal solder joints:

the voltage drop on the internal SCR or an external MOS-

The time required to blow the protection fuse, or clear-

ing time, depends upon the power dissipation in the

heater resistor and the ambient temperature. Fuse man-

ufacturers typically provide a curve of clearing time vs.

voltage, and the clearing time vs. ambient temperature.

The greater the power dissipation in the heater resistor,

the quicker the fuse blows. Clearing time is also inverse-

ly proportional to ambient temperature. The heater resis-

tance for different operating current specifications can

range from a few ohms to a few hundred ohms. The

resistance should be selected based on the acceptable

clearing time and operating temperature range.

For a battery pack requiring 4A of operating current, a

fuse with a 5A nominal current rating is appropriate. An

SFD-145B device made by Sony Chemical Corp. is

selected, which has a 22

Ω fusible resistor. Based on

safety considerations, the clearing time should be no

more than 1s or 2s. This is commensurate with the

delay time required to detect the fault condition. The

power dissipated in the SCR when the fuse is blown is

junction temperature in the MAX1906 never exceeds

150°C at 60°C ambient temperature, the required ther-

mal resistance must be:

area, and is shown in the Typical Operating

Characteristics. Even though a combined thermal resis-

copper area, it is advisable to include some margin to

per area is conservative, and is available in most designs.

RR

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2

Li+ Battery-Pack Protector with

Integrated Fuse Driver

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