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LTC2910 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2910 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC2960 11 2960f 2960 F06 DVCC OUT IN+ LTC2960-3 R6 6.81M VB VA R4 681k R5 48.7k Figure 6. External Hysteresis The addition of R6 allows OUT to sink or source current to the summing junction at IN+. Neglecting internal switch resistances and providing that R6 >> R5, the externally modified hysteresis (referred to VA) becomes: VHEXT ≈ VHYS(VA) + VB R4 R6 ⎛ ⎝⎜ ⎞ ⎠⎟ Since the amount of hysteresis is to be doubled, the second term in the above expression needs to be about 300mV. With a logic supply, VB, equal to 3V, the ratio R4/ R6 should be about 0.1. Choosing R6 to be 6.81M satis- fies the design criteria. The addition of R6 modifies the rising and falling thresholds originally determined by R4 and R5. The modified rising threshold becomes: VR = VTH + V+HYS ()• 1+ R4 R5 + R4 R6 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = 400mV + 20mV ()• 1+13.98 + 0.1 () = 6.3336V It is apparent that the R4/R6 term does not affect the ris- ing threshold significantly resulting in a change of only +0.645%. The falling threshold incorporating R6 is: VF = VTH 1+ R4 R5 + R4 R6 VTH –VB VTH ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ = 0.4V • 1+ 13.98 – 0.65 () = 5.732V APPLICATIONS INFORMATION The falling threshold can be restored to the original value by reducing the value of R5. Under the assumption that the addition of R6 has a negligible impact on the rising threshold, a new R4/R5 ratio can be calculated as shown: R4 R5 = VR VTH + V+HYS () –1 = 6.6V 420mV –1 = 14.71 Given the ratio of R4/R5, the closest 1% resistor value for R5 is 46.4k. With the actual resistor values now known, the final thresholds can be calculated by plugging the values into the equations above for VR and VF to obtain: VR = 6.626V, VF = 6.010V, VHYST = 616mV As a result of the added current component through R6 an error term exists that is a function of the pull-up volt- age, VB in Figure 6. Operation with Supply Transients over 40V and Hot Swapping The circuit in Figure 7(a) allows the LTC2960 to withstand high voltage transients. The magnitude of the voltage transients that can be absorbed is set by the voltage rat- ing of RZ. A TT-IRC pulse-withstanding surface mount 1206 resistor with a nominal voltage rating of 200V is used. The external 30V Zener diode (Z1) and the 143kΩ current limiting resistor (RZ) protect the VIN supply pin of the LTC2960. Note that there is a speed penalty which is the time constant determined by RZ and C1, 14.3ms in this example. If VIN is below 30V, there is a voltage drop across RZ that is dependent on the quiescent current of the LTC2960 which is nominally less than 150mV but can be as high as 290mV if MR is pulled low. The maximum voltage drop is determined by the maximum specified ICC and MR pull-up currents. For conditions where the Zener conducts current, it can be biased in the microamp range owing to the low quiescent current of the LTC2960. For a supply voltage of 150V, the Zener is biased <1mA. When input pins are used to sense VIN, the input pins ADJ/IN+/ IN– absolute maximum rating of 3.5V must not be exceeded. VIN can be a maximum of 8.75x the lowest programmed threshold to satisfy this condition. For a maximum VIN of 150V, the lowest programmable threshold is >17V. |
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