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LM27761: How to understand "2.5-Ω Inverter Output Impedance" ?

Part Number: LM27761

Hi Team,

LM27761 have 2.5-Ω Inverter Output Impedance parameter. There have some questions need to confirm:

1.  Is 2.5-Ω Output Impedance a pull-down resistor?

2. Is 2.5-Ω Output Impedance a fixed value?  Does it correspond to a fixed output current?

3. For example, the 2.5-Ω Output Impedance stand that it can max output 50mA current?

  • Hi Kevin,

    Section in the datasheet describes the output impedance. The charge pump part of the device can be approximated by an ideal voltage source of value - VIN in series with a resistance which is the output impedance. This impedance is due to the resistance of the charge pump MOSFET switches and ESR of the external caps. 

    1. This is not a pull down resistance. It is a series resistance

    2. Equation 1 in the datasheet describes the output impedance. The output impedance depends on the switching frequency. The switching frequency of the converter varies with output current till a certain value. At higher output currents switching frequency is constant (2MHz). So you will see a similar variation of output impedance with output current.

    3. The device can output 250mA max. What you need to take care of while designing is that you have enough margin between the absolute values of VIN and VOUT to take care of the voltage drop across the charge pump output impedance and the LDO dropout voltage.

    Best regards,