MAX11080/MAX11081
the SMCJ70 on the DCIN connection point. This protec-
tion circuit also helps to reduce power spikes that can
occur during the insertion of the battery cells. During
negative voltage excursions, the protection circuit stores
enough charge to power the regulator through the tran-
sient. Figure 14 shows the clamp configuration to protect
the DCIN supply input.
The DCIN input contains a comparator circuit to detect an
open circuit on this pin for fault-management purposes.
Whenever a nominal voltage of two silicon diode drops
appears between C12 and DCIN following the power-up
sequence, the ALRM
L
output is asserted as a fault indica-
tion. This voltage drop must appear for at least the delay
time set by C
DLY
to result in a fault. The voltage drop from
C12 to DCIN during normal operation should be kept at
no more than 0.5V to prevent erroneous tripping of the
DCIN open-circuit comparator under worst-case circum-
stances (lowest silicon diode forward bias voltage). The
diode D
DCIN
is used to supply the transient current
demanded at startup by the decoupling circuit. In parallel
with this diode, R
DCIN
provides the supply path during
normal operation. It is selected to be 5kΩ so that the max-
imum voltage drop between C12 and DCIN is about
0.25V with nominal supply currents.
High-power batteries are often used in noisy environ-
ments subject to high dV/dt or dI/dt supply noise and
EMI noise. For example, the supply noise of a power
inverter driving a high horse-power motor produces a
large square wave at the battery terminals, even though
the battery is also a high-power battery. Typically, the
battery dominates the task of absorbing this noise, since
it is impractical to put hundreds of farads at the inverter.
The MAX11080/MAX11081 are designed with several
mechanisms to deal with extremely noisy environments.
First, the major power-supply inputs that see the full
battery-stack voltage are 80V tolerant. This is high
enough to handle the large voltage changes on the bat-
tery stack that can occur when the batteries transition
between charge and discharge conditions. Next, the
linear regulator has high PSRR to produce a clean low-
voltage power supply for the internal circuitry. This
allows DCIN to be connected directly to the stack volt-
age. Finally, GND
U
serves two purposes. It supplies the
internal charge pump with its power and acts as the ref-
erence ground for the upper alarm communication port.
The charge pump creates a secondary low-voltage
supply that is referenced to GND
U
. Because the level-
shifted supply VDD
U
is referenced to GND
U
, the entire
upper alarm communication port glides smoothly on
GND
U
and it is effectively immune to noise on GND
U
.
The upper alarm signal is internally shifted down to
AGND level where it is processed by the digital logic.
There are two connection methods that can be used for
GND
U
depending on application requirements.
For the top module in a system, or where GND
U
cannot
be DC-coupled to the next higher module for other rea-
sons, GND
U
should be connected to the same location
as DCIN. This connection is valid as long as the voltage
difference between the top of Stack(n) and the bottom
of Stack(n+1) during worst-case conditions does not
exceed the margin of the alarm pin signaling levels.
When GND
U
is not DC-coupled to the far side of the
bus bar, it can be AC-coupled to the far side to main-
tain alarm communication when the bus bar is open-cir-
cuit. In that case, the two sides of the AC-coupling
capacitor can be at different DC potentials, but the
alarm communication signal continues to be passed
across the capacitor connection. It is recommended
that an AC- or DC-coupled version of GND
U
is paired
with the alarm signal through the communication bus
wiring, possibly by twisted pair wire, for maximum noise
immunity and minimum emissions.
The preferred connection to reject noise between mod-
ules is when a DC connection can be made from GND
U
to AGND of the next module. It is again recommended
that the DC-coupled GND
U
signal is routed adjacent to
the alarm signal as part of the communication bus for
maximum noise immunity and minimum emissions.
Shutdown Control
The SHDN pin connections of the MAX11080/MAX11081
operate in a manner that allows the shutdown/wake-up
command to trickle up through the series of daisy-
chained packs. Because the internal linear regulator is
powered down during shutdown, the shutdown function
must operate when V
AA
is absent and, therefore, it can-
not depend on a Schmitt trigger input. A special low-cur-
rent, high-voltage circuit is used to detect the state of the
SHDN pin. The shutdown pin has a +2.1V minimum
threshold for the inactive state. When SHDN > 2.1V, the
MAX11080/MAX11081 turn on and begin regulating V
AA
,
12-Channel, High-Voltage
Battery-Pack Fault Monitors
18 ______________________________________________________________________________________
