Datasheet LTC3619B (Analog Devices) - 9

LTC3619B
operaTion Short-Circuit Protection Programming Input Current Limit
When either regulator output is shorted to ground, the Selection of one external RLIM resistor will program the corresponding internal N-channel switch is forced on for input current limit. The current limit can be programmed a longer time period for each cycle in order to allow the from 200mA up to IPEAK current. As the input current inductor to discharge, thus preventing inductor current increases, RLIM voltage will follow. When RLIM reaches runaway. This technique has the effect of decreasing the internal comparator threshold of 1V, channel 2’s switching frequency. Once the short is removed, normal power PFET on-time will be shortened, thereby, limiting operation resumes and the regulator output will return to the input current. its nominal voltage. Use the following equation to select the RLIM resistance
Input Current Limit
that corresponds to the input current limit. Internal current sense circuitry in each channel measures 55kΩ− A RLIM = the inductor current through the voltage drop across the IDC(A) power PFET switch and forces the same voltage across the small sense PFET. The voltage across the small sense IDC is the input current (at VIN) to be limited. The following PFET generates a current representing 1/55,000th of the are some RLIM values with the corresponding current limit. inductor current during the on-cycle. The current out of
RLIM IDC
RLIM pin is the summed representation of the inductor 91.6k 600mA currents from both channels, which can be expressed in 110k 500mA the following equation. 137.5k 400mA IRLIM = IOUT1 • D1 • K1 + IOUT2 • D2 • K2, where D1 = VOUT1/VIN and D2 = VOUT2/VIN are the duty
Selection of CLIM Capacitance
cycle of channel 1 and 2, respectively. Since IRLIM current is a function of the inductor current, K1 is the ratio RDS(ON) (power PFET)/RDS(ON)(sense PFET) its dependency on the duty cycle cannot be ignored. Thus, of channel 1, and K2 is the ratio RDS(ON)(power PFET)/ a CLIM capacitor is needed to integrate the IRLIM current RDS(ON) (sense PFET) of channel 2. The ratio of the power and smooth out transient currents. The LTC3619B is stable PFET to the sense PFET is trimmed to within 2%. with any size capacitance >100pF at the RLIM pin. Given that both PFETs are carefully laid out and matched, Each application input current limit will call for different their temperature and voltage coefficient effects will be CLIM value to optimize its response time. Using a large CLIM similar and their terms be canceled out in the equation. In capacitor requires longer time for the RLIM pin voltage that case, the constants K1 and K2 will only be dependent to charge. For example, consider the application 500mA on area scaling, which is trimmed to within 2%. Thus, the input current limit, 5V input and 1A, 2.5V output with a IRLIM current will track the input current very well over 50% duty cycle. When an instantaneous 1A output pulse is varying temperature and VIN. applied, the current out of the RLIM pin becomes 1A/55k = 18.2µA during the 50% on-time or 9.1µA full duty cycle. The RLIM pin can be grounded to disable input current With a C limit function. LIM capacitor of 1µF, RLIM of 116k, and using I = CdV/dt, it will take 110ms for CLIM to charge from 0V to 1V. This is the time after which the LTC3619B will start input current limiting. Any current within this time must be considered in each application to determine if it is tolerable. 3619bfb 9 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 Package Description Revision History Typical Applications Related Parts