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SC1437ISK-B42TR Datasheet(PDF) 5 Page - Semtech Corporation |
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SC1437ISK-B42TR Datasheet(HTML) 5 Page - Semtech Corporation |
5 / 8 page 5 2000 Semtech Corp. www.semtech.com POWER MANAGEMENT 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. N O I T C E N N O C L E SY A L E D + VY A L E D O N - VS m 0 4 N E P OS m 0 8 TTTTTABLE 1 ABLE 1 ABLE 1 ABLE 1 ABLE 1 CHAR CHAR CHAR CHAR CHARGER O GER O GER O GER O GER OVER VER VER VER VERVVVVVOL OL OL OL OLTTTTTAAAAAGE PR GE PR GE PR GE PR GE PRO O O O OTECTION TECTION TECTION TECTION TECTION 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 FIGURE 1 VTRIP = 2.1(1+R2/ R1) Applications Information 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. U1 SC1437 1 2 3 5 4 SEL V- OV V+ SENSE C1 0.1uF C2 0.1uF R3 100 R4 100 VSENSE+ R1 VSENSE- VOLTAGE DETECTOR: TRIP DELAY = 0 R2 VTRIP SC1437 CHOOSING THE EXTERNAL MOSFET CHOOSING THE EXTERNAL MOSFET CHOOSING THE EXTERNAL MOSFET CHOOSING THE EXTERNAL MOSFET 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. 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. 0.476 < (R2/ R1) < 2 |
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