The LM25137 contains MOSFET gate drivers and associated high-side level shifters to drive the external N-channel power MOSFETs. The high-side gate driver works in conjunction with the integrated bootstrap diode and external bootstrap capacitor C_BOOT. During the conduction interval of the low-side MOSFET, the SW voltage is approximately 0V and C_BOOT charges from VCC through the diode.

The LM25137 controls the HO and LO outputs with an adaptive dead-time methodology such that both outputs (HO and LO) are never enabled at the same time, preventing cross conduction. When the controller commands LO to be enabled, the adaptive dead-time logic first disables HO and waits for the HO-to-GND voltage to drop below 2V (typical). LO is then enabled after a small delay (HO falling to LO rising delay). Similarly, the HO turn-on is delayed until the LO voltage has dropped below 2V. HO is then enabled after a small delay (LO falling to HO rising delay). This technique provides adequate dead-time for any size N-channel MOSFET component or parallel MOSFET configurations. Caution is advised when adding series gate resistors, as series gate resistors can decrease the effective dead-time. The selected high-side power MOSFET determines the appropriate bootstrap capacitance value C_BOOT in accordance with Equation 10.

Equation 10.

where

• Q_G is the total gate charge of the high-side MOSFET at the applicable gate drive voltage, normally 5V
• ΔV_CBOOT is the voltage variation of the high-side MOSFET driver after turn-on

To determine C_BOOT, choose ΔV_CBOOT so that the available gate drive voltage is not significantly impacted. An acceptable range of ΔV_CBOOT is 100mV to 200mV. The bootstrap capacitor must be a low-ESR ceramic capacitor, typically 0.1μF. Given the nominal VCC voltage of 5V, using logic-level power MOSFETs with R_DS(on) rated at V_GS = 4.5V is imperative.