LTC3446EDE Datasheet, PDF (11/20 Pages) Linear Technology – Monolithic Buck Regulator with Dual VLDO Regulators

LTC3446
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 }
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 )}


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.
V_IN
2.7V TO 5.5V _CIN
V_IN
SW
DIGITAL
CONTROL
BUCKFB
LTC3446
LV_IN
PGOOD LV_OUT1
MODESEL
ENBUCK
ENLDO1 LV_FB1
ENLDO2
R_TH
I_TH
LV_OUT2
C_ITH
LV_FB2
GND
L1
D1
OPT R2
R1
V_OUTB
C_F
OPT
C_OUTB
V_OUT1
R4
C_OUT1
R3
V_OUT2
R6
C_OUT2
R5
3446 F01
Figure 1. General LTC3446 Application Circuit
3446ff
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