Datasheet LTC3785 (Analog Devices) - 10

LTC3785 operaTion switch A and D. In most applications a 0.1µF to 0.47µF, with both comparators having 1.5% hysteresis. During X5R or X7R dielectric capacitor is adequate. an overvoltage fault, all output switching stops until the fault ceases. During an undervoltage fault, the IC is com-
RUN/SOFT-START (RUN/SS)
manded to run fixed frequency only (disabled Burst Mode operation). If the design requires a tightened threshold to The RUN/SS pin serves as the enable to the LTC3785, one of the comparator thresholds the voltage divider on soft-start function, and fault programming. A 1µA current the V source charges the external capacitor. Once the RUN/SS SENSE pin can be skewed to achieve the threshold. Since the range is a constant, tightening the UV threshold voltage is above a diode drop(~0.7V) the IC is enabled. Once will loosen the OV threshold and vice versa. the IC is enabled, the RUN/SS voltage minus a diode drop (RUN/SS – 0.7V) clamps the output of the error amp (VC) to limit duty cycle. The range of the duty cycle clamping is
FORWARD CURRENT LIMIT
approximately 0.7V to 1.7V. The RUN/SS pin is clamped The LTC3785 is designed to sense the input current by sam- to approximately 2.2V. If current limit is reached the pin pling the voltage across MOSFET A during the on time of the will begin to discharge with a current determined by the switch (TG1 = High). The sense pins are ISVIN and ISSW1. A magnitude of inductor current overcurrent limit, but not current sense resistor can be used if increased accuracy is to exceed 10µA. This function will be described in more required. The current limit threshold can be programmed detail in the Forward Current Limit section. with a resistor on the ILSET pin. Once the desired current limit has been chosen, RILSET can be determined by the
OSCILLATOR
following formula: The frequency of operation is set through a resistor from 6000 R Ω the RT pin to ground where: ILSET = R DS(ON)A •ILIMIT  25000 where R f ≅ MHz DS(ON)A = RDS(ON) of N-channel MOSFET switch A OSC and ILIMIT = current limit in Amps.  RT  Once the voltage between ISVIN and ISSW1 exceeds the threshold, current will be sourced out of FB to take control
ERROR AMP
of the voltage loop, resulting in a lower output voltage The error amplifier is a voltage mode amplifier with a to regulate the input current. This fault condition causes reference voltage of 1.225V internally connected to the the RUN/SS capacitor to begin discharging. The level of non-inverting input. The loop compensation components the discharge current depends on how much the current are configured around the amplifier to provide loop com- exceeds the programmed threshold. Figure 3 is a simpli- pensation for the converter. The RUN/SS pin will clamp the fied diagram of the current sense and fault circuitry. If the error amp output, VC, to provide a soft-start function. current limit fault duration is long enough to discharge the RUN/SS capacitor below 1.225V, the fault latch is set and
UNDERVOLTAGE AND OVERVOLTAGE PROTECTION
will cycle the RUN/SS capacitor 16 times (1µA charging and 1µA discharging of the RUN/SS capacitor) to create an The LTC3785 incorporates overvoltage (OV) and off time of 32 times the soft-start time before the outputs undervoltage (UV) functions for fault protection and are allowed to switch to restart the output voltage. If the transient limitation. Both comparators are connected current limit fault level exceeds 150% of the programmed to the VSENSE pin, which usually has a similar voltage ILIMIT level at any time, the IMAX comparator is tripped and divider as the error amplifier without the compensation. output switches B and D are turned on to discharge the The overvoltage threshold is 10% above the reference. inductor current for the remainder of the cycle. The undervoltage threshold is 6.5% below the reference 3785fc 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts