Technical Note

8/22

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2011.08 - Rev.C

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BD8112EFV-M

●

Buck-Boost DC/DC controller

・

Number of LEDs in series connection

Output voltage of the DCDC converter is controlled such that the forward voltage over each of the LEDs on the output is

set to 1.0V (Typ.). DCDC operation is performed only when the LED output is operating. When two or more LED outputs

are operating simultaneously, the LED voltage output is held at 1.0V (Typ.) per LED over the column of LEDs with the

highest VF value. The voltages of other LED outputs are increased only in relation to the fluctuation of voltage over this

column. Consideration should be given to the change in power dissipation due to variations in VF of the LEDs. Please

determine the allowable maximum VF variance of the total LEDs in series by using the description as shown below:

VF variation allowable voltage 3.7V (Typ.) = short detecting voltage 4.5V (Typ.)－LED control voltage 1.0V (Typ.)

The number of LEDs that can be connected in series is limited due to the open-circuit protection circuit, which engages at

85% of the set OVP voltage.

Therefore, the maximum output voltage of the under normal operation becomes

30.6 V (= 36 V x 0.85, where (30.6 V – 1.0 V) / VF > N [maximum number of LEDs in series]).

・

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 (output voltage side), ROVP2 (GND side), and DCDC voltages 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 =

x α [kHz]

30 x 10

to RT:

{ RT: α = 50kΩ: 0.94, 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 47kΩ～523kΩ is recommended. Settings that deviate from the frequency range shown below

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

Fig.15 RT versus switching 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, if external input frequency is less than the internal oscillation frequency, the internal

oscillator will engage after the above-mentioned 30 µs (typ.) delay; thus, does not input a synchronization signal with a

frequency less than the internal oscillation frequency.

30 × 10

6

RT [Ω]

k

100k

200k

300k

400k

500k

600k

0

100

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500

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RT[kΩ]