What this controller brings to a boost or flyback rail
The Texas Instruments LM3478MAX/NOPB is a current-mode PWM controller that handles boost, flyback, and SEPIC topologies from a single 8-SOIC package. It drives an external N-channel MOSFET with a 100% duty-cycle capability. The supply range is 2.97 V to 40 V. Switching frequency is programmable between 100 kHz and 1 MHz. The junction temperature rating is -40°C to 125°C.
Switching frequency — what it means for the magnetics and the board
The 100 kHz to 1 MHz programmable range is the spec that drives transformer and inductor selection. At the low end, 100 kHz lets you use a larger core with lower AC losses — useful when the design is cost-optimised and board space is not the constraint. At 1 MHz, the magnetics shrink significantly, but you pay for it in gate-drive losses and tighter layout to keep the switch node clean. The controller does not have clock sync, so if your system already runs a fixed switching frequency for EMI management, you will need to either accept the free-running frequency or look at a synchronisable part like the LM5150.
Supply voltage and temperature — where this part fits
The Vcc range is 2.97 V to 40 V. The junction temperature range is -40°C to 125°C. No synchronous rectifier inside.
Control features and output configuration
The enable pin and frequency control are the only on-chip adjustments. There is no soft-start pin, so the designer must account for inrush during startup through the external compensation network. The single transistor-driver output is configured for positive output voltages — standard for boost and flyback. Duty cycle goes to 100%, which is useful for boost converters that need to pass the input straight to the output during dropout or for pre-biased loads. The part does not include a current-sense amplifier; the current limit is set by an external sense resistor feeding the internal comparator.
Lifecycle and sourcing
The LM3478MAX/NOPB carries an Active lifecycle status and is ROHS3 compliant. For a pin-compatible alternative with automotive qualification, the LM5150QURUMRQ1 offers clock-sync capability and a 2.3 MHz switching ceiling, but it operates from a lower 1.5 V minimum supply and tops out at 87% duty cycle.