Datasheet

6
LTC1772
1772fb
APPLICATIONS INFORMATION
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The basic LTC1772 application circuit is shown in Figure 1.
External component selection is driven by the load re-
quirement and begins with the selection of L1 and R
SENSE
(= R1). Next, the power MOSFET, M1 and the output diode
D1 are selected followed by C
IN
(= C1) and C
OUT
(= C2).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
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
.
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
RIPPLE
= (0.4)(I
OUT
). Rearranging the above equation, it
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
of current. Using Figure 2, the value of R
SENSE
is:
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
current. The ripple current, I
RIPPLE
, decreases with higher
inductance or frequency and increases with higher V
IN
or
V
OUT
. The inductor’s peak-to-peak ripple current is given
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
SF = I
OUT
/I
OUT(MAX)
(%)
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110
100
90
80
70
60
50
40
30
20
10
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
Figure 2. Maximum Output Current vs Duty Cycle
OPERATIO
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(Refer to Functional Diagram)