2MHz DC-DC converters put MLCCs into USB-PD designs

Technology News |
By Nick Flaherty

Silanna Semiconductor has launched two wide voltage, high-frequency point of load DC-DC converters targeting USB-PD applications.

The DC-DC converter designs are configurable to 2MHz using a silicon controller, and the different designs are optimised for different USB-PD controllers. The 3x3mm QFN chip support smaller designs for USB-PD chargers using either UAB-A or USB-C connectors from 18W up to 65W with a power density of 15W/in3.  

The design contains an internal feedback path that allows for clean and well controlled start-up operation until external USB-PD controllers can bias themselves and take over control of the output voltage.

“We operate from 660kHz up to 2MHz so we are up at the upper end to reduce the size of the output filter,” said Tim Wilhelm, Director of Marketing.

This also reduces the capacitance needed in the design and allows a move from electrolytic to multi layer ceramic capacitors (MLCCs) for higher reliability, he says.

“A 150kHz design needs a 22uH toroidal inductor and additional FETs with a 220uF electrolytic capacitor,” he said. “Up at 2MHz enables us to get a much smaller output filter, 12% the volume, with a 1.8 to 4.7uH molded inductor and 50uF MLCC. Just getting rid of electrolytics is tantamount to success to avoiding dried up capacitors,” he said.

The design uses a dual input LDO (low drop out) regulators that create the 3.3V bias voltage and can take the input from the input voltage line or the output. “USB always comes up to 5V as the safe voltage so we have two inputs, one at power up from Vin and when we cross 4V we move to Vout and that gives higher efficiency,” he said.

The different chips support the USB-PD controllers from Weltrend and Cypress (now part of Infineon).

The SZPL3102A supports the Weltrend controller and is designed to avoid overvoltage protection modes, using an internal resistor divider that switches in at the start and switches out after 5V to avoid an open loop. “The Cypress PD controllers don’t have an internal resistor divider and have a smooth startup,” said Wilhelm.

The highly integrated design and small output filter also helps with EMI, avoiding loops, he says.

The SZPL3102A/3103A DC-DC converters are being sampled to key accounts and will be fully released later this year.

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