AN8049 データシートの表示（PDF） - Panasonic Corporation

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AN8049

1.8-volt 3-channel step-up, step-down, and polarity inverting DC-DC converter control IC

Panasonic Corporation 

Voltage Regulators

AN8049SH

I Application Notes (continued)

[6] Setting the maximum duty

The maximum duty is set to 86% internally to the IC. However, this setting can be changed to be any value in the

range 0% to 100% by adding an external resistor.

1. To use a duty lower than the current duty (80% to 92%)

Insert the resistor RDT between the DT pin and ground.

Determine the DT pin voltage for the required duty from the provided DT pin voltage vs. maximum on-duty

characteristics in the "[2] Main characteristics" section and determine the value of the external resistor RDT from

formula A.

Note that there is a sample-to-sample variation of −19% to +33% due to temperature characteristics and sample-

to-sample variations of the internal resistors R1 and R2. (However, the direction of the sample-to-sample varia-

tions is identical for R1 and R2.) Determine the size of the sample-to-sample variations in the DT pin voltage VDT

from formula B, and estimate the size of the sample-to-sample variation in the duty from the provided DT pin

voltage vs. maximum on-duty characteristics in the "[2] Main characteristics" section .

RDT

=

VREF

R1

−

(

VDT

11

R1 + R2 )

×

VDT

VDT

=

R2 / RDT

R1 + R2 / RDT

× VREF

··········A

··········B

VREF

1.26 V

R1

DT

R2

RDT

ch.1, 2 ch.3

R1 45 kΩ 44 kΩ

R2 55 kΩ 56 kΩ

2. To use a duty higher than the current duty (80% to 92%)

Insert the resistor RDT between the DT pin and the VREF pin.

Use formulas C and D to determine the value of the external resistor RDT and the size of the sample-to-sample

variations in the same manner as in item 1 above.

RDT

=

(

1

R1 +

VREF − VDT

1

R2 ) × VDT −

VREF

R1

··········C

R1

VREF

1.26 V

RDT

VDT

=

R2

+

R2

R1 /

RDT

× VREF

··········D

R2

DT

[7] Setting the soft start time

The soft start time is determined by the value of the capacitor connected between the DT pin and ground.

OSC pin waveform

DT pin waveform

VDT

Soft start time, tD

0.2 V

Use the following formula to set the soft start time tD.

tD = − R2 × CDT × ln (1 −

VDT

VREF − VDT

×

R1

R2

)

R1

R2

VREF

1.26 V

DT

CDT

ch.1, 2 ch.3

R1 45 kΩ 44 kΩ

R2 55 kΩ 56 kΩ

19

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