in(on)

UVLO

R6 R8

17.4

2

V

V

1.25

12.125V

R8

2

+

+

=

´

=

´

=

2

2

2

2

max

SW(rms)

L(pk pk)

peak

peak

L(pk pk)

D

0.637

I

I

3 I

3 I

I

5.51

3 8.26

3 8.26 5.51

4.57A

3

3

-

-

=

´ D

+ ´

- ´

´ D

=

´

+ ´

- ´

´

=

[

]

OUT(max)

min

OUT

L(peak)

SW

OUT

I

(1 D

)

4 (1 0.077)

V

I

ESR

8.26 0.08

711mV

2 f

C

2 123k 300

´ -

é

ù

é

ù

´ -

é

ù

D

=

+

´

=

+

´

=

ê

ú

ê

ú

ë

û

´

´

´

´

m

ë

û

ë

û

IN(min)

SW(on)

max

min

SW

L(pk pk)

(V

V

D

(18 0.5) 0.637

L

16.4 H

f

I

123kHz 5.51

-

-

´

-

´

=

=

=

m

´ D

´

L(pk pk)

L(peak)

L(avg)

I

I

I

8.26A

2

-

D

=

+

=

L(pk pk)

L(avg)

I

I

5.51A

-

D

=

=

out

L(avg)

max

50% I

0.5 4

5.51A

1 D

1 0.637

´

´

=

=

=

-

-

OUT

d

IN(max)

min

OUT

d

sw(on)

V

V

V

48

0.7

45

D

7.7%

V

V

V

48

0.7 0.5

+

-

+

-

=

=

=

+

-

+

-

OUT

d

IN(MIN)

max

OUT

d

sw(on)

V

V

V

48

0.7 18

D

63.7%

V

V

V

48

0.7 0.5

+

-

+

-

=

=

=

+

-

+

-

LM5032

www.ti.com

SNVS344B – MARCH 2005 – REVISED DECEMBER 2014

(13)

(14)

The highest average inductor current in each phase is calculated at highest load and minimum input voltage,

where each phase is assumed to carry 50% of the total load current:

(15)

Allowing the peak-to-peak inductor current ripple to be 100% of the average:

(16)

And the peak inductor current IL(peak) will be:

(17)

Knowing the switching frequency, maximum duty cycle and target peak-peak ripple current, the required

inductance can be calculated:

(18)

Off-the-shelf available inductors of 15 µH were used, resulting in slightly higher peak-peak inductor ripple current,

and slightly higher inductor peak current.

8.2.2.2.2

Output Capacitor Selection

The output ripple across the boost output capacitor can be approximated from the following equation, where

COUT is the value of output capacitance, and ESR is the equivalent-series-resistance of the output capacitance.

For this design, the chosen electrolytic output capacitors are 150

μF with 160-mΩ ESR, so the net capacitance is

300

μF and net ESR is 80 mΩ.

(19)

This meets the target 2% specification. However, the extra ceramic output capacitors will also absorb a

significant percentage of the switching frequency ripple, so the resulting output peak-to-peak ripple voltage

should be lower than the value calculated above, and should be comfortably less than the 2% specification.

8.2.2.2.3

Boost MOSFET Selection

The boost MOSFET should be rated for at least the rated output voltage plus some margin for voltage ringing. A

60-V device was selected. Since the boost inductor value was chosen to achieve peak-to-peak ripple current

equal to 100% of the average current, the RMS MOSFET current at maximum load and minimum Vin is:

(20)

The chosen 60-V rated MOSFET SUD50N06-9L has 9.3-m

Ω Rds(on), resulting in approximately 200-mW

conduction loss.

8.2.2.2.4

Boost Diode Selection

The boost diode must have a reverse voltage rating of at least VOUT, plus some margin for ringing. Thus a 60-V

rated part was selected. Since fast reverse recovery is important, a Schottky device can be used at this voltage

rating. A common-cathode dual-diode MBR1560 was selected, with each diode connected to one or other of the

interleaved phases.

8.2.2.3 UVLO Setting

To ensure start-up below the required minimum system input voltage of 18 V, the UVLO divider resistors R6 and

R8 are set to 17.4 k

Ω and 2 kΩ, respectively. This sets the input UVLO turn-on level to:

(21)

Copyright © 2005–2014, Texas Instruments Incorporated

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