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ACT4455

36V/5A Step Down DC/DC Converter

Active-Semi, Inc 

ACT4455

Rev 2, 21-Nov-12

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

With a selected inductor value the peak-to-peak

inductor current is estimated as:

( ) ILPK _PK

=

VOUT

×

VIN

V_

OUT

L ×VIN × fSW

(4)

The peak inductor current is estimated as:

1

ILPK = ILOADMAX

+

2

I

LPK

_ PK

(5)

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:

R FB 1

=

R

FB

2

⎜⎛

⎝

0

V OUT

.808

V

− 1 ⎟⎞

⎠

(1)

Over Current Protection Setting

The output over current threshold is calculated by:

IOCP1 = IOCP 2 = 116 mV / RSENSE

(2)

It is recommended that 1% or 0.5% high-accuracy

current sensing resistor is selected to achieve high-

accuracy over current protection. Two over current

protection thresholds can be different based on

different current sensing resistance.

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

(3)

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.

The selected inductor should not saturate at ILPK.

The maximum output current is calculated as:

IOUTMAX

=

I_

LIM

1

2

I

LPK

_ PK

(6)

ILIM is the internal current limit, which is typically

6.5A, as shown in Electrical Characteristics Table.

Input Capacitor

The input capacitor needs to be carefully selected

to maintain sufficiently low ripple at the supply input

of the converter. A low ESR capacitor is highly

recommended. Since large current flows in and out

of this capacitor during switching, its ESR also

affects efficiency.

The input capacitance needs to be higher than

10µF. The best choice is the ceramic type,

however, low ESR tantalum or electrolytic types

may also be used provided that the RMS ripple

current rating is higher than 50% of the output

current. The input capacitor should be placed close

to the IN and G pins of the IC, with the shortest

traces possible. In the case of tantalum or

electrolytic types, they can be further away if a

small parallel 0.1µF ceramic capacitor is placed

right next to the IC.

Output Capacitor

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple

voltage is:

VRIPPLE

= IOUTMAX K R RIPPLE ESR

+

28

VIN

× fSW 2 LCOUT

(7)

Where IOUTMAX is the maximum output current,

KRIPPLE is the ripple factor, RESR is the ESR of the

output capacitor, fSW is the switching frequency, L is

the inductor value, and COUT is the output

capacitance. In the case of ceramic output

capacitors, RESR is very small and does not

contribute to the ripple. Therefore, a lower

capacitance value can be used for ceramic type. In

the case of tantalum or electrolytic capacitors, the

ripple is dominated by RESR multiplied by the ripple

Innovative PowerTM

-6-

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Copyright © 2012 Active-Semi, Inc.

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