Datasheet LT8611 (Analog Devices) - 16

LT8611
APPLICATIONS INFORMATION Enable Pin
output capacitor to sense the output current or may be The LT8611 is in shutdown when the EN pin is low and placed between the VIN bypass capacitor and the input active when the pin is high. The rising threshold of the EN power source to sense input current. The current loop comparator is 1.0V, with 40mV of hysteresis. The EN pin modulates the internal cycle-by-cycle switch current limit can be tied to V such that the average voltage across ISP-ISN pins does IN if the shutdown feature is not used, or tied to a logic level if shutdown control is required. not exceed 50mV. Adding a resistor divider from V Care must be taken and filters should be used to assure IN to EN programs the LT8611 to regulate the output only when V the signal applied to the ISN and ISP pins has a peak-to- IN is above a desired voltage (see the Block Diagram). Typically, this peak ripple of less than 30mV for accurate operation. In threshold, V addition to high crest factor current waveforms such as IN(EN), is used in situations where the input supply is current limited, or has a relatively high source the input current of DC/DC regulators, another cause of resistance. A switching regulator draws constant power high ripple voltage across the sense resistor is excessive from the source, so source current increases as source resistor ESL. Typically the problem is solved by using a voltage drops. This looks like a negative resistance load small ceramic capacitor across the sense resistor or using to the source and can cause the source to current limit or a filter network between the ISP and ISN pins. latch low under low source voltage conditions. The VIN(EN) The ICTRL pin allows the ISP-ISN set point to be linearly threshold prevents the regulator from operating at source controlled from 50mV to 0mV as the ICTRL pin is ramped voltages where the problems might occur. This threshold from 1V down to 0V, respectively and as shown in Figure 3. can be adjusted by setting the values R3 and R4 such that When this functionality is unused the ICTRL pin may be they satisfy the following equation: tied to INTVCC or floated. In addition the ICTRL pin includes   a 2µA pull-up source such that a capacitor may be added V (10) IN(EN) = R3  +1 • 1.0V for soft-start functionality. R4  The IMON pin is a voltage output proportional to the voltage where the LT8611 will remain off until VIN is above VIN(EN). across the current sense resistor such that VIMON = 20 • Due to the comparator’s hysteresis, switching will not stop (ISP-ISN) as shown in Figure 4. This output can be used until the input falls slightly below VIN(EN). to monitor the input or output current of the LT8611 or When operating in Burst Mode operation for light load may be an input to an ADC for further processing. currents, the current through the VIN(EN) resistor network 60 can easily be greater than the supply current consumed by the LT8611. Therefore, the V 50 IN(EN) resistors should be large to minimize their effect on efficiency at low loads. 40
Current Control Loop
VOLTAGE (mV) 30 ISN In addition to regulating the output voltage the LT8611 -V ISP 20 includes a current regulation loop for setting the average input or output current limit as shown in the Typical Ap- MAX V 10 plications section. 0 0 500 1000 1500 2000 The LT8611 measures voltage drop across an external ICTRL VOLTAGE (mV) current sense resistor using the ISP and ISN pins. This 8611 F03 resistor may be connected between the inductor and the
Figure 3. LT8611 Sense Voltage vs ICTRL Voltage
8611fa 16 For more information www.linear.com/LT8611 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts