Many industrial and automotive applications have widely varying input voltage (VIN) rails and often need a buck-boost DC/DC converter to regulate the output voltage (VOUT). Buck-boost DC/DC converters can be either cascaded buck and boost stages or single stage. Cascading buck and boost stages result in double conversion, leading to higher size, cost and power losses.
Figure 1 shows the single-stage buck-boost converter choices from TI for different power levels. At relatively low power levels, the non-synchronous buck-boost topology (for example, the LM5118) is the simplest solution. Traditional single-stage buck-boost topologies such as a single-ended primary inductor converter (SEPIC) or flyback use two inductors and therefore become bulky and less efficient as power level increases. For the highest efficiency, the four-switch buck-boost is the topology of choice for 25W to 250W power levels.
Figure 1: Buck-Boost topologies for different power levels
The LM5175 is a four-switch buck-boost controller that uses a single inductor and integrates MOSFET drivers for all four switches. It delivers higher power and high efficiency compared to other topologies because of the following advantages:
Figure 2: LM5175 four-switch buck-boost converter
In addition to enabling high-power, high-efficiency buck-boost solutions, the LM5175 has additional features to aid designers. The cycle-by-cycle current limit with optional hiccup mode reduces thermal stress in case of an overload (Figure 3).
Figure 3: Hiccup mode protection for reduced thermal stress
In addition, the converter has an average current limit function that can accurately limit either the source or the load current. This feature can be especially useful for battery charging or LED current control.
Figure 4: LM5175 average current loop used to control current in an LED string
The LM5175 has an optional dither feature that can help with EMI by allowing users to modulate the switching frequency around its nominal value. A dither capacitor sets the modulation frequency.
Figure 5: The dithering feature in the LM5175 helps with EMI
The LM5175 four-switch controller enables high-power and high-efficiency buck-boost solutions with applications in automotive (start/stop for infotainment, LED lighting), industrial (industrial PC, Ethernet switches), communication (power amplifier supplies) and other areas. Additional features such as hiccup mode, average current loop and frequency dithering make it a powerful tool in DC/DC designs.
I'm currently using the LM5175EVM-HP development board configured for an output voltage of 25.2V. The output capacitors have been changed accordingly. Our intent of using this converter is for charging a Li-ion battery composed of Panasonic 18650 batteries. A problem that we're having is when we connect a slightly discharged battery to the output of the converter. When our fully charged 25.2V battery gets down to around 24.6V, the converter completely shuts off. This is in contrast to when the battery voltage is around 25V and the battery charges perfectly as expected. The converter input is from a DC power supply set to 15V with a current limit at 3A.
Any advice would be great.
Thank you for considering LM5175. It is not clear if any changes were made to the board other than for VOUT. My first guess is that your power supply is going in current limit.
For proper tracking of this issue, I suggest opening a new topic in e2e forum with LM5175 and/or LM5175EVM-HP in the heading.
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