NCP346
http://onsemi.com
7
Normal Operation
Figure 1 illustrates a typical configuration. The external
adapter provides power to the protection system so the
circuitry is only active when the adapter is connected. The
OVP monitors the voltage from the charger and if the
voltage exceeds the overvoltage threshold, V
th
, the OUT
signal drives the gate of the MOSFET to within 1.0 V of
V
CC
, thus turning off the FET and disconnecting the source
from the load. The nominal time it takes to drive the gate to
this state is 400 nsec (1.0 msec maximum for gate
capacitance of < 12 nF). The CNTRL input can be used to
interrupt charging and allow the microcontroller to measure
the cell voltage under a normal condition to get a more
accurate measure of the battery voltage. Once the
overvoltage is removed, the NCP346 will turn on the
MOSFET. The turn on circuitry is designed to turn on the
MOSFET more gradually to limit the in−rush current. This
characteristic is a function of the threshold of the MOSFET
and will vary depending on the device characteristics such
as the gate capacitance.
There are two events that will cause the OVP to drive the
gate of the FET to a HIGH state.
• Voltage on IN Rises Above the Overvoltage Detection
Threshold
• CNTRL Input is Driven to a Logic HIGH
Adjusting the Overvoltage Detection Point with
External Resistors
The separate IN and V
CC
pins allow the user to adjust the
overvoltage threshold, V
th
, upwards by adding a resistor
divider with the tap at the IN pin. However, R
in
does play a
significant role in the calculation since it is several
10’s of kW. The following equation shows the effects of R
in
.
V
CC
+ V
x
(1 ) R
1
ń(R
2
ńńR
in
))
(eq. 1)
GND
Figure 4. Voltage divider input to adjust overvoltage
detection point
V
CC
R
1
R
2
R
in
I
N
which equates to:
V
CC
+ V
x
(1 ) R
1ń
R
2
) R
1
ńR
in
)
(eq. 2)
So, as R
in
approaches infinity:
V
CC
+ V
x
(1 ) R
1
ńR
2)
(eq. 3)
This shows that R
in
shifts the V
th
detection point in
accordance to the ratio of R
1
/ R
in
. However, if R
1
<< R
in
,
this shift can be minimized. The following steps show this
procedure.
Designing around the Maximum Voltage Rating
Requirements, V(V
CC
, IN)
The NCP346’s maximum breakdown voltage between
pins V
CC
and IN is 15 V. Therefore, care must be taken that
the design does not exceed this voltage. Normally, the
designer shorts V
CC
to IN, V(V
CC
, IN) is shorted to 0 V, so
there is no issue. However, one must take care when
adjusting the overvoltage threshold.
In Figure 4, the R1 resistor of the voltage divider divides
the V(V
CC
, IN) voltage to a given voltage threshold equal to:
(V
CC
,IN)+ V
CC
*(R1ń(R1 ) (R2ńńR
in
)))
(eq. 4)
V(V
CC
, IN) worst case equals 15 V, and V
CC
worst case
equals 30 V, therefore, one must ensure that:
R1ń(R1 ) (R2ńńR
in
)) t 0.5
(eq. 5)
Where 0.5 = V(V
CC
, IN)max/V
CCmax
Therefore, the NCP346 should only be adjusted to
maximum overvoltage thresholds which are less than 15 V.
If greater thresholds are desired than can be accommodated
by the NCP346, ON Semiconductor offers the NCP345
which can withstand those voltages.
