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LTC3446
11
3446ff
applicaTions inForMaTion
A general LTC3446 application circuit is shown in Figure 1.
External component selection is driven by output voltage
and load requirements. The following text is divided into
two sections: the first covers Buck regulator design and
the second covers use of the linear VLDO regulators.
BUCK REGULATOR DESIGN
Buck regulator design begins with the selection of the
L1 inductor based on desired ripple current. Once L1 is
chosen, C
IN
 and C
OUTB
 can be selected based on output
voltage ripple requirements. Output voltage is programmed
through R1 and R2, and loop response can be optimized
by choice of R
ITH
 and C
ITH
.
Inductor Selection
Although the inductor does not influence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current I
L
 decreases
with higher inductance and increases with higher V
IN
 or
V
OUTB
:
 
I
L
=
V
OUTB
f
O
"L
" 1
V
OUTB
V
IN
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
Accepting larger values of I
L
 allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
I
L
 = 0.3 " I
MAXP
, where I
MAXP
 is the peak switch current
limit. The largest ripple current I
L
 occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
 
L=
V
OUTB
f
O
"I
L
" 1
V
OUTB
V
IN(MAX)
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values will cause the burst frequency
to increase.
Figure 1. General LTC3446 Application Circuit
V
IN
R2
L1
R1
SW
BUCKFB
LV
IN
LTC3446
GND
C
IN
C
ITH
DIGITAL
CONTROL
C
OUTB
C
F
OPT
D1
OPT
V
OUTB
R4
R3
C
OUT1
V
OUT1
V
IN
2.7V TO 5.5V
LV
OUT1
PGOOD
MODESEL
ENBUCK
ENLDO1
ENLDO2
LV
FB1
R6
R5
C
OUT2
3446 F01
V
OUT2
LV
OUT2
R
TH
I
TH
LV
FB2
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