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LTC2908IDDB-B1 Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LTC2908IDDB-B1 Datasheet(HTML) 11 Page - Linear Integrated Systems |
11 / 16 page 11 LTC2908 sn2908 2908fs APPLICATIO S I FOR ATIO RST Output Characteristics The DC characteristics of the RST pull-up and pull-down strength are shown in the Typical Performance Character- istics section. The RST output has a weak internal pull-up to VCC = Max(V1, V2) and a strong pull-down to ground. The weak pull-up and strong pull-down arrangement allows this pin to have open-drain behavior while possess- ing several other beneficial characteristics. The weak pull-up eliminates the need for external pull-up resistors when the rise time on these pins is not critical. On the other hand, the open-drain RST configuration allows for wired-OR connections and can be useful when more than one signal needs to pull down on the RST line. As noted in the discussion of power-up and power-down, the circuits that drive RST are powered by VCC. During fault condition, VCC of at least 0.5V guarantees a maxi- mum VOL = 0.15V at RST. Output Rise and Fall Time Estimation The following formula estimates the output fall time (90% to 10%) for a particular external load capacitance (CLOAD): tFALL ≈ 2.2 • RPD • CLOAD where RPD is the on-resistance of the internal pull-down transistor estimated to be typically 40Ω at room tempera- ture (25°C) and CLOAD is the external load capacitance on the pin. Assuming a 150pF load capacitance, the fall time is about 13ns. The rise time on the RST pin is limited by a weak internal pull-up current source to VCC. The following formula esti- mates the output rise time (10% to 90%) at the RST pin: tRISE ≈ 2.2 • RPU • CLOAD where RPU is the on-resistance of the pull-up transistor. Notice that this pull-up transistor is modeled as a 6µA current source in the Block Diagram as a typical representation. The on-resistance as a function of the VCC = Max(V1, V2) voltage (for VCC > 1V) at room temperature is estimated as follows: R MAX V V V PU = () Ω 610 12 1 5 • ,– At VCC = 3.3V, RPU is about 260k. Using 150pF for load capacitance, the rise time is 86µs. A smaller external pull- up resistor may be used if the output needs to pull up faster and/or to a higher voltage. For example, the rise time reduces to 3.3µs for a 150pF load capacitance when using a 10k pull-up resistor. |
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