Datasheet LTC7860 (Analog Devices) - 8

LTC7860
operaTion High Efficiency Switching Surge Stopper Overview
regulation when the power loss is high, the components The LTC7860 is designed for use as a high efficiency and thermal design can be optimized for normal operation switching surge stopper and/or input inrush current lim- and safely operate through high voltage input surges and/ iter. Normal operation for the LTC7860 is in "dropout" or or overcurrent faults. SWITCH-ON mode. The LTC7860 switches during start-up In a switching surge stopper the insertion loss in normal or in response to either an input over-voltage or output operation or SWITCH-ON mode is the primary consider- short-circuit event (PROTECTIVE PWM mode). If the time ation and not efficiency in switching. The LTC7860 circuit spent switching exceeds the time programmed by the must operate without damage during a surge or fault timer the LTC7860 will shut down. event. The Switching Surge Stopper is effectively a wire A high efficiency surge stopper or input inrush current in normal operation where the insertion loss is determined limiter can be thought of as a pre-regulator. As an example by multiplying the input current by the effective resistance. of a MIL1275 application, the input voltage connects to
LTC7860 Main Control Loop
a 28V vehicle power bus. The 28V power bus can go as high as 100V with a surge profile lasting up to 500mS. The The LTC7860 uses a peak current-mode control output must be pre-regulated or limited to 34V maximum architecture to regulate the output in a step-down DC/ and can go as low as 12V during engine cranking. The DC switching regulator. The VFB input is compared to an LTC7860 limits the voltage seen at the output and pro- internal reference by a transconductance error amplifier tects any load connected to the 28V bus from potentially (EA). The internal reference can be either a fixed 0.8V destructive voltage levels. The LTC7860 timer limits reference VREF or the voltage input on the SS pin. In the time spent switching where excessive and thermally normal operation VFB regulates to the internal 0.8V destructive power loss can occur. reference voltage. In soft-start, when the SS pin voltage is less than the internal 0.8V reference voltage, V For both a linear surge stopper such as the LTC4363 and FB will regulate to the SS pin voltage. The error amplifier output the LTC7860 switching surge stopper, the power loss connects to the ITH (current [I] threshold [TH]) pin. The increases significantly once regulation begins. In a linear voltage level on the ITH pin is then summed with a slope surge stopper, the power loss is the power loss of the compensation ramp to create the peak inductor current regulating MOSFET. In a high efficiency surge stopper or set point. switching surge stopper, internal power loss is determined by conversion efficiency. A switching surge stopper will The peak inductor current is measured through a sense allow higher output current and power levels than a com- resistor, RSENSE, placed across the VIN and SENSE pins. parable linear solution by virtue of reduced power loss. In The resultant differential voltage from VIN to SENSE is a switching surge stopper the internal surge power loss proportional to the inductor current and is compared to can increase by as much as 10 times the normal power the peak inductor current setpoint. During normal opera- loss. If the time spent in PWM mode regulation is limited, tion the P-channel power MOSFET is turned on when the the operating power can be pushed beyond what can be clock leading edge sets the SR latch through the S input. achieved in steady state operation. This is precisely the The P-channel MOSFET is turned off through the SR latch same concept as utilized in linear surge stoppers but R input when the differential voltage from VIN to SENSE extended to a switching supply. The use of the timer is greater than the peak inductor current setpoint and the improves reliability and reduces component size when current comparator, ICMP, trips high. compared to a continuous solution. By limiting the time in 7860f 8 For more information www.linear.com/LTC7860