BD8381EFV-M 查看數據表（PDF） - ROHM Semiconductor

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BD8381EFV-M

LED Drivers for Automotive Light

ROHM Semiconductor 

BD8381EFV-M

Technical Note

●Buck-Boost DC/DC controller

・Over-voltage protection circuit (OVP)

The output of the DCDC converter should be connected to the OVP pin via a voltage divider. In determining an

appropriate trigger voltage of for OVP function, consider the total number of LEDs in series and the maximum variation in

VF. Also, bear in mind that over-current protection (OCP) is triggered at 0.85 x OVP trigger voltage. If the OVP

function engages, it will not release unless the DCDC voltage drops to 72.5% of the OVP trigger voltage. For example, if

ROVP1 (out put voltage side), ROVP2 (GND side), and DCDC voltage VOUT are conditions for OVP, then:

VOUT ≥ (ROVP1 + ROVP2) / ROVP2 x 2.0 V.

OVP will engage when VOUT ≧ 32 V if ROVP1 = 330 kΩ and ROVP2 = 22 kΩ.

・Buck-boost DC/DC converter oscillation frequency (FOSC)

The regulator’s internal triangular wave oscillation frequency can be set via a resistor connected to the RT pin (pin 5).

This resistor determines the charge/discharge current to the internal capacitor, thereby changing the oscillating frequency.

Refer to the following theoretical formula when setting RT:

fosc =

60 × 106

RT [Ω]

x α [kHz]

60 x 106 (V/A/S) is a constant (±5%) determined by the internal circuitry, and α is a correction factor that varies in relation to RT:

{ RT: α = 50kΩ: 0.98, 60kΩ: 0.985, 70kΩ: 0.99, 80kΩ: 0.994, 90kΩ: 0.996, 100kΩ: 1.0,

150kΩ: 1.01, 200kΩ: 1.02, 300kΩ: 1.03, 400kΩ: 1.04, 500kΩ: 1.045 }

A resistor in the range of 62.6kΩ～523kΩ is recommended. Settings that deviate from the frequency range shown

below may cause switching to stop, and proper operation cannot be guaranteed.

Fig.13 RT versus switching frequency

SYNC frequency[kHz]

Fig.14 RT versus SYNC frequency

・External DC/DC converter oscillating frequency synchronization (FSYNC)

Do not switch from external to internal oscillation of the DC/DC converter if an external synchronization signal is present

on the SYNC pin. When the signal on the SYNC terminal is switched from high to low, a delay of about 30 µs (typ.)

occurs before the internal oscillation circuitry starts to operate (only the rising edge of the input clock signal on the SYNC

terminal is recognized). Moreover, the external synchronizing signal is given to priority when an external input frequency

is used. And in the case of using external input frequency, follow the Fig.14.

・Soft Start Function

The soft-start (SS) limits the current and slows the rise-time of the output voltage during the start-up, and hence leads to

prevention of the overshoot of the output voltage and the inrush current. The SS voltage is made Low when OVP of the

overcurrent and the excess voltage is detected, and the switching is stopped. Resume operation is begun.

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2011.04 - Rev.A

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