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PDF SC1437ISK-B45TR Data sheet ( Hoja de datos )

Número de pieza SC1437ISK-B45TR
Descripción Precision Voltage Detector With Programmable Trigger Voltage & Timer Delay
Fabricantes Semtech Corporation 
Logotipo Semtech Corporation Logotipo



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SC1437
Precision Voltage Detector With
Programmable Trigger Voltage & Timer Delay
POWER MANAGEMENT
Description
Features
The SC1437 is a battery over voltage detection circuit with driver
for external MOSFET. The trip point is adjustable with an external
resistor divider connected to the SENSE pin. The trip point is
preset to a nominal trip voltage of 4.2V if no external divider is
used. Other internal trip voltages of 4.5V and 4.7V are available,
specified by different part numbers. An internal delay, with two
selectable times or bypass, is on-board to suppress accidental
over voltage conditions due to glitches on the battery supply volt-
age, V+. The delay is externally adjustable by pulling the SEL pin
high, low, or left floating. During an actual over voltage condition,
the internal driver will pull the over voltage pin (OV) down to the
voltage present at the V- pin. This insures the external N-Channel
MOSFET will be completely off during an over voltage condition.
Supply current during the monitoring mode of operation is ap-
proximately 10µA. If an over voltage condition is detected, supply
currents increase when the timer is started. A voltage comparator
will engage at voltages greater than 6.8V, which bypasses the
timer and open circuits the battery by turning off the external
MOSFET. This important function protects the battery during the
timer delay in the event of a charger failure. There are three
functional options available. With the B option the OV pin is active
low. With the L option the OV pin is active high. With the H option
the OV pin is open drain, active high. All options are available in a
5 lead SOT-23 package.
! Low quiescent current, less than 10µA typical
! Input voltage range 1.8V to 6.8V
! Adjustable or preset voltage trip point
! Selectable timer delays of 0, 40 and 80ms without use of
external resistor
! SOT-23 5 lead package
! Accuracy is 2% over specified temperature range
! 30V maximum rating
Applications
! Battery packs
! Portable electronics
! Lithium Ion protection circuits
! Programmable Fuse
! Power Monitoring
! CPU Reset Circuits
Typical Application Circuit
BATTERY
R1
R2
Revision 01/10/2001
+
100
0.1uF
1 SEL
4
SENSE
3 OV
V+ 5
V- 2
SC1437
100
0.1uF
VTRIP = 2.1(1+R2/R1)
0.476 < (R2/R1) < 2
-
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SC1437ISK-B45TR pdf
SC1437
POWER MANAGEMENT
Applications Information
The SC1437 can be used as a battery over voltage detection
circuit with driver for an external MOSFET. In this configuration
the SC1437 is capable of protecting the battery if the charger
circuit fails. Because of its low cost, the SC1437 is an excellent
choice in cases when primary discharge protection is not
required. The part is an excellent choice for secondary over
voltage protection where the battery has primary fault and
discharge protection, but could benefit by adding another line of
defense against charger failure.
The SC1437 comes in three voltage trip options and three
function options. The voltage options are 4.2V, 4.5V or 4.7V. In
addition, the trip voltage levels can be adjusted with an
external resistive divider.
The SC1437 also has three separate functional options that
affect the OV pin action:
With the B option, the OV pin is active low and is used to drive
an N-Channel MOSFET.
With the H option, the OV pin is active high with open drain to
drive a P-Channel MOSFET.
Finally, the L option is active high and it's output is 5V logic high
for specific use as a voltage detector flag.
Any part can be configured as a voltage detector as shown in
Figure 1. A programmable trip delay can be set as indicated by
Table 1. In Figure 1, the trip voltage is set externally by R1 & R2.
while the trip delay is set to 0 seconds plus any propagation
delay of the SC1437 to allow the MOSFET to turn off, specified
by parameter toff.
TABLE 1
SEL CONNECTION
DELAY
V+ NO DELAY
V- 40mS
OPEN
80mS
CHARGER OVERVOLTAGE PROTECTION
The SC1437 configuration for secondary overvoltage protection
is shown in Figure 2. This is the Evaluation Board schematic.
The Evaluation Board is set up to work with either internal trip
voltage or external trip voltage via R1 and R2. It also can set the
trip delay to all three values through jumper selection of JP2
and JP3.
With CHARGER+ and CHARGER- input applied, the charger
voltage will be filtered by R3 and C1. The filtered voltage will
then be sensed by the V+ input. In this case the trip voltage can
be set to a value other than the internal setting of 4.2V. This is
done by using the on-board external resistor divider and
removing JP1 shunt jumper that will allow the resistors R1 and
R2 to program a trip voltage of 5.75V. The Evaluation Board also
allows the trip delay to be set to 0S, 40mS or 80mS by applica-
tion of a jumper JP2, JP3 or leaving the jumper off, respectively.
Configuration of timing delays are shown in Table 1. Once the
trip delay is set to the required value the SC1437 will monitor
the charger voltage. If the charger voltage equals or exceeds
5.5V the OV pin will transition from high to low (Version B)
turning Q1 off, thereby breaking the circuit and protecting the
battery from overvoltage.
Figure 3 shows the trip delay of 80mS, while Figure 4 shows the
trip delay set to 40mS and Figure 5 shows no delay set.
FIGURE 1
VSENSE+
R4
100
VTRIP
C2
0.1uF
U1
1 SEL V+ 5
2 V-
34
OV SENSE
SC1437
R3 R1
100
C1
0.1uF
R2
VSENSE-
VOLTAGE DETECTOR: TRIP DELAY = 0
VTRIP = 2.1(1+R2/R1)
0.476 < (R2/R1) < 2
When using the program resistors R1 & R2, Figure 2, to set an
alternate trip voltage, be sure these resistors are placed across
the battery on the drain side of the MOSFET. This will eliminate
any voltage drop errors caused by the MOSFET on resistance.
Figure 6 and Figure 7 shows the normalized trip voltage
variation with respect to temperature. Note that in all cases the
trip voltage remains within its 2% specified range. This is true for
both the internal trip voltage and externally set trip voltage,
however when using an external resistor divider, the tolerance
and temperature coefficient of the resistors will add to the
overall device error. Choice of the resistor tolerance and
temperature coefficient will depend on your error budget. We
also recommend using high value resistors to limit battery
current drain. Values between 100K and 1M should suffice
without introducing excessive noise into the IC.
CHOOSING THE EXTERNAL MOSFET
The external switch must be a logic level MOSFET. Most logic
level MOSFETs have threshold voltages in the 1V range and can
handle a gate to source voltage (VGS) of 12V which is well
above the clamp voltage of the SC1437. In addition, the
selection should be able to handle the charging current of the
battery over all temperature ranges. Whether to use a P-Channel
or N-Channel MOSFET depends upon the option chosen. The B
option requires the use of a N-Channel MOSFET while the H
option uses a P-Channel MOSFET.
2000 Semtech Corp.
5
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