Datasheet LTC3606B (Analog Devices) - 8

LTC3606B
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 input corresponding internal N-channel switch is forced on for current limit. The current limit can be programmed from a longer time period for each cycle in order to allow the 200mA up to IPEAK current. As the input current increases, inductor to discharge, thus preventing inductor current RLIM voltage will follow. When RLIM reaches the internal runaway. This technique has the effect of decreasing comparator threshold of 1V, the power PFET on-time will switching frequency. Once the short is removed, normal be shortened, thereby, limiting the input current. operation resumes and the regulator output will return to Use the following equation to select the R its nominal voltage. LIM resistance that corresponds to the input current limit.
Input Current Limit
RLIM = 55k / IDC Internal current sense circuitry measures the inductor IDC is the input current (at VIN) to be limited. The following are current through the voltage drop across the power PFET some RLIM values with the corresponding current limit. switch and forces the same voltage across the small sense
R
PFET. The voltage across the small sense PFET generates
LIM IDC
a current representing 1/55,000th of the inductor current 91.6k 600mA during the on-cycle. The current out of RLIM pin is the 110k 500mA representation of the inductor current, which can be 137.5k 400mA expressed in the following equation.
Selection of CLIM Capacitance
IRLIM = IOUT • D1 • K1 Since IRLIM current is a function of the inductor current, where D1 = VOUT1/VIN is the duty cycle. its dependency on the duty cycle cannot be ignored. Thus, K1 is the ratio R a C DS(ON) (power PFET)/RDS(ON)(sense PFET). LIM capacitor is needed to integrate the IRLIM current The ratio of the power PFET to the sense PFET is trimmed and smooth out transient currents. The LTC3606B is stable 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 coeffi cient 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 to that case, the constant K1 will only be dependent on area charge. For example, consider the application 500mA input scaling, which is trimmed to within 2%. Thus, the I current limit, 5V input and 1A, 2.5V output with a 50% duty RLIM current will track the input current very well over varying cycle. When an instantaneous 1A output pulse is applied, temperature and V the current out of the RLIM pin becomes 1A/55k = 18.2μA IN. during the 50% on-time or 9.1μA full duty cycle. With a The RLIM pin can be grounded to disable input current CLIM capacitor of 1μF, RLIM of 116k, and using I = CdV/dt, limit function. it will take 110ms for CLIM to charge from 0V to 1V. This is the time after which the LTC3606B will start input current limiting. Any current within this time must be considered in each application to determine if it is tolerable. 3606bfb 8