SNVSB14C April 2018 – October 2021 LM5036
PRODUCTION DATA
The LM5036 device contains a high-voltage VCC start-up regulator that allows the input pin (VIN) to be connected directly to an input voltage up to 100-V. Higher input voltages can be accommodated by adding some additional external parts, as described in Section 8.2.2.4. When the UVLO pin voltage is greater than VSD (0.38-V typical), the start-up regulator is enabled to charge an external capacitor connected to the VCC pin. The output voltage of the VCC regulator is regulated at VCC (7.8-V typical). The VCC regulator provides power to the reference (REF) regulator. The regulator output at VCC is internally current limited to ICC(Lim) (81-mA typical) . The value of the VCC capacitor depends on the total system design, and its start-up characteristics. The recommended range of values for the VCC capacitor is 0.47-µF to 10-µF.
LM5036 can power itself using its internal high-voltage start-up linear regulator, but internal power dissipation can be reduced by powering VCC from an auxiliary switched mode supply. LM5036 device integrates all of the functions needed to implement a low-cost and easy-to-design isolated fly-buck auxiliary supply based on the constant-on-time (COT) control scheme. The primary output VAUX1 of the auxiliary supply must be connected through a diode to the VCC pin, as shown in Figure 7-1. The auxiliary supply must raise the VCC voltage above the internally generated VCC voltage in order to shut off the internal start-up regulator. Powering VCC from an auxiliary switched mode supply improves efficiency while reducing the power dissipation of the controller IC. The VCC under-voltage (UV) circuit will still function in this mode, requiring that VCC never falls below its UV threshold during the start-up sequence. The VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased in normal operation. Therefore, the auxiliary VCC voltage should never exceed the VIN voltage.