Datasheet LTC1696 (Analog Devices) - 9

LTC1696
APPLICATIONS INFORMATION Feedback Inputs
VS The LTC1696 has two feedback inputs that allow monitor- LTC1696 R2 ing of two output voltages. The trip point of the internal FB1 + comparator is set by an internal reference of 0.88V with R1 ±2% accuracy. The output voltage, VS, is sensed through – an external resistor divider network (Figure 1). The resis- tors R1 and R2 values are calculated with the typical trip + VREF = 0.88V – point of 0.88V. R1 • V
Figure 1
S = 0.88 R1+R2 (V The chosen values for R1 and R2 yield an overvoltage R2 = S − 0.88) •R1 threshold of 3.608V (+9.3%). With worst-case tolerances 0.88 applied, the minimum overvoltage threshold is 3.481V As an example, let’s calculate values for R1 and R2 for a (+5.5%) and the maximum overvoltage threshold is 3.3V supply in which an overvoltage indication is required 3.738V (+13.3%). at +10% (3.63V). First, a value for R1 is chosen based on
Reset Function
the allowable resistor divider string current. This is de- termined by power dissipation requirements and possible In the event of an overvoltage condition, the OUT pin of sensitivity to noise coupling into the resistor divider. In the LTC1696 is latched into an active high state. The in- this exercise, assume the resistor divider current is 20µA. ternal latch is reset by pulling the TIMER/RESET pin low R1 is calculated from: through an external N-channel MOSFET switch or pulling V V 0.88V CC voltage below the UVLO trip point of 1.94V. R1= FB = = 44k IDIVIDER 20µA The nearest 1% value for R1 is 44.2k. Now, calculating for R2 yields: 44.2k •(3.63v − 0.88V) R2 = = 138.1k 0.88V Choosing the nearest 1% value yields 137k. 1696fb For more information www.linear.com/LTC1696 9