The LM51770 features two high voltage current sensors. The first one maintains the peak current sensing between the CSA and CSB pins. The second current sensor inputs are connected to the ISNSP and ISNSN pins.

This optional current sensing provides the capability to monitor or to limit either the input or the output current of the DC/DC converter If the optional current sense amplifier is not used, the user can disable it to reduce the bias current consumption of the whole device by connecting the IMONOUT pin to VCC. Do not do this dynamical during the operation of the device because the configuration gets latched at start-up of the converter. Use the CFG pin to select one of the following desired operation modes.

Current Monitor Operation:

In case the current sense amplifier is configured as a monitor, the output voltage on the IMONOUT pin is a linear relation between the sense voltage between ISNSP and ISNSN pins and the sense amplifier transcendence as well as the resistor placed on the IMONOUT pin:

The output voltage of the IMONOUT pin is clamped to the values given in specifications section.

If the user intends to reduce the bandwidth of the current monitor, the user can place an optional capacitor in parallel to the IMONOUT pin like it is indicated in Figure 8-18.

Current Limit Operation:

In this configuration, the current sense gm amplifier monitors the voltage across the sense resistor and compares it with an internal reference voltage. If the drop across the sense resistor is greater than the reference threshold the gm amplifier gradually reduces the peak current capability of the DC/DC converter until the differential voltage is equal the reference voltage. This function of the LM51770 can be used to do the following:

• Regulate the current into the load from the power stage
• Regulate the current from the output into the power stage
• Regulated the current from the input supply to the power stage
• Regulated the current into the device input from the power stage

To select a negative current limit direction, the SYNC pin needs to be pulled low for the time when EN/UVLO goes above the EN rising threshold until the soft-start ramp starts the converter operation. The configuration gets latched and the SYNC pin can be used for the synchronization afterward. If the synchronization function is not used it can be pulled low continuously. For a positive current limit protection the SYNC pin can be pulled high or connected to a valid synchronization signal during the time when EN/UVLO goes above the EN rising threshold until the soft-start ramp starts the converter operation

Once the current limit operation mode is selected, a RC compensation network must be placed on the IMONOUT pin. For most applications, a compensation bandwidth with a factor of 3× to 5× faster than the compensation of the output voltage loop has given good results.