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ISL88706IB844Z Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL88706IB844Z Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 15 page 7 FN8092.5 January 12, 2009 Low Voltage Monitoring These devices monitor both the voltage level of VDD and an auxiliary voltage on PFI. When IC is initially biased reset is asserted until the VDD voltage is greater than the specific IC fixed-voltage trip point for the tPOR duration of 200ms. At any subsequent time that VDD does not exceed its voltage threshold, reset is once again asserted, i.e. RST is high and RST is low (see Figure 1). . Power Failure Monitor These devices also have a Power-Failure Monitor that helps to monitor an additional critical voltage on the Power-Fail Input (PFI) pin. For example, the PFI pin could be used to provide an early power-fail warning, detect a low-battery condition, presence detection or simply monitor a power supply other than +5V. The 1.25V threshold detector can be adjusted using an external resistor divider network to provide custom voltage monitoring of voltages greater than 1.25V, according to Equation 1 (see Figure 2). PFO goes low whenever PFI is less than the 1.25V (or user-set) threshold voltage. . If using a voltage divider on the PFI input to critique an external voltage and intending to use the MR input to initiate resets then avoid having the PFI voltage less than PFI Vth +2.2V as unintended PFO transition may occur when MR is transitioning high. Adjusting tPOR On the ISL88707 and ISL88708, users can adjust the Power-On Reset time-out delay (tPOR) to many times the nominal tPOR of 200ms. To do this, connect a capacitor between CPOR and ground (see Figure 3). For example, connecting a 50pF capacitor to CPOR will increase tPOR from 200ms to ~1.4s. Care should be taken in PCB layout and capacitor placement in order to reduce stray capacitance as much as possible, which contributes to tPOR error. FIGURE 1. POWER-SUPPLY MONITORING TIMING DIAGRAM (WDI TRI-STATED) VDD MR RST tPOR VTH1 1V tPOR tPOR >tMR tRPD RST PFI V TH 1.25 R 1 R 2 + R 2 --------------------- ⎝⎠ ⎜⎟ ⎛⎞ = (EQ. 1) FIGURE 2. CUSTOM VTH WITH RESISTOR DIVIDER ON PFI VIN R1 R2 PFI ISL8870x ISL88705, ISL88706, ISL88707, ISL88708, ISL88716, ISL88813 |
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