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# LM5161: Can a positive pressure BUCK circuit be directly used to achieve the BUCK circuit function of input negative pressure and output negative pressure

Part Number: LM5161

As shown in Figure 1, it is a BUCK circuit with a positive input and a positive output, with an input of 80V and an output of 3V; Is it possible to achieve an output voltage of -77V by swapping GND and input positions as shown in Figure 2 (PSPICE for TI simulation has no results)

• Hello,

Under no circumstances should the input of the buck converter be applied from GND to Vin. The schematic, as shown, will violate the absolute maximum ratings of the device specified in the datasheet.

If you need a negative output voltage from a buck converter, then you should use the Inverting Buck Boost Topology. Here is an application note that discusses IBB in detail: https://www.ti.com/lit/an/snvaa61/snvaa61.pdf

However, you will not be able be able to achieve a -77V output with an 80V input. As described in the application note, the IC becomes referenced to the negative output voltage. This means that the voltage at the input of the buck IC is Vin - (-Vout). With your conditions, that would mean 80V - (-77V) = 157V, which violates the maximum voltage rating of the device.

Best regards,

Ridge

• Is it impossible to directly reduce the voltage to output -30V from a negative source(-80V) ? And if it means i have to invert negative source to positive source, then use inverting buck boost topology?

• Hello,

Please refer to the absolute maximum ratings table in the datasheet where Vin to GND has a minimum rating of -0.3V. Your application schematic will cause damage to the device. A negative input voltage cannot be used on this IC.

If you want to invert the output from a positive supply, you will have to use the IBB topology. However, a Vin=80V to Vout=-30V is not possible for this device as it would violate the absolute maximum input voltage for the device. Please refer to the above comment for detail.

Best regards,

Ridge