The LM5143-Q1 contains an internal high-voltage VCC bias regulator that provides the bias supply for the PWM controller and the gate drivers for the external MOSFETs. The input voltage pin (VIN) can be connected directly to an input voltage source up to 65 V. However, when the input voltage is below the VCC setpoint level, the VCC voltage tracks VIN minus a small voltage drop.

The VCC regulator output current limit is 170 mA (minimum). At power up, the regulator sources current into the capacitors connected at the VCC pin. When the VCC voltage exceeds 3.3 V, both output channels are enabled (if EN1 and EN2 are connected to a voltage greater than 2 V) and the soft-start sequence begins. Both channels remain active unless the VCC voltage falls below the VCC falling UVLO threshold of 3.1 V (typical) or EN1/2 is switched to a low state. The LM5143-Q1 has two VCC pins that must be connected together on the PCB. TI recommends that two VCC capacitors are connected from VCC to PGND1 and from VCC to PGND2. The recommended range for each VCC capacitor is from 2.2 µF to 10 µF.

An internal 5-V linear regulator generates the VDDA bias supply. Bypass VDDA with a 470-nF ceramic capacitor to achieve a low-noise internal bias rail. Normally VDDA is 5 V, but there are two operating conditions where it regulates at 3.3 V. The first is in skip cycle mode when V_OUT1 is set to 3.3 V and V_OUT2 is disabled. The second is in a cold-crank start-up where V_IN is 3.8 V and V_OUT1 is 3.3 V.

Internal power dissipation of the VCC regulator can be minimized by connecting VCCX to a 5-V output at VOUT1 or VOUT2 or to an external 5-V supply. If the VCCX voltage is above 4.3 V, VCCX is internally connected to VCC and the internal VCC regulator is disabled. Tie VCCX to AGND if it is unused. Never connect VCCX to a voltage greater than 6.5 V or less than –0.3 V. If an external supply is connected to VCCX to power the LM5143-Q1, V_IN must be greater than the external bias voltage during all conditions to avoid damage to the controller.