ACT4523AYH-T データシートの表示（PDF） - Active-Semi, Inc

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ACT4523AYH-T

Wide-Input Sensorless CC/CV Step-Down DC/DC Converter

Active-Semi, Inc 

ACT4523A

Rev 0, 25-Jul-13

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

Figure 1 shows the connections for setting the

output voltage. Select the proper ratio of the two

feedback resistors RFB1 and RFB2 based on the

output voltage. Adding a capacitor in parallel with

RFB1 helps the system stability. Typically, use RFB2 ≈

10kΩ and determine RFB1 from the following

equation:

R FB1

=

RFB 2

⎜⎛

⎝

VOUT

0.808V

− 1 ⎟⎞

⎠

(1)

CC Current Setting

ACT4523A constant current value is set by a

resistor connected between the ISET pin and GND.

The CC output current is linearly proportional to the

current flowing out of the ISET pin. The voltage at

ISET is roughly 1.1V and the current gain from

ISET to output is roughly 21000 (21mA/1µA). To

determine the proper resistor for a desired current,

please refer to Figure 2 below.

Figure 2:

Curve for Programming Output CC Current

Output Current vs. RISET

4500

4000

3500

3000

2500

2000

1500

1000

VIN = 24V, VOUT = 4V

500

2

6

10

14

18

22

26

RISET (kΩ)

Inductor Selection

The inductor maintains a continuous current to the

output load. This inductor current has a ripple that is

dependent on the inductance value:

Higher inductance reduces the peak-to-peak ripple

current. The trade off for high inductance value is

the increase in inductor core size and series

resistance, and the reduction in current handling

capability. In general, select an inductance value L

based on ripple current requirement:

( ) L =

VOUT

×

VIN

V_

OUT

(2)

V f I K IN SW LOADMAX RIPPLE

where VIN is the input voltage, VOUT is the output

voltage, fSW is the switching frequency, ILOADMAX is

the maximum load current, and KRIPPLE is the ripple

factor. Typically, choose KRIPPLE = 30% to

correspond to the peak-to-peak ripple current being

30% of the maximum load current.

With a selected inductor value the peak-to-peak

inductor current is estimated as:

( ) ILPK _PK

=

VOUT

×

VIN

V_

OUT

L ×VIN × fSW

(3)

The peak inductor current is estimated as:

1

ILPK

=

ILOADMAX

+

2

I

LPK

_PK

(4)

The selected inductor should not saturate at ILPK.

The maximum output current is calculated as:

IOUTMAX

=

I_

LIM

1

2

ILPK _ PK

(5)

LLIM is the internal current limit, which is typically

4.5A, as shown in Electrical Characteristics Table.

External High Voltage Bias Diode

It is recommended that an external High Voltage

Bias diode be added when the system has a 5V

fixed input or the power supply generates a 5V

output. This helps improve the efficiency of the

regulator. The High Voltage Bias diode can be a

low cost one such as IN4148 or BAT54.

Figure 3:

External High Voltage Bias Diode

Innovative PowerTM

-6-

www.active-semi.com

Copyright © 2013 Active-Semi, Inc.

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