MV1012SC

12

3.2.6

Resistors for Svin and Svout pins(R5, R6, R7 and R8)

The Svin and Svout pins are used for comparator input to detect voltage inversions on both ends of L1 to determine

switching timing on (zero current detection). To ensure advantageous comparator characteristics, the input voltage into

the pins should not exceed 3.5 V. The voltages on both ends of L are very high; voltage dividing resistors (R5, R6, R7, and

R8) are required, as shown in the circuit diagram (Figure 7).

Due to the configuration of the basic circuit, an LED leak current flows via the voltage dividing resistors (R5 to R8) and

the IC internal resistor (25 kΩ). To reduce output current when the IC is not operating and the dimming ratio is at its

minimum, use a winding voltage supply, as described in Section 3.3.

Figure 7 shows the voltage waveforms of the Svin and Svout pins and the internal circuit diagram.

Figure 7 Zero current detecting operation waveforms and internal circuit diagram

Select R7 and R8 so that the Svout pin voltage is around 3 V when the input voltage is at maximum. Use the following

formula:

Select R5 and R6 with resistance about 5% lower than R7 and R8. (See Section 6.2 [LED short circuits] on page 28.)

Select highly accurate resistors as R5, R6, R7, and R8 (accuracy ±1% or better).

When the output voltage Vo does not exceed 10% of the maximum input voltage Vi, zero current detection may be

disabled. In that case, use the winding voltage supply, as described in Section 3.3 on page 15.

3.2.7

CS pin filter (R3 and C4)

R3 and C4 are filters used to shield the CS pin from noise. Adjusting R3 to range between 0 Ω and several kΩ and C4

between 10 pF and 100 pF will reduce false detections of off-timing due to switching-on noise in the off-time modulation

region. When adjusting the filter constants, check for false detections on the actual apparatus using as many different

5.1.2 [[B] Off-time modulation range] on pages 21 to 22.)

If the filter constants set are too large, the detection delay will grow, along with the change in output current due to the

output current setting and the input voltage. Reselect current detection resistors (R1 and R2) or the inductor, if necessary.

R8

C8

C9

1

2

R6

R7

R5

4

25kΩ

25kΩ

Comp

L1

-

+





k

25

3

V

max

_

Vi

k

25

8

R

7

R

－







CAT.No.1A0602-1E