Datasheet LTC3785-1 (Analog Devices) - 10

LTC3785-1
OPERATION RUN/SOFT-START (RUN/SS)
Since the range is a constant, tightening the UV threshold will loosen the OV threshold and vice versa. The RUN/SS pin serves as the enable to the LTC3785-1, soft-start function, and fault programming. A 1μA current source charges the external capacitor. Once the RUN/SS
POWER GOOD COMPARATOR
voltage is above a diode drop(~0.7V) the IC is enabled. Once The PGOOD pin is an open-drain output which indicates the IC is enabled, the RUN/SS voltage minus a diode drop the status of the buck-boost converter output. The output (RUN/SS – 0.7V) clamps the output of the error amp (VC) voltage is monitored at VSENSE via a resistor divider tap to limit duty cycle. The range of the duty cycle clamping is from VOUT to GND. The values used for this resistor divider approximately 0.7V to 1.7V. The RUN/SS pin is clamped are typically selected to be the same as those used in the to approximately 2.2V. If current limit is reached the pin error amplifi er feedback divider. If the voltage on VSENSE will begin to discharge with a current determined by the either falls 7.5% below (UV condition) or rises 7.5% magnitude of inductor current overcurrent limit, but not above (OV condition) the regulation voltage, the PGOOD to exceed 10μA. This function will be described in more open-drain output will pull low signaling the output is detail in the Forward Current Limit section. out of regulation. Once an out of regulation condition is triggered, the voltage on VSENSE must rise 1.5% above
OSCILLATOR
the UV threshold or fall 2% below the OV threshold before the pull-down will turn off. In addition, there is a 15μs The frequency of operation is set through a resistor from deglitch delay to help prevent false trips due to voltage the RT pin to ground where f ≅ (2.5e10/RT)Hz. transients caused by line or load steps. Depending upon the application, this delay may be insuffi cient. A capaci-
ERROR AMP
tor can be placed from VSENSE to GND to add additional The error amplifi er is a voltage mode amplifi er with a deglitch fi ltering, ensuring PGOOD doesn’t trip during reference voltage of 1.225V internally connected to the a transient. The PGOOD output will also pull low during non-inverting input. The loop compensation components shutdown and input undervoltage lockout to indicate these are confi gured around the amplifi er to provide loop com- fault conditions. pensation for the converter. The RUN/SS pin will clamp the error amp output, VC, to provide a soft-start function.
FORWARD CURRENT LIMIT
The LTC3785-1 is designed to sense the input current by
UNDERVOLTAGE AND OVERVOLTAGE PROTECTION
sampling the voltage across MOSFET A during the on time of The LTC3785-1 incorporates overvoltage (OV) and the switch (TG1 = High). The sense pins are ISVIN and ISSW1. undervoltage (UV) functions for fault protection and A current sense resistor can be used if increased accuracy transient limitation. Both comparators are connected is required. The current limit threshold can be programmed to the V with a resistor on the I SENSE pin, which usually has a similar voltage LSET pin. Once the desired current divider as the error amplifi er without the compensation. limit has been chosen, RILSET can be determined by the The overvoltage threshold is 7.5% above the reference. following formula: The undervoltage threshold is 7.5% below the reference with both comparators having 1.5% hysteresis. During RILSET = 6000 Ω R an overvoltage fault, all output switching stops until the DS(ON)A •ILIMIT fault ceases. During an undervoltage fault, the IC is com- where RDS(ON)A = RDS(ON) of N-channel MOSFET switch A manded to run fi xed frequency only (disabled Burst Mode and ILIMIT = current limit in Amps. operation). If the design requires a tightened threshold to Once the voltage between I one of the comparator thresholds the voltage divider on SVIN and ISSW1 exceeds the threshold, current will be sourced out of FB to take control the VSENSE pin can be skewed to achieve the threshold. 37851fa 10