Datasheet LT8471 (Analog Devices) - 10

LT8471
operaTion
and a gradual ramp-up of duty cycle for the Skyhook can achieve, thus allowing better control of the switch channel. When the part is brought out of shutdown, the current during start-up. The slope compensation func- external SS capacitors are first discharged (providing tion is disabled during foldback to increase the available protection against OV/UV pin glitches and slow ramp- current that can be delivered to the output. ing). Next, internal 250k resistors pull the SS pins up to ~2.15V. By connecting an external capacitor to each
Thermal Shutdown Operation
of the SS pins, the voltage ramp rates on the pins can Not shown in the Block Diagram is the thermal shutdown be set. Typical values for the soft-start capacitor range circuit. If the temperature of the part exceeds approximately from 100nF to 1μF. 164°C, the SR21 and SR22 latches are set. A full soft-start • Finally, the primary channels’ switching frequency is cycle will then be initiated after the temperature drops folded back by 2, 4, or 8 times when the correspond- below approximately 162.5°C. The thermal shutdown ing FB pin voltage is below certain thresholds (see circuit protects the power switches as well as the external the Typical Performance Characteristics section). This components connected to the LT8471. feature reduces the minimum duty cycle that the part
applicaTions inForMaTion Input Supply Requirements
In the low side configurations, the E pin is typically tied V to ground and the respective C pin toggles. VIN for the IN1 is the main power supply of the LT8471. It powers channel 1, the Skyhook channel and most of the internal respective channel must operate in a range of 2.6V to 40V. control and bias circuits for both channels. It must be High side configurations require that the C pin be tied to a powered up to enable any channel of the LT8471. VIN2 positive DC voltage supply and the respective E pin toggles. is the power supply for channel 2, and only needs to be The channel’s VIN pin should be at least 2.2V (typical) higher powered up when channel 2 is used. When VIN2 is not than the respective C pin to provide adequate drive to the powered up, channel 2 will shut down. base of the NPN power switch. When configured with a high side switch, the channel’s VIN can operate up to 50V
Switch Configurations and the Skyhook Regulator
above ground, 60V above the respective E pin voltage, and The primary channel NPN power switches can be connected 40V above the respective C pin voltage. in low side or high side configurations. A low side connec- The Skyhook boost regulator is available to provide ad- tion is when the power switch is on the lower voltage side ditional VIN voltage when it is needed to support high side of the inductor while the switch is on. The boost, SEPIC, switch topologies. When enabled, the Skyhook output flyback and dual-inductor inverting configurations use low voltage (SHOUT) is regulated to 4.25V (typical) above side power switches. Conversely, a high side connection is the C2 pin voltage. Figure 1 shows an example where the when the power switch is on the higher voltage side of the application input voltage is 6V to 32V. The Skyhook boost inductor when it is on. The buck, ZETA and single-inductor converter regulates SHOUT, VIN1 and VIN2 to 10.25V to inverting configurations use high side power switches. 36.25V insuring that the VIN1 and VIN2 pins are typically Channels 1 and 2 can be configured for high side or low 4.25V above the C1 and C2 pins. More information about side switching and do not need to be configured in the the Skyhook regulator is available in later sections. same way as the other. 8471fd 10 For more information www.linear.com/8471 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 Applications Package Description Related Parts