6

LTC1772

1772fb

The basic LTC1772 application circuit is shown in Figure 1.

External component selection is driven by the load re-

(= R1). Next, the power MOSFET, M1 and the output diode

With the current comparator monitoring the voltage devel-

determines the inductor’s peak current. The output cur-

rent the LTC1772 can provide is given by:

I

R

I

OUT

SENSE

RIPPLE

=

−

012

2

.

(see Inductor Value Calculation section).

A reasonable starting point for setting ripple current is

becomes:

R

I

SENSE

OUT

=<

1

10

()(

)

for Duty Cycle

40%

However, for operation that is above 40% duty cycle, slope

compensation effect has to be taken into consideration to

select the appropriate value to provide the required amount

R

SF

I

SENSE

OUT

=

()(

)(

)

10

100

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses.

The inductance value also has a direct effect on ripple

by:

I

VV

fL

VV

VV

RIPPLE

IN

OUT

OUT

D

IN

D

=

−

+

+

⎛

⎝⎜

⎞

⎠⎟

()

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

I

V

R

PK

ITH

SENSE

–.

07

10

when the LTC1772 is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

tor current. The amount of reduction is given by the curves

in Figure 2.

DUTY CYCLE (%)

0

10 20 30 40 50 60 70 80 90 100

1772 F02

110

100

90

80

70

60

50

40

30

20

10

AT 5% DUTY CYCLE

AT 5% DUTY CYCLE

Figure 2. Maximum Output Current vs Duty Cycle

OPERATIO

(Refer to Functional Diagram)