The LM5177, LM51770, and LM51772 are asynchronous buck-boost controllers, which offer current monitoring features with a current sense resistor connected to the ISNSP and ISNSN pins. With the current monitor feature, a control system, such as a uController, measures the current into the application or from the supply and use this information for additional functions, such as observing and sending warn signals to a host system, or adding an additional control loop.
Depending on the device, the sensed current is represented as an easy to measure voltage on the IMONOUT, or the CDC pin of the device.
The integrated transconductance amplifier converts the voltage measured on the sense resistor into a proportional current provided on the IMONOUT or CDC pin. With the attached resistor, this signal is transformed into voltage which is measured with an ADC and processed by a uController.
The integrated transconductance amplifier works in both directions, providing positive and negative current signals. For negative output current, a bias voltage need to be supplied on the IMONOUT or CDC pin, instead of a resistor to the AGND. This bias signal is generated by using the VCC signal, VCC2 signal, or a Reference Voltage, such as a uController.
Figure 2 and Figure 3 show the connection of the resistor divider on the current monitor output. In Figure 3 the VCC of the controller is used as reference voltage but RIMON1 can also connect to a reference voltage, such as from the uController, if higher accuracy is required.
The corresponding monitor output voltage is calculated with Equation 1 or Equation 2.
With:
Equation 1 results in:
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