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ACT4501

1.25A CC/CV Step-Down DC/DC Converter

Active-Semi, Inc 

ACT4501

Rev 2, 14-Nov-12

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. Typically, use RFB2 ≈ 10kΩ and

determine RFB1 from the following equation:

RFB1

=

RFB2

⎜⎛

⎝

VOUT

0.808V

−1⎟⎞

⎠

(1)

CC Current Setting

ACT4501 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 1V and the current gain from ISET to output

is roughly 25000 (25mA/1µA). To determine the

proper resistor for a desired current, please refer to

Figure 2 below.

Figure 2:

Curve for Programming Output CC Current

1800

1600

1400

1200

1000

800

600

400

200

0

0

Output Current vs. RISET

10 20 30 40 50 60 70 80 90

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 ILPK _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

2.5A, 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.

Innovative PowerTM

-6-

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

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