Simple SOT23 Boost Controllers

_______________________________________________________________________________________

7

under light loads. The selection of 30% ripple current

causes this to happen at loads less than approximately

1/6th of maximum load.

There are two common reasons not to run in CCM:

1) High output voltage. In this case, the output-to-

input voltage ratio exceeds the level obtainable

by the MAX1522/MAX1523/MAX1524s’ maximum duty

factor. Calculate the application’s maximum duty cycle

using the equation in the Calculate the Maximum Duty

Cycle section. If this number exceeds 80%, you will

have to design for DCM.

2) Small output current. If the maximum output current

is very small, the inductor required for CCM may be

disproportionally large and expensive. Since I

are not a concern, it may make sense to use a smaller

inductor and run in DCM. This typically occurs when

the load current times the output-to-input voltage ratio

drops below a few hundred milliamps, although this

also depends on the external components.

Calculate the Maximum Duty Cycle

The maximum duty cycle of the application is given by:

diode (about 0.5V).

Design Procedure for CCM

On-Time Selection

For CCM to occur, the MAX1522/MAX1523/MAX1524

must be able to exceed the application’s maximum

duty cycle. For applications up to 45% duty cycle, con-

nect SET to GND for 0.5µs on-time to get fast switching

and a smaller inductor. For applications up to 80% duty

on-time. For applications greater than 80% duty cycle,

CCM operation is not guaranteed; see the Design

Procedure for DCM section.

Switching Frequency

A benefit of CCM is that the switching frequency

remains high as the load is reduced, whereas in DCM

the switching frequency varies directly with load. This is

important in applications where switching noise needs

to stay above the audio band. The medium- and heavy-

load switching frequency in CCM circuits is given by:

varies primarily with input voltage. However, when the

load is reduced, a CCM circuit drops into DCM, and

the frequency becomes load dependent:

Calculate the Peak Inductor Current

For CCM, the peak inductor current is given by:

I

VV

V

I

PEAK

OUT

D

IN MIN

LOAD MAX

=×

+

×

115

.

()

()

ƒ≈

×

+−

+

×

×

−

SWITCHING LIGHT LOAD

ON

OUT

D

IN

OUT

D

LOAD

LOAD MAX

t

VV

V

VV

I

I

()

()

.

1

018

ƒ=

×

+−

+

SWITCHING

ON

OUT

D

IN

OUT

D

t

VV

V

VV

1

DutyCycle

VV

V

VV

MAX

OUT

D

IN MIN

OUT

D

+−

+

×

()

%

100

MAX1522

MAX1523

INPUT

2.7V TO 4.2V

C3

0.1

µF

OFF

ON

6

3

4

5

2

1

EXT

SET

FB

SHDN

GND

C1

10

µF

6.3V

L1

33

µH

CDR74B-330

D1

MBR0530T3

Q1

FDC633N

R1

130k

Ω

1%

OUTPUT

12V

C2

33

µF

TPSD336M020R0200

220pF

220pF

R1

15.0k

Ω

1%

Figure 1. MAX1522/MAX1523 Standard Operating Circuit