500kHz, 36V Output, SOT23,

Step-Up DC-DC Converters

_______________________________________________________________________________________

9

Determining Peak Inductor Current

If the boost converter remains in the discontinuous

mode of operation, then the approximate peak inductor

η is

the efficiency of the boost converter.

Determining the Inductor Value

47µH is the recommended inductor value when the out-

put voltage is 30V and the input voltage is 5V. In gener-

al, the inductor should have a current rating greater

than the current-limit value. For example, the inductor’s

current rating should be greater than 150mA to support

a 4mA output current. Equivalent series resistance

(ESR) should be below 1Ω for reasonable efficiency.

Due to the MAX5025–MAX5028’s high switching fre-

quency, inductors with a ferrite core or equivalent are

recommended. Powdered iron cores are not recom-

mended due to their high losses at frequencies over

500kHz. Table 1 shows a list of vendors and 47µH

inductor parts.

For 4mA output current and output voltages other than

30V, the inductor can be simply scaled in value

according to the following formula:

Use the following formula to calculate the upper bound

of the inductor value at different output voltages and

output currents. This is the maximum inductance value

for discontinuous mode operation.

inductance value using the following formula, which will

allow the maximum output current to be delivered with-

out reaching the peak current limit:

the inductor should be greater than the switch current

limit.

L

TV

V

I

V

mA

LOWER

S

OUT

IN

OUT

IN

=

−

()

()

⎛

⎝⎜

⎞

⎠⎟

2

5

260

2

η

L

VV

V

T

IV

UPPER

OUT

IN

S

OUT

OUT

IN

=

−

()

2

2

2

η

L

HV

V

V

OUT

IN

−

()

()

47

25

μ

I

TV

V

I

L

LPEAK

S

OUT

IN

OUT

=

−

()

2

η

RR

V

V

OUT

REF

12

1

=

⎛

⎝⎜

⎞

⎠⎟

-

CONTROL

LOGIC

MAIN PWM

COMPARATOR

CURRENT-

LIMIT

COMPARATOR

FB

REF

-A

+A

+C

-C

MAX5025

MAX5026

THERMAL

SHUTDOWN

OSCILLATOR

ULVO

PGND

3

GND

2

5

SHDN

4

1

6

LX

N

Table 1. Inductor Vendors

VENDOR

PHONE

FAX

PART NUMBER OF 47µH INDUCTOR

Coilcraft

847-639-6400

847-639-1469

DT1608C-473

Sumida

847-545-6700

847-545-6720

CDRH4D28-470

Toko

847-297-0070

847-699-7864

A915BY-470M

Figure 1. Functional Diagram