The LTC3850 features a fully differential comparator to sense current through the inductor. The SENSE+ and SENSE- pins can be connected to a current sense resistor in series with the inductor, or to an RC network in parallel with the output inductor for energy efficient sensing across the inductor's DC resistance (DCR sensing). DCR sensing prevents power loss through sense resistors, but has a typical built in error of 5% - 10%, which is acceptable for most applications. The LTC3850 can use either method for design flexibility and has a choice of three pin-selectable current limit thresholds.

When an output experiences a short circuit, the LTC3850 protects the input supply and power components by limiting peak current cycle by cycle. The main MOSFET turns off when 130% of inductor's peak current sense threshold of VSENSE (MAX)/RSENSE is reached. VSENSE(MAX) can be set to 30mV, 50mV, or 75mV for a wide variety of output current levels. Duty cycle has very little effect on current limit. For load currents greater than the programmed maximum but less than a hard short, the LTC3850 gracefully folds back the top MOSFET's on-time, reducing the output voltage. The LTC3850 also protects against undervoltage input and overvoltage output voltages. The RUN pins can be referenced to a voltage divider from VIN, so that their precision thresholds control the state of the outputs. If the output voltage is more than 7.5% above its target, the bottom MOSFET can remain on until regulation is recovered. If the LTC3850 is allowed to operate with input voltage approaching the programmed output voltage, its duty cycle can be as high as 97%.

The LTC3850 provides several features in small 4mm x 4mm QFN-28 (0.4mm pitch), 4mm x 5mm QFN-28 (0.5mm pitch), or 28-pin narrow SSOP packages. It operates at high efficiency with or without DCR current sensing. Tracking, very fast response time, strong on-board MOSFET gate drivers, 2-phase operation, external synchronization and low cost allows the LTC3850 to be used for set-top boxes and many other applications.

Conclusion

POL regulation can add performance to STB designs by being more efficient than traditional multi-output off-line custom flyback power supplies. It allows a higher output power without needing extra PFC circuitry and enables a quicker time to market by using an off-the-shelf regulatory approved single output power supply with highly efficient POL downstream regulators. With its variety of features, high efficiency and small package size, the LTC3850 offers the power supply designer an ideal solution for STBs.