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MIC841 Datasheet(PDF) 6 Page - Micrel Semiconductor |
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MIC841 Datasheet(HTML) 6 Page - Micrel Semiconductor |
6 / 8 page MIC841/2 Micrel MIC841/2 6 January 2005 Once the desired trip points are determined, set the V IN(HI) threshold first. For example, use a total of 1MΩ = R1 + R2 + R3. For a typical single-cell lithium ion battery, 3.6V is a good “high threshold” because at 3.6V the battery is moderately charged. Solving for R3: V 3.6V 1.24 1M R3 IN(HI) == Ω R3 344k = Ω Once R3 is determined, the equation for V IN(LO) can be used to determine R2. A single lithium-ion cell, for example, should not be discharged below 2.5V. Many applications limit the drain to 3.1V. Using 3.1V for the V IN(LO) threshold allows calculation of the two remaining resistor values. V 3.1V 1.24 1M R2 344k IN(LO) == Ω + R2 56k = Ω 1M R2 R3 R1 Ω− − () = R1 600k = Ω The accuracy of the resistors can be chosen based upon the accuracy required by the system. Programming the MIC842 Thresholds The voltage threshold is calculated using: VV R1 R2 R2 IN(LO) REF = + where: V 1.240V REF = INP OUT VDD GND MIC842N 470k VIN R1 R2 VOUT 4 2 1 5 Figure 2. MIC842 Example Circuit In order to provide the additional criteria needed to solve for the resistor values, the resistors can be selected such that they have a given total value, that is, R1 + R2 = R TOTAL. A value such as 1MΩ for RTOTAL is a reasonable value because it draws minimum current but has no significant effect on accuracy. Applications Information Output The MIC841N and MIC842N outputs are an open-drain MOSFET, so most applications will require a pull-up resistor. The value of the resistor should not be too large or leakage effects may dominate. 470kΩ is the maximum recommended value. Note that the output of “N” version may be pulled up as high as 6V regardless of the ICs supply voltage. The “H” and “L” versions of the MIC841 and MIC842 have a push-pull output stage, with a diode clamped to V DD. Thus, the maxi- mum output voltage of the “H” and “L” versions is V DD. See “Electrical Characteristics.” When working with large resistors on the input to the devices, a small amount of leakage current can cause voltage offsets that degrade system accuracy. The maximum recommended total resistance from V IN to ground is 3MΩ. The accuracy of the resistors can be chosen based upon the accuracy re- quired by the system. The inputs may be subjected to voltages as high as 6V steady-state without adverse effects of any kind regardless of the ICs supply voltage. This applies even if the supply voltage is zero. This permits the situation in which the IC’s supply is turned off, but voltage is still present on the inputs. See “Electrical Characteristics.” Programming the MIC841 Thresholds The low-voltage threshold is calculated using: VV R1 R2 R3 R2 R3 IN(LO) REF = ++ + The high-voltage threshold is calculated using: VV R1 R2 R3 R3 IN(HI) REF = ++ where, for both equations: V 1.240V REF = In order to provide the additional criteria needed to solve for the resistor values, the resistors can be selected such that they have a given total value, that is, R1 + R2 + R3 = R TOTAL. A value such as 1MΩ for RTOTAL is a reasonable value because it draws minimum current but has no significant effect on accuracy. HTH OUT VDD LTH GND MIC841N 604k 1% 56k 1% 340k 1% 470k VIN R1 R2 R3 VOUT VDD 5 3 1 4 2 Figure 1. MIC841 Example Circuit |
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