External component setting procedure

Connecting a resistor and capacitor to the RT pin (pin 2) and the CT pin (pin 3) will set the triangular wave oscillation

frequency. The RT determines the charge/discharge current to the capacitor. Referring to Fig. 18, set RT resistor and

the CT capacitor. Recommended setting ranges are 5 to 100 kΩ for the CT resistor, 33 to 1000 pF for the CT capacitor,

and 20 kHz to 800 kHz for the oscillation frequency. Any setting outside of these ranges may turn OFF switching,

thus impairing the operation guarantee.

(3) Setting of DTC voltage

transistor (FET) from being FULL ON. Fig. 19 shows the relationship between the DTC voltage and the maximum duty

Furthermore, the maximum duty ratio should be designed so as not to become a maximum duty for the normal use. The

following section shows ranges for the normal use.

VOMAX

VCCMIN

ONDutyMAX =

10000

1000

100

10

1

2

10

100

200

RT [kΩ]

Fig. 18 RT/CT vs. Frequency

Ta=25˚C

Vcc=12V

CT=33pF

CT=33pF

CT=100pF

CT=100pF

CT=200pF

CT=200pF

CT=470pF

CT=470pF

CT=1000pF

CT=1000pF

CT=33pF

CT=100pF

CT=200pF

CT=470pF

CT=1000pF

120

100

80

60

40

20

0

-20

1.4

1.5

2

1.9

1.8

1.7

1.6

Fig. 19 DTC voltage vs. Maximum duty

Ta=25˚C

Vcc=12V

Fosc=220kHz

Fig. 15 Step-down voltage

Fig.16 Step-up voltage

Fig. 17 Inverting voltage

R8+R9

R9

Vo =

X 1.25 [V]

R8+R9

R9

Vo =

X 1.25 [V]

R8

R9

(

)

Vo = 1.25 1–

[V ]

8/16

12

14

Vo

R8

R9

ERR

+

–

Reference voltage: 1.25 V

12

14

Vo

R8

R9

ERR

+

–

Reference voltage: 1.25 V

12

14

11

Vo (Negative)

R8

R9

ERR

+

–

Reference voltage: 1.25 V

VREF

(1) Design of feedback resistance constant

(2) Setting of oscillation frequency

Set step-down, step-up, and inverting feedback resistance as shown below. Set resistance in the range of 1 kΩ to

330 kΩ. Setting the resistance to 1 kΩ or less will result in degraded power efficiency, while setting it to 330 kΩ or

more will increase the offset voltage due to the input bias current of 0.1μA (TYP) of the error amplifier.

VOMAX

ONDutyMAX =

VOMAX

ONDutyMAX =