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SC1437ISK-LXXTR

Precision Voltage Detector With Programmable Trigger Voltage & Timer Delay

Semtech Corporation 

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-

3

4

OV SENSE

SC1437

R3

R1

100

C1

R2

0.1uF

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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