Datasheet LTC3619 (Analog Devices) - 10

LTC3619
operaTion
When the output is loaded heavily, for example, with Given that both PFETs are carefully laid out and matched, millifarad of capacitance, it may take longer than 950µs their temperature and voltage coefficient effects will be to charge the output to regulation. If the output is still low similar and their terms be canceled out in the equation. In after the soft-start time, the LTC3619 will try to quickly that case, the constants K1 and K2 will only be dependent charge the output capacitor. In this case, the input current on area scaling, which is trimmed to within 2%. Thus, the limit (after it engages) can prevent excessive amount of IRLIM current will track the input current very well over instantaneous current that is required to quickly charge varying temperature and VIN. the output. See the Channel 2 Start-Up from Shutdown The RLIM pin can be grounded to disable input current curve in the Typical Performance Characteristics section. limit function. After input current limit is engaged, the output slowly ramps up to regulation while limited by its 500mA of
Programming Input Current Limit
input current. Selection of one external RLIM resistor will program the
Short-Circuit Protection
input current limit. The current limit can be programmed from 200mA up to I When either regulator output is shorted to ground, the PEAK current. As the input current increases, RLIM voltage will follow. When RLIM reaches corresponding internal N-channel switch is forced on for the internal comparator threshold of 1V, channel 2’s a longer time period for each cycle in order to allow the power PFET on-time will be shortened, thereby, limiting inductor to discharge, thus preventing inductor current the input current. runaway. This technique has the effect of decreasing switching frequency. Once the short is removed, normal Use the following equation to select the RLIM resistance operation resumes and the regulator output will return to that corresponds to the input current limit. its nominal voltage. RLIM = 55k / IDC
Input Current Limit
IDC is the input current (at VIN) to be limited. The following are some RLIM values with the corresponding current limit. Internal current sense circuitry in each channel measures the inductor current through the voltage drop across the
RLIM IDC
power PFET switch and forces the same voltage across 91.6k 600mA the small sense PFET. The voltage across the small sense 110k 500mA PFET generates a current representing 1/55,000th of the 137.5k 400mA inductor current during the on-cycle. The current out of RLIM pin is the summed representation of the inductor
Selection of CLIM Capacitance
currents from both channels, which can be expressed in Since IRLIM current is a function of the inductor current, the following equation. its dependency on the duty cycle cannot be ignored. Thus, I a C RLIM = IOUT1 • D1 • K1 + IOUT2 • D2 • K2, LIM capacitor is needed to integrate the IRLIM current and smooth out transient currents. The LTC3619 is stable where D1 = VOUT1/VIN and D2 = VOUT2/VIN are the duty with any size capacitance >100pF at the RLIM pin. cycle of channel 1 and 2, respectively. Each application input current limit will call for different K1 is the ratio RDS(ON) (power PFET)/RDS(ON)(sense PFET) CLIM value to optimize its response time. Using a large CLIM of channel 1, and K2 is the ratio RDS(ON)(power PFET)/ capacitor requires longer time for the RLIM pin voltage to RDS(ON) (sense PFET) of channel 2. The ratio of the power charge. For example, consider the application 500mA input PFET to the sense PFET is trimmed to within 2%. current limit, 5V input and 1A, 2.5V output with a 50% duty 3619fa 10 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Functional Diagram Operation Applications Information Package Description Typical Application Related Parts