MAX1906VEEE Maxim Integrated, MAX1906VEEE Datasheet - Page 11

• Manufacturer Part Number: MAX1906VEEE
• Description: Battery Management
• Manufacturer: Maxim Integrated
• Series: MAX1906r

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
ratings. When greater than 1in
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 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.
______________________________________________________________________________________
Protection Fuse Selection
Design Procedure
2
Fuse Drive Options
of copper plane is avail-
Li+ Battery-Pack Protector with
The fuse blows when sufficient power is dissipated in the
heater resistor to melt the fuse’s internal solder joints:
V
condition, which is typically 4.45V per cell. V
the voltage drop on the internal SCR or an external MOS-
FET. R
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 22Ω 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
approximately 1.3V
junction temperature in the MAX1906 never exceeds
150°C at 60°C ambient temperature, the required ther-
mal resistance must be:
where R
case, and R
ambient. R
5mm
area, and is shown in the Typical Operating
Characteristics. Even though a combined thermal resis-
tance of 90°C/W is achievable with less than 0.04in
copper area, it is advisable to include some margin to
reduce the rise in device temperature. Using 0.25in
per area is conservative, and is available in most designs.
Integrated Fuse Driver
BATT_OV
✕
HEATER
5mm QFN package. R
θJC
is the battery-pack voltage in the overvoltage
θJC
θCA
is the thermal impedance from junction to
is fixed, and is about 5°C/W for the 16-lead
is the resistance of the heater resistor.
P
R
(
<
<
HEATER
V
is the thermal impedance from case to
θ
(
90
BATT OV
150
CA
°
✕
C W
+
°
C
R
0.75A or 1W. To ensure that the
/
_
R
θ
-
HEATER
=
JC
60
V
−
<
HEATER
°
C
(
V
T
) / (
SWITCH
MAX
θCA
1
W
-
T
)
×
varies with copper
A
)
I
) ( )
2
HEATER
/
Pd
SWITCH
=
2
cop-
2
11
of
is