Rainbow Electronics MAX16915 - Manual (Page 8)

Ideal Diode, Reverse-Battery, and Overvoltage Protection
Switch/Limiter Controllers with External MOSFETs
MAX16914/MAX16915
8 ______________________________________________________________________________________
Overvoltage Indicator Output (OV)
The MAX16914/MAX16915 include an active-low,
open-drain overvoltage-indicator output (OV). For the
MAX16914, OV asserts low when V
CC exceeds the pro-
grammed overvoltage threshold. OV deasserts when the
overvoltage condition is over.
For the MAX16915, OV asserts if V
OUT exceeds the
programmed overvoltage threshold. OV deasserts when
V
OUT drops 4% (typ) below the overvoltage threshold
level. If the overvoltage condition continues, OV may
toggle with the same frequency as the overvoltage limiter
FET (P2). If the P2 device is turned on for a very short
period (less than t
OVBPD ), the OV pin may not toggle.
To obtain a logic-level output, connect a 45kI pullup
resistor from OV to a system voltage less than 44V. A
capacitor connected from OV to GND helps extend the
time that the logic level remains low.
Applications Information
Load Dump
Most automotive applications run off a multicell “12V”
lead-acid battery with a nominal voltage that swings
between 9V and 16V (depending on load current, charg-
ing status, temperature, battery age, etc.). The battery
voltage is distributed throughout the automobile and is
locally regulated down to voltages required by the differ-
ent system modules. Load dump occurs when the alter-
nator is charging the battery and the battery becomes
disconnected. The alternator voltage regulator is tem-
porarily driven out of control. Power from the alternator
flows into the distributed power system and elevates the
voltage seen at each module. The voltage spikes have
rise times typically greater than 5ms and decays within
several hundred milliseconds but can extend out to 1s
or more depending on the characteristics of the charg-
ing system. These transients are capable of destroying
sensitive electronic equipment on the first “fault event.”
Setting Overvoltage Thresholds
TERM and SET provide an accurate means to set the
overvoltage level for the MAX16914/MAX16915. Use a
resistive divider to set the desired overvoltage condition
(see the
Typical Operating Circuit). V SET has a rising
1.20V threshold with a 4% falling hysteresis. Begin by
selecting the total end-to-end resistance:
R TOTAL = R1 + R2
For high accuracy, choose R
TOTAL
to yield a total cur-
rent equivalent to a minimum 100 x I SET where I SET is the
input bias current at SET.
For example:
With an overvoltage threshold (V
OV
) set to 20V, R
TOTAL
< 20V/(100 x I SET ), where I SET = 1FA (max).
R TOTAL < 200kI
Use the following formula to calculate R2:
R2 = (V TH x R TOTAL )/V OV
where V TH is the 1.20V SET rising threshold and V OV is
the desired overvoltage threshold.
Then, R2 = 12.0kI.
Use the nearest standard-value resistor lower than the
calculated value. A lower value for total resistance dissi-
pates more power but provides slightly better accuracy.
To determine R1:
R
TOTAL
= R2 + R1
Then, R1 = 188kI.
Use the nearest standard-value resistor lower than the
calculated value. A lower value for total resistance dissi-
pates more power but provides slightly better accuracy.
MOSFET Selection
Output p-Channel MOSFET (P2)
Select the external output MOSFET according to the
application current level. The MOSFET’s on-resistance
(R
DS(ON) ) should be chosen low enough to have a
minimum voltage drop at full load to limit the MOSFET
power dissipation. Determine the device power rating to
accommodate an overvoltage fault when operating the
MAX16915 in overvoltage-limiting mode. During normal
operation for either IC, the external MOSFET dissipates
little power. The power dissipated in the MOSFET during
normal operation is:
P NORM = I LOAD
2 x R DS(ON)
where P NORM is the power dissipated in the MOSFET
in normal operation, I LOAD is the output load current,
and R DS(ON) is the drain-to-source resistance of the
MOSFET. Worst-case power dissipation in the output
MOSFET occurs during a prolonged overvoltage event
when operating the MAX16915 in voltage-limiting mode.
The power dissipated across the MOSFET is as follows:
P OVLO = V DS x I LOAD
where P OVLO is the power dissipated in the MOSFET in
overvoltage-limiting operation, V DS is the voltage across
the MOSFET’s drain and source, and I LOAD is the load
current.