Datasheet LTC3606B (Analog Devices) - 9

LTC3606B
OPERATION
Figure 1a shows VIN (IIN) current below input current and the output must deliver the required current load. limit with a CLIM capacitor of 0.1μF. When the load pulse This may cause the input voltage to droop if the current is applied, under the specifi ed condition, ILIM current is compliance is exceeded. Depending on how long this time 1.1A/55k • 0.66 = 13.2μA, where 0.66 is the duty cycle. is, the VIN supply decoupling capacitor can provide some It will take a little more than 7.5ms to charge the CLIM of this current before VIN droops too much. In applications capacitor from 0V to 1V, after which the LTC3606B begins with a bigger VIN supply decoupling capacitor and where to limit input current. The IIN current is not limited during VIN supply is allow to droop closer to dropout, the CLIM this 7.5ms time and is more than 725mA. This current capacitor can be increased slightly. This will delay the transient may cause the input supply to temporarily start of input current limit and artifi cially regulated VOUT droop if the supply current compliance is exceeded, but before input current limit is engaged. In this case, within recovers after the input current limit engages. The output the 577μs load pulse, the VOUT voltage will stay artifi cially will continue to deliver the required current load while the regulated for 92μs out of the total 577μs before the input output voltage droops to allow the input voltage to remain current limit activates. This approach may be used if a regulated 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. In this example, a 577μs, 0A to 2A with normal operation and input current limit in every output pulse is applied once every 4.7ms. A CLIM capacitor few switching cycles. This may be undesirable in terms of 2.2nF requires 92μs for VRLIM to charge from 0V to 1V. of noise. Use 2πRC >> 100/clock frequency (2.25MHz) as During this 92μs, the input current limit is not yet engaged 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 3606B F01a 1ms/DIV 3606B 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 ILIM = 475mA
Figure 1a. Input Current Limit Within 100ms Load Pulses Figure 1b. Input Current Limit Within 577μs, 2A Repeating Load Pulses
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