This reference design is a small SEPIC auxiliary power supply using the LM5157-Q1 device with integrated FET. A highlight of this design is the small board space by achieving a reasonable efficiency, so switching frequency has been set to a tradeoff of 440 kHz. Output voltage can be set from 5 V to 7 V, depending on the load requirements.

Though the design has been tested up to 2-A peak current the continuous current is around 1 A, resulting in a reasonable temperature rise +30 K at the SMA rectifier (hottest spot). The maximum output current depends on the minimum input voltage due to peak current limitation.

Optional ORing diodes at the BIAS pin to supply the controller with output voltage are allowing cold cranking as low as 2 V after start up V_IN > 3 V. The undervoltage lockout UVLO allows a flexible set up.

An optional tiny LC input filter attenuates reflected ripple. The inductor was selected for high self-resonance frequency to act beyond the FM band.

• 6-V bias supply for automotive applications and general purposes, steps up and steps down
• Cost-effective with a single driver, single FET, and single rectifier
• High integration due to powerful internal FET
• Provides input filter to prevent FM band noise
• Supports ultra-low cold-cranking as low as 1.5-V input voltage by ORing diodes
• This reference design is completely built and has completed testing in a lab

• Steering wheel control

Top Board Photo

Unless otherwise indicated, all measurements were done with 2-A output current. Resistors were used as load.

For all measurements dithering is disabled, R10 = 0 Ω.

The prototype on the bench showed a switching frequency of 437 kHz.

The ON threshold is 6 V and the OFF threshold is 4.7 V.

For lower output currents, LM51571-Q1 with the same package and the same pin out is more cost-effective.

The size of the board is 68.6 mm × 39.4 mm. The board itself has two layers, using single side assembly. The copper thickness is 70 µm on each layer.

The bottom layer is a solid ground plane to achieve low noise at the radio frequency (RF) range. Images in this document do not reflect actual size.

Figure 1-1 Layout Top Side

Figure 1-2 Layout Bottom Side

The input voltage of the power stage (TP8) was used for calculation of the efficiency and loss.

Figure 2-1 Efficiency Graph

The irregularity in the curve for 5-V input voltage at 0.8-A and 0.9-A output current is related to the measurement range change from the input current.

At nominal input voltage, 12 V and efficiency of 88% can be achieved at nominal output current 1 A.