Datasheet LTC3619B (Analog Devices) - 10

LTC3619B
operaTion
Figure 1a shows VIN (IIN) current below input current limit this 92µs, the input current limit is not yet engaged and with a CLIM capacitor of 0.1µF. Channel 1 is unloaded to the output must deliver the required current load. This simplify calculations. When the load pulse is applied, may cause the input voltage to droop if the current com- under the specified condition, ILIM current is 1.1A/55k • pliance is exceeded. Depending on how long this time is, 0.66 = 13.2µA, where 0.66 is the duty cycle. It will take a the VIN supply decoupling capacitor can provide some of little more than 7.5ms to charge the CLIM capacitor from this current before VIN droops too much. In applications 0V to 1V, after which the LTC3619B begins to limit input with a bigger VIN supply decoupling capacitor and where current. The IIN current is not limited during this 7.5ms VIN supply is allow to droop closer to dropout, the CLIM time and is more than 725mA. This current transient may capacitor can be increased slightly. This will delay the cause the input supply to temporarily droop if the supply start of input current limit and artificially regulated VOUT current compliance is exceeded, but recovers after the before input current limit is engaged. In this case, within input current limit engages. The output will continue to the 577µs load pulse, the VOUT voltage will stay artificially deliver the required current load while the output voltage regulated for 92µs out of the total 577µs before the input droops to allow the input voltage to remain regulated current limit activates. This approach may be used if a during input current limit. faster recovery on the output is desired. For applications with short load pulse duration, a smaller Selecting a very small CLIM will speed up response time CLIM capacitor may be the better choice as in the example but it can put the device within threshold of interfering shown in Figure 1b. Channel 1 is unloaded for simplifi- with normal operation and input current limit in every cation. In this example, a 577µs, 0A to 2A output pulse few switching cycles. This may be undesirable in terms is applied once every 4.7ms. A CLIM capacitor of 2.2nF of noise. Use 2πRC >> 100/clock frequency (2.25MHz) as requires 92µs for VRLIM to charge from 0V to 1V. During a starting point, R being RLIM, C being CLIM. VOUT VOUT 2V/DIV 200mV/DIV VIN IIN AC-COUPLED 500mA/DIV 1V/DIV VRLIM 1V/DIV IOUT 500mA/DIV IL IIN 1A/DIV 500mA/DIV 50ms/DIV 3619B F01a 1ms/DIV 3619B F01b VIN = 5V, 500mA COMPLIANT VIN = 5V, 500mA COMPLIANT RLIM = 116k, CLIM = 0.1µF RLIM = 116k, CLIM = 2200pF ILOAD = 0A to 1.1A, COUT = 2.2mF, VOUT = 3.3V ILOAD = 0A to 2A, COUT = 2.2mF, VOUT = 3.3V ILIM = 475mA, CHANNEL 1 NOT LOADED ILIM = 475mA, CHANNEL 1 NOT LOADED
Figure 1a. Input Current Limit Within 100ms Load Pulses Figure 1b. Input Current Limit Within 577µs, 2A Repeating Load Pulses
3619bfb 10 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