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LT8697 Datasheet(PDF) 17 Page  Linear Dimensions Semiconductor 

LT8697 Datasheet(HTML) 17 Page  Linear Dimensions Semiconductor 
17 / 28 page LT8697 17 8697p For more information www.linear.com/LT8697 applicaTions inForMaTion bedesignedtolimitloadcurrent.Also,anelectronicswitch may be necessary to prevent an output overcurrent condi tion on the USB5V output from bringing down the SYS output. See the Inductor Selection and Maximum Output Current discussion below to determine how much total load current can be drawn from the outputs for a given LT8697 application. Setting the Current Limit In addition to regulating the output voltage, the LT8697 includes a current regulation loop for setting the average outputcurrentlimit.TheLT8697measuresthevoltagedrop across an external current sense resistor RSENSE using the ISP and ISN pins. This resistor should be connected in series with the load current after the output capacitor. The current loop modulates the cyclebycycle top switch switch current limit such that the average voltage across the ISP–ISN pins does not exceed its regulation point. The LT8697 current limit can be programmed by forcing a voltage on the ICTRL pin between 0V and 1V. Program the current limit using the following equation: ILIM = VCTRL RSENSE • 20.3 The preceding ILIM equation is valid for VISP – VISN < 48mV. At 48mV VSENSE, the internal current limit loop takes over output current regulation from the ICTRL pin. The maximum programmable output current (ILIM(MAX)) is therefore found by the following equation: ILIM(MAX) = 48mV RSENSE The internal 2μA pullup on the ICTRL pin allows this pin to be floated if unused, in which case the ILIM(MAX) would be the output current limit. When in forced continuous mode, the LT8697’s ability to regulate the output current is limited by its tON(MIN). In this scenario, at very low output voltage the output current can exceed the programmed output current limit and is limited by the bottom switch current limit of 4.5A plus 1/2 the ripple current. To help mitigate this effect, at low output voltage the LT8697 folds back the switching frequency by 10:1 to allow regulation at very low duty cycle. Also, above VIN = 29V the LT8697 disables forced continuous mode so the part can pulse skip to maintain regulation at any low VOUT to VIN ratio. For VIN < 29V, use the following equation to find the minimum output voltage (VOUT(MIN)) where the LT8697 can regulate the output current limit: VOUT(MIN) = 0.1 • fSW • tON(MIN) • (VIN – VSW(TOP) + VSW(BOT)) – VSW(BOT) – VSENSE – VL where fSW is the switching frequency, tON(MIN) is the minimum ontime, VSW(TOP) and VSW(BOT) are the in ternal switch drops (~0.3V and ~0.15V) respectively at maximum load), VSENSE is voltage across the RSENSE at the programmed output current and VL is the resistive drop across the inductor ESR at the programmed output current. If the calculated VOUT(MIN) is negative or is less than the IR drop across the resistive short on the output at the programmed current limit, then the LT8697 can regulate the output current limit. In practical applications, the resistances of the cable, inductor and sense resistor are more than adequate to allow the LT8697 to regulate to the output current limit for any switching frequency and input voltage. For a 400kHz application in a worstcase condition, the programmed output current can be regulated into VOUT = 0V for any input voltage up to 42V. For a 2MHz application in a worst case condition, the programmed output current can be regulated into VOUT = 0.3V or higher. Refer to Figure 6 to see how the front page application circuit responds to a short directly on the regulator output without a cable. IOUT (A) 1 0.4 0.6 3 8697 F06 0.2 0 1.5 2 2.5 1.0 0.8 VCTRL = OPEN, VIN = 27V VCTRL = 0.5V, VIN = 16V VCTRL = OPEN, VIN = 16V VCTRL = 0.5V, VIN = 27V Figure 6. Output Current Regulation vs VOUT at fSW = 2MHz, RSENSE = 18mΩ 
