ACT4533 datasheet(6/16 Pages) ACTIVE-SEMI

6 / 16 page

ACT4533

Rev 1, 25-Feb-13

Innovative Power

- 6 -

www.active-semi.com

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, RFB2 ≈

10kΩ and determine RFB1 from the equation:

CC Current Setting

ACT4533 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

CC Current Line Compensation

When operating at constant current mode, the

current limit increase slightly with input voltage. For

wide input voltage applications, a resistor RC is

added to compensate line change and keep output

high CC accuracy, as shown in Figure 3.

Figure 3:

Iutput Line Compensation

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:

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:

The peak inductor current is estimated as:

(3)

(

)

OUT

LPK

LOADMAX

LPK

(4)

Output Current vs. RISET

3000

2500

2000

1500

1000

500

3500

RISET (kΩ)

VIN = 24V, VOUT = 4V

(2)

(

)

RIPPLE

LOADMAX

OUT

⎟

⎠

⎞

⎜

⎝

⎛

−

808

OUT

(1)

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