(4) Setting of soft start time

Adding a capacitor to the DTC resistance divider will enable the soft start function activation.

The soft start function will be required to prevent an excessive increase in the coil current and overshoot of the output voltage,

while in startup operation. Fig. 20 shows the relationship between the capacitor and the soft start time. Referring to this Figure,

set the capacitor. It is recommended to set the capacitance value in the range of 0.01 to 10 μF. Setting the capacitance value to

0.01 μF or less, may cause overshoot to the output voltage, while setting it to 10 μF or more may cause an inverse current

in the internal parasitic diode when the power supply is grounded, thus resulting in damage to the internal element.

the internal element.

Since the PNP Tr is generally slow in switching, in terms of the sat characteristics , the ON/OFF peak circuit is used as

an acceleration circuit. The D1 and the C7 generate an ON peak current, while the Q1 and the C7 forms an OFF peak

circuit.Set pull-up resistance to 510 Ω as a guide at VCC=12 V. It is recommended to set this resistance in the range of

100 kΩ to 10 kΩ. In order to make adjustment of the R6 and R7, however, pay attention of the points listed in table below.

Take 1000 pF as a guide for the C7 setting. If the ON/OFF peak currents are inadequate, increase the C7 capacitance

value. It is recommended to set capacitance values in the range of 100 pF to 10000 pF. Setting the capacitance value

to 10000 pF or more may increase the peak current and degrade the power efficiency.

9/16

1.00E+02

1.00E+01

1.00E+00

1.00E–01

Fig. 20 Soft start capacitance vs. Delay time

Ta=25˚C

Vcc=12V

Vo=5V

Fosc=220kHz

L=33 μH

R1=20k

R2=33k

1.00E–09

1.00E–08

1.00E–07

1.00E–06

1.00E–05

Fig. 21 ON/OFF peak circuit

NO.

1

2

3

4

Item

Efficiency

Tr Turn ON / Turn OFF

Switching frequency

Load current capacitance

To reduce R6

Degraded

Faster Turn OFF

Increasable

Degraded

To reduce R7

Degraded

Faster Turn OFF

Increasable

Degraded

Vcc

C5

C6

C7

R6

R7

D1

Q2

Q1

L

D2

OUT