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LT3742EUF-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3742EUF-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3742 11 3742f APPLICATIONS INFORMATION characteristic to notice about this method is that if the output of controller 1 goes out of regulation enough to trip the power good comparator, controller 2 will be disabled. In Figure 2b, a slightly larger capacitor on RUN/SS2 delays the turn-on of controller 2 with respect to controller 1. The start-up waveforms for this method look very similar to the one shown in Figure 6a, but here controller 2 is not disabled if controller 1 goes out of regulation. In Figure 2c, both RUN/SS pins share a single capacitor and start up at the same time. By sharing the same soft- start signal, this method provides ratiometric tracking of the two outputs. Undervoltage Lockout (UVLO) An external resistor divider can be used to accurately set the minimum input voltage at which the LT3742 will operate. Figure 3 shows the basic UVLO operation. Once the UVLO pin drops below 1.25V, an undervoltage lock- out event is signaled, turning on a 3μA current source to provide hysteresis. During a UVLO event, both controllers and the gate drive boost regulator are disabled. For the LT3742, all RUN/SS pins are discharged to get ready for a new soft-start cycle. For each controller that is enabled, it’s RUNSS pin will be held to ~700mV until the input voltage rises above the upper UVLO trip voltage. The UVLO function is only active when one or more of the controllers are enabled using the RUN/SS pin. The UVLO pin can not be used to directly start the part. Do not leave the UVLO pin unconnected; tie it to VIN if not used. A separate, less accurate, internal undervoltage lockout will disable the LT3742 when VIN is less than 3.5V. The UVLO resistor values are chosen to give the desired minimum operating voltage (VIN(MIN)) and the desired amount of hysteresis (VHYST). The LT3742 will turn on when the input voltage is above (VIN(MIN) + VHYST), and once on, will turn off when VIN drops below VIN(MIN). Select the value for RUV1 first, then select the value for RUV2. R V A RR V V UV HYST UV UV IN MIN 1 21 27 125 125 = μ = . • . –. () V V Input Voltage Range The minimum input voltage is determined by either the LT3742’s minimum operating voltage of 4V, UVLO or by the output voltages of a given application. The LT3742 can operate at 100% duty cycle, so if the input voltage drops close to or equal to one of the output voltages, the control- ler will go into low dropout operation (100% duty cycle). The duty cycle is the fraction of the time the N-channel MOSFET is on every switch cycle, and is determined by the input and output voltages: DC VV VV V OUT D IN DS D = + + ⎛ ⎝⎜ ⎞ ⎠⎟ – where VD is the forward drop of the catch diode (~0.4V) and VDS is the typical MOSFET voltage drop (~0.1V). Figure 3. Undervoltage Lockout – + 3 2 1.25V RUV1 RUV2 VIN VIN UVLO 3 μA 3742 F03 UVLO |
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