MV1012SC

11

3.2.4

Inductor (L1)

Inductance is calculated, ignoring toff2, as follows, where the input voltage is represented by Vi, the output voltage

(LED voltage) by Vo, the switching frequency by f, the inductance by L, the forward voltage of the fly-wheel diode D1 by

The switching frequency changes with changes in input voltage and with dimming.

A general inductor has the DC bias characteristics shown in Figure 6(a). As current increases, inductance decreases.

This results in the MOSFET current waveform indicated by a solid line in Figure 6(b). The output current Io is slightly

smaller than the calculated value.

The inductor receives the peak current Ip about twice larger than the output current Io and thus pay attention to a

decrease in inductance with Ip when selecting the inductor.

Figure 6(a) Inductor’s DC bias characteristics

Figure 6(b) MOSFET current waveform at inductance drop

3.2.5

Gate drive circuit (R4, R9, and D2)

In the IC, the gate charging current (IG_source) is limited to about 40 mA and the discharge current (IG_sink) to about 425

mA. Thus, the circuit can be used with R4 = 0 Ω—i.e., with a direct connection. Using R4 enables adjustment for delays,

noise reduction, and improved dimming characteristics. However, if the resistance of R4 is too large, zero current

detection may not be performed once the restart operation starts. The restart operation may continue. To determine the

resistance of R4, be sure to confirm that zero current detection can be performed even when oscillation starts with the

REF voltage slowed up

When R4 is added to the circuit, the discharge current is limited. To achieve advantageous constant-current

characteristics, the delay needs to be small; this means a discharge diode is required, D2. The discharge current can be

adjusted with resistor R9.

The charge and discharge currents above are levels with Vcc = 9 V. They will vary depending on Vcc value.

)

V

Vi

(

Io

f

2

)

V

Vo

(

)

Vo

Vi

(

L

F

F

















The inductance falls

when a current is applied.

CAT.No.1A0602-1E