BD7682FJ-LB_16 Datasheet, PDF (9/31 Pages) Rohm – Low Noise Quasi-Resonant Control DC/DC converter IC for AC/DC Converter

BD768xFJ-LB Series
Datasheet
(a) Determination of ON Width (Turn OFF)
ON width is controlled via the FB pin and CS pin.
The ON width is determined by comparing FB pin voltage at 1/ AV_CS (typ = 1/2) with the CS pin voltage.
In addition, it is compared with the IC's internally generated V_LIM1A (1.0V typ) voltage and the comparator level
changes linearly, as is shown in Figure 5.
The CS pin is also used for the per-pulse over-current limiter circuit.
Changes at the FB pin result in changes in the maximum blanking frequency and over-current limiter level.
 mode1: Burst operation
 mode2: Frequency reduction operation (reduces maximum frequency)
 mode3: Maximum frequency operation (operates at maximum frequency)
 mode4: Overload operation (pulse operation is stopped when overload is detected)
MAX
FfsSWw[[kkHHzz]]
mode1
120kHz
mode2
mode3
mode4
30kHz
0.0V 0.5V
1.25V
CS
Limiter[V] _{mode1 mode2}
V_Lli_Im_M11
2.0V
mode3
2.8V _{FB [V]}
mode4
VVlLimIM22
0.0V 0.5V
1.25V
2.0V
2.8V
FB [V]
Figure 5. Relationship of FB Pin to Over-Current Limiter and Maximum Frequency
The over-current limiter level is adjusted for soft start function (section 5) and over-current protection of the input
voltage compensation (section 4 (c))
In this case, the V_LIM1 and V_LIM2 values are as listed below.
Table 1 Over-Current Protection Voltage
Soft Start
Start to 1ms
1ms to 4ms
I_ZT ≥ -1.0mA
V_LIM1
V_LIM2
0.250V (25.0%)
0.063V (6.0%)
0.500V (50.0%)
0.125V (12.0%)
I_ZT < -1.0mA
V_LIM1
V_LIM2
0.175V (17.5%)
0.047V (4.5%)
0.350V (35.0%)
0.094V (9.0%)
>4ms
1.000V (100.0%)
0.250V (25.0%)
0.700V (70.0%)
0.188V (18.8%)
(Note) Values in parentheses are relative values when compared to V_LIM1 (1.0V typ) during I_ZT ≥ -1.0mA.
(b) LEB (Leading Edge Blanking) Function
When the switching MOSFET is turned ON, surge current occur at each capacitor component and drive
current. Therefore, when the CS pin voltage rises temporarily, detection errors may occur in the over-current
limiter circuit.
To prevent detection errors, BD768xFJ has the blanking function. This function masks the CS voltage for T_LEB
(typ = 250ns) after the OUT pin changes from low to high.
This blanking function reduces CS pin filter.
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29.Jun.2016. Rev.002