Technical Information Site of Power Supply Design

Product Key Points

BD9S Series: Ultra-small Step-down DC/DC Converters for Automotive, ADAS Applications with Power-good Pin

Ultra-small Packages 2 mm On a Side
1 A/0.6 A Versions with Internal Phase
Compensation Circuits

  • ADAS (Advanced Driver Assistance System)
  • BD9S series
  • Power Good
  • Adjustable soft start
  • AEC-Q100 grade 1
  • 2.2 MHz
  • Compact inductor
  • BD9S400MUF-C
  • BD9S300MUF-C
  • BD9S200MUF-C
  • BD9S100NUX-C
  • BD9S000NUX-C
  • Output discharge
  • 100% duty cycle

ROHM recently announced the "BD9S Series" of secondary step-down DC/DC converters, intended for use in safe driving assistance modules of automobiles such as ADAS (Advanced Driver Assistance Systems) sensors, cameras, radar and the like, in which compactness, low power consumption, and high reliability are required.

The "BD9S series" are miniaturized high-efficiency single-channel synchronous rectification step-down DC/DC converters provided with power good and adjustable soft start functions, and are compliant to AEC-Q100 grade 1 for automotive applications. A power good signal is output as a flag to indicate whether the output voltage is within a preset tolerance range, in order to enhance system stability and reliability. Adjustable soft start can be set using one capacitor, providing a high degree of freedom and enabling safe startup by preventing inrush currents on startup and the like. 2.2 MHz is used as the switching frequency to avoid interference with AM radio, and at the same time a compact inductor can be used. These features, together with the small-size package, contribute to miniaturization of the power supply circuit.

The new products have internal power MOSFETs. Newly released are two series: the 4 A output BD9S400MUF-C, 3 A BD9S300MUF-C, and 2 A BD9S200MUF-C with Nch+Nch configurations and adopting 3×3 mm wettable-flank QFN packages, and the 1 A BD9S100NUX-C and 0.6 A BD9S000NUX-C, with Pch+Nch configurations in 2×2 mm packages. These devices have adjustable outputs; fixed-output counterparts are currently in development. The following table lists the BD9S series.

Part No. Output Voltage Max.
Package Common Specifications
BD9S400MUF-C 0.8V~Vin×0.8V 4.0A VQFN16FV3030
・Input voltage range: 2.7 V to 5.5 V
・Output voltage accuracy: ±1.5%
・Switching frequency:
・Operating temp. range:
   –40°C to 125°C
BD9S300MUF-C 3.0A
BD9S200MUF-C 2.0A
BD9S100NUX-C 0.8V~Vin 1.0A VSON008X2020
BD9S000NUX-C 0.6A
**BD9S110NUX-C 1.2V 1.0A Same as above
**BD9S111NUX-C 1.8V 1.0A

** The BD9S110NUX-C and the BD9S111NUX-C are currently in development.

Features of the 1 A/0.6 A Version Products BD9S100NUX-C/BD9S000NUX-C

The 1 A BD9S100NUX-C and 0.6 A BD9S000NUX-C employ internal output stage power MOSFETs, with Pch and Nch devices on the high and low sides respectively. Features are summarized below. Boldface indicates features present only in the models BD9S100NUX-C and BD9S000NUX-C.

Pch+Nch synchronous rectifying step-down DC/DC converter
2×2 mm ultra-small package
Support for 100% duty cycle
Output discharge function
Internal phase compensation circuits
2×2 mm ultra-small package
・AEC-Q100 grade 1 compatible
・2.2 MHz PWM current mode operation
・Adjustable soft start function: Set with one capacitor
・Power Good output (Nch open drain)
・Enable control
・Various protection functions: Input undervoltage lockout (UVLO), load short-circuit protection (SCP) function, output overvoltage protection (OVP) function, Overcurrent protection (OCP) function, thermal shutdown (TSD) function

BD9S000NUX-C Block Diagram (Click to enlarge)

BD9S000NUX-C: Example of Efficiency Curve at 5Vin/3.3Vout

■Support for 100% Duty Cycles
The output stage adopts a Pch+Nch configuration, so that when the input voltage drops and approaches the preset output voltage, the high-side MOSFET on cycle reaches 100%, and a voltage continues to be supplied to the output. The output voltage at this time is approximately the result of subtracting the voltage drop due to the load current times (Pch MOSFET on-resistance + inductor DCR) from the input voltage.

■Output Discharge Function
When even just one of the following conditions obtains, the SW pin is pulled down to GND level by an internal resistor (650 Ω typ), to discharge the electric charge on the output line. As a result, the output pre-bias is reduced or brought to zero in order to ensure a safe restart.

   ・VEN is 0.4 V or lower
   ・VIN is 2.45 V (Typ) or lower (UVLO operation condition)
   ・VFB is 0.56 V (Typ) or lower, and this state persists for 1 ms (Typ) (SCP operation condition)
   ・VFB is 0.92 V (Typ) or higher (OVP operation condition)
   ・Tj is 175°C (Typ) or higher (TSD operation condition)

When all of these conditions no longer obtain, output discharge stops.

The 4 A/3 A/2 A BD9S400MUF-C/BD9S300MUF-C/BD9S200MUF-C models, in 3×3 mm wettable-flank packages with a Nch+Nch configuration, are explained in a separate article.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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