LTC3780 High Efficiency, Synchronous Buck Boost DC-DC Converter
Prevalent on ebay and Amazon is the "LTC3780 Automatic lifting pressure constant voltage step up step down 10A 130W" DC to DC Converter.
I'm not quite sure what the "Automatic lifting pressure" is about, but it is a fairly well designed DC-DC switcher based on Linear's LTC3780 - High Efficiency, Synchronous, 4-Switch Buck-Boost Controller. It sells for about $20 to $25 USD.
My interest in this DC-DC switcher is to operate 19VDC Laptops/notebooks and LCD TVs from 12V batteries/solar.
I took the plunge and purchased two units. Here are the specs:
- Input Voltage: 5-32V DC
- Output Voltage: 1-30V DC via trimpot.
- Output Current: 10A maximum, 7A continuous
- Output Power: 80W continuous, peak 130W
- Input Fused: 15A SMD
- Short Circuit Protected
- Low Voltage Disconnect
- Size 77.6 (if snapped at V-Groove) x 46.5 x 15mm
You will notice there are three trimpots and an absence of documentation. Well that's not quite true, I did find the product datasheet if you can read Chinese.
Upon delivery, it was evident one unit was touched up on the production line. There was evidence of flux around the LTC3780, and in the process of touching up the joints, one of the trimpots were burnt.
Pushing ahead, the next task was to connect the converters to a bench power supply and DC load and put it through their paces.
During testing of the adjustable current limiting, I fried one of the boards. No output, just the red fault LED steadily illuminated.
The next stage was to investigate the cause. In doing so, I sketched out the following schematics of the supervisory components of the DC-DC switcher.
The low voltage disconnect/cut-off and adjustable current limit is controlled by a commodity LM358 Dual Operational Amplifier running from a 5 volt rail generated by a LM7805 linear regulator.
The red Fault LED is connected to the LTC3780 power good (PG) output. According to the LTC3780 data sheet, the PG output is pulled low when the output voltage is outside +/- 7.5% of the regulation point.
The low voltage disconnect/cut-off consists of a comparator circuit connected to the LTC3780's run pin. When 'run' is below 1.5V, the LTC3780 shuts down. The data sheet warns not to apply greater than 6V to this pin.
The VIN voltage divider is compared to a 2.5V reference. The voltage divider consisting of a 500K trimmer and 36K resistor allows a low voltage cutout in the range of 2.5V to 37V, greater than the maximum input voltage.
VR3 and R9 makes up the external voltage divider for the error amplifier feedback input. The LTC3780 will adjust the output to ensure the voltage on VOsense is 0.800V. VOut can be calculated by 0.8V * (1 + R2/R1). The 500K trimmer and 14K resistor gives an output voltage range of 0.8V to 29V.