Datasheet LTC3649 (Analog Devices) - 10

link to page 10 LTC3649
OPERATION
If current monitoring is needed but current limiting is not, The general idea behind this setup is that once the induc- simply pick an RIMON resistor small enough such that VIMON tor current rises, the current out of the IMON pin will rise will never approach 2V. A 10k resistor along with a 10nF proportionally. As a result, the ISET voltage will increase, capacitor is typically a good RC pair to use in this case. thus increasing the regulated output voltage. This rise of If current limiting is useful for the application, it is im- VOUT offsets the voltage drop across the cable, RCABLE, portant to carefully pick the value of the capacitor from thus keeping VPOL constant. IMON to GND, CIMON. If CIMON is picked to be too large, RSET2 should be sized to account for the amount of cable then the switching regulator will be slow to react to a resistance: large output transients, and the average inductor current R will rise above the programmed level until the loop can SET2 = 40000 • RCABLE react. If CIMON is picked to be too small, then the loop can Furthermore, in order to regulate VPOL at the desired become unstable. Typically, an RC time constant that is voltage: at least 10 times slower than the switching frequency is (R a good place to start. SET1 + RSET2) • 50µA = VPOL(DESIRED) CSET1 is still required if soft-start is desired for the ap- 10 1.59 C plication, and C IMON • RIMON ≥ = SET2 is required to filter out the AC ripple 2π • f f noise of the inductor current. Once again, typically C SW SW SET2 and RSET2 should be sized to have a RC time constant 10
Cable Drop Compensation
times slower than the switching frequency. In certain applications, the point of load will be separated
Input Voltage Regulation
from the switching regulator with a significant amount of wire resistance. Thus, the voltage at the point of load, In certain applications, the input supply to the power regu- V lator can exhibit fairly high output impedance. As a result, POL, will be reduced from VOUT near the regulator by the resistance of the trace/wire multiplied by the current. when the regulator is running at heavy loads, VIN might In those applications, it is useful to adjust for the V droop more than desired. The input voltage regulation loop OUT regulation point depending on the average output current allows the application to be programmed to decrease the to maintain an accurate V peak inductor current level, and consequently the input POL. current draw, when it senses that the input voltage has The IMON feature of the LTC3649 along with its single dropped below a programmed threshold. This threshold is resistor output voltage programmability allows this fea- programmed by connecting a resistor divider from VIN to ture to be implemented with the following configuration GND with its intermediate node fed back to VINREG. With shown in Figure 1. this setup, if VVINREG ever falls below 2V, the regulator will decrease the output current level in order to maintain RCABLE VPOL(DESIRED) a 2V level at the pin. If this feature is not required, tie the SW = VOUT – IOUT • RCABLE LTC3649 VINREG pin to INTVCC to prevent this control loop from VOUT LOAD C interfering with normal operation. OUT ISET IMON Another useful application for the input voltage regulation RSET1 loop is for momentary hold up supplies. Suppose an input CSET1 supply is suddenly removed from the application, VIN will R immediately start to drop until it reaches the programmed SET2 CSET2 input voltage regulation point. When this happens and 3649 F01 CCM operation is selected, the regulator will actually take
Figure 1. Cable Drop Compensation Application
charge from the output capacitor and boost charge back to 3649fb 10 For more information www.linear.com/LTC3649 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts