Data Sheet G14309EJ2V0DS00

20

µµµµPC1909

Note that when the rising of VREF is later than the rising of VCC at startup, OUT1 and OUT2 become high level

simultaneously when VREF is in a range of about 0.45 to 0.5 V, which may result in the external power MOS FET being

inadvertently turned on. To prevent this, speed up the rising of VREF by pulling it up to VCC with a resistor. Note, however,

that the standby current (ICC(SB)) will increase by only the current that flows through the connected resistor.

The pull-up resistance value (R) can be calculated from the following equation.

R [k

Ω] =

0.1 [mA]

VCC(MAX.) [V]

− 0.5

Remark VCC(MAX.): The highest power supply voltage that can be applied at startup without causing malfunction.

For the same reason, if VCC drops below the operation stopped voltage (VCC(L to H) – VH), VREF and the constant current

circuit will be cut-off, which weakens the drive capacity of the OUT1 and OUT2 outputs when the power supply is cut-off.

If this drive capacity is weakened, the charge that has accumulated at the power MOS FET gates may not be sufficiently

discharged, blunting the falling section of the power MOS FET gate drive waveform. In this case, connecting a capacitor

of at least 0.47

µF between the VREF pin and GND allows sufficient time and output block drive capacity to discharge the

charge accumulated at the gates of the power MOS FET.

Because the operation start and stop voltages in the

µPC1909 are 9 V TYP. and 5 V TYP. respectively, VREF is output

while operation is stopped until VCC is 5 V or lower. However, when VCC reaches about 6.5 V, VREF falls together with

VCC, as can be seen in Figure 4-3. When VREF falls, the constant current value of the triangle wave oscillator decreases,

causing the oscillation frequency to drop.

Figure 4-3. VREF vs. VCC Characteristics

10

9

8

7

6

5

4

3

2

1

0

1

2

3

4

5

6

IREF = 10 mA

IREF = 0 A

TA = 25

°C

VH

VCC(L to H)

VCC - Supply Voltage - V

In standard applications, VREF is resistance-divided to create the DTC1, DTC2, and FB voltages. In addition, because

the levels of the triangle wave (CT) and internally level-shifted triangle wave (dCT) are also generated internally by

dividing the resistance of VREF, if VREF drops, each of the above will drop in proportion to VREF. As a result, even if the

oscillation frequency drops, standard applications are not affected. Further study will be required, however, including for

transient states.