Figure 1-1 highlights the key differences in device pin-out between the LM614xx and LM624xx device set using the LM61440-Q1 and LM62440-Q1 as device examples.

The primary distinctions between the LM614xx and LM624xx are listed below:

• Switching frequency: The LM624xx is internally set to a default switching frequency of 2.1MHz. The LM614xx enables control of the switching frequency through an adjustable RT resistor.
• Mode selection: The LM624xx enables control of the mode operation at light load. Pulling MODE/SYNC high results in FPWM mode setting while pulling MODE/SYNC low results in Auto mode setting. The LM614xx has the mode feature factory set and fixed per device option.
• External synchronization: To synchronize to a desired switching frequency the LM624xx requires a valid switching signal into the “MODE/SYNC” (Pin 6) for synchronization while the LM614xx uses “EN/SYNC” (Pin 7) for synchronization. Note that when using the external synchronization feature of the device spread-spectrum is disabled.
• Output voltage setting: The LM624xx is available in both adjustable and fixed output options. The LM614xx is only available in adjustable output option. For adjustable output options a top and bottom resistor is required while for fixed output options the feedback pin can be tied directly to the output rail for proper regulation.

Table 1-1 and Table 1-2 show the device comparison table of the LM61440-Q1 and LM62440-Q1 example devices and illustrates the difference between a LM614xx and LM624xx. The table provides the orderable device numbers and details the variety of different options the device is factory set to. Figure 1-2 and Figure 1-3 are example application circuits from the data sheet. To create a universal footprint to interchange between the LM614xx and LM624xx device set, several components must be considered.

In the case where FPWM mode is desired, the conversion from LM61440AFS-Q1 to LM62440APP-Q1 will require a layout change. This is because the change from the LM614xx to the LM624xx requires the resistor (RT resistor for LM614xx and MODE resistor for LM624xx) on Pin 6 to be connected to VIN or an external high voltage for the FPWM mode feature. To have a future-proof PCB layout that enables easy interchange between LM614xx to LM624xx, a top RT resistor between Pin 6 to VIN can be left DNP on the board.

If the FPWM variant of the LM614xx is used then that top RT resistor will be DNP. Alternatively if the LM624xx is used and FPWM mode is required then the bottom MODE resistor to GND is left DNP and the top MODE resistor to VIN is populated. For the case of Auto mode, the Auto mode variant of the LM614xx is used and the bottom RT resistor is populated to set the output voltage. Alternatively if the LM624xx is used and Auto mode is required then the top MODE resistor is left DNP and the bottom MODE resistor is populated.