Datasheet LTC7860 (Analog Devices) - 10

LTC7860
applicaTions inForMaTion
The LTC7860 is a high efficiency switching surge stopper to the LTC7860 to keep the VFB trace short and without which provides input voltage surge protection, input inrush VIAS. The planes for VOUT and GND are then routed to current limiting and output short protection. High Efficiency the desired regulation point. Detailed layout suggestions Switching permits high output current capability and small are discussed in the Layout sections later in the data solution size. During an input overvoltage event, such as sheet. The feed-forward capacitor CFF is added to improve a load dump in vehicles, the LTC7860 controls the gate of transient response. an external MOSFET to act as a switching DC/DC regulator V (PROTECTIVE PWM mode). This operation regulates the OUT LTC7860 RZ1 RFB2 CFF output voltage to a safe level, allowing the loads to operate VFB through the input over-voltage event. VFBN RFB1 During normal operation (SWITCH-ON mode), the LTC7860 PGND SGND Z1 turns on the external MOSFET continuously, passing the input voltage through to the output. An internal comparator 7860 F01a limits the voltage across the current sense resistor and
Figure 1a. Switching Surge Stopper for VIN Operation of
regulates the maximum output current to protect against
60V and BELOW (VFBN > 2V)
over current faults.
Operation for VIN ABOVE 60V OUTPUT VOLTAGE PROGRAMMING
For operation at VIN = 60V, a floating ground must be cre- ated by a bootstrapped shunt regulator such as a Zener or The LTC7860 is highly flexible and offers application similar element. To establish shunt DC bias to the LTC7860, options to address a variety of input and output voltage connect the floating ground to the LTC7860 PGND and ranges. These options are best divided into two categories. SGND pins. The Zener voltage or Shunt DC bias limit is The first category is operation at or below VIN = 60V. The typically 12V to minimize internal power dissipation but can second category is operation above 60V. The LTC7860 be extended up to 60V. The VIN Operational Input voltage input and output voltage operation in the second category ranges for these applications are limited only by external depends only on external components and can be reliably components and can reliably extend to 200V and beyond. extended up to 200V. There are two feedback options for operation above 60V which are Inverting and Non-Inverting Feedback.
Operation for VIN of 60V and BELOW
The Inverting Feedback Option uses fewer components with For operation at VIN of 60V and BELOW, the output voltage slightly reduced accuracy (Figure 1b). The Non- Inverting is programmed by connecting a feedback resistor divider Feedback Option uses additional components but with bet- from the output to the VFB pin as shown in Figure 1a. The ter accuracy (Figure 1c). It has the additional advantage front page application is an example of this configuration. of reducing VIN quiescent current in normal operation. The output voltage in steady state operation is set by the feedback resistors RFB2 and RFB1 according to the equation:
Inverting Feedback Option
 R  In the Inverting Feedback Option for V V FB1 IN ABOVE 60V, the OUT = 0.8V • 1+ voltage is programmed by connecting a feedback resistor  RFB2  divider from the output to ground as shown in Figure 1b. Great care should be taken to route the VFB line away from VOUT is divided down and the voltage presented to the gate noise sources, such as the inductor or the GATE signal of QFB. The gate voltage is then translated into a signal that drives the external P MOSFET. The best practice is current by QFB and RFB3 and sent to the LTC7860 Floating to locate resistors RFB2 and RFB1 and capacitor CFF local Ground Inverting Feedback Pin VFBN. The resistor RFB4 7860f 10 For more information www.linear.com/LTC7860