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LTC2916HDDB-1-TRPBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC2916HDDB-1-TRPBF Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC2915/LTC2916 9 29156fa Typically, the user will pick a value of R1 based on accept- able current draw. Current used by the resistor divider will be approximately R V I 1 05 = ⎛ ⎝⎜ ⎞ ⎠⎟ . Recommended range of R1 is 1k to 1M. Higher values of resistance exacerbate the degradation of threshold accuracy due to leakage currents. If the nominal value of the supply being monitored is VNOM, then R2 = R1(2VNOM – 1) Resistor tolerances must be taken into account when determining the overall accuracy. Selecting the Reset Timing Capacitor The reset timeout period can be set to one of two fixed internal timers or set with a capacitor in order to accom- modate a variety of applications. Connecting a capacitor, CRT, between the RT pin and ground sets the reset timeout period, tRST. The following formula approximates the value of capacitor needed for a particular timeout: CRT = tRST • 110 [pF/ms] For example, using a standard capacitor value of 2.2nF would give 20ms delay. Figure 2 shows the reset timeout period as a function of the value of the timer capacitor. Leaving RT open with no external capacitor generates a reset timeout of approximately 400μs. Shorting RT to VCC generates a reset timeout of approximately 200ms. RST Output Characteristics The DC characteristics of the RST pull-down strength are shown in the Typical Performance Characteristics section. RST is an open-drain pin and thus requires an external pull-up resistor to the logic supply. RST may be pulled above VCC, providing the voltage limits of the pin are observed. The open-drain of the RST pin allows for wired-OR con- nection of several LTC2915/LTC2916s. As noted in the discussion of power up and power down, the circuits that drive RST are powered by VCC. During a fault condition, VCC of at least 1V ensures that RST pulls low. Manual Reset (LTC2916 Only) The LTC2916 includes the MR pin for applications where a manual reset is desired. MR is internally pulled up, al- lowing it to interface with a push button with no external components required. Asserting MR low when RST is high, initiates a reset, resulting in RST being asserted low for the set reset timeout. Shunt Regulator The LTC2915 and LTC2916 contain an internal 6.2V shunt regulator on the VCC pin to allow operation from a high voltage supply. To operate the part from a supply higher than 5.7V, the VCC pin must have a series resistor, RCC, to the supply. This resistor should be sized according to the following equation: VV mA R VV A SMAX CC SMIN () ( ) . − ≤≤ − 57 5 7 250 μ where VS(MIN) and VS(MAX) are the operating minimum and maximum of the supply. As an example, consider operation from an automobile battery which might dip as low as 10V or spike to 60V. We must then pick a resistance between 10.86k and 12k. APPLICATIONS INFORMATION Figure 2. Reset Timeout Period vs RT Capacitance RT PIN CAPACITANCE, CRT (nF) 0.001 0.1 10 10000 0.1 0.01 1 10 100 1000 29156 F02 1 100 1000 |
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