LTC3417A-2 Datasheet, PDF (9/20 Pages) Linear Technology – Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator

LTC3417A-2
APPLICATIONS INFORMATION
A general LTC3417A-2 application circuit is shown in
Figure 4. External component selection is driven by the
load requirement, and begins with the selection of the
inductors L1 and L2. Once L1 and L2 are chosen, C_IN,
C_OUT1 and C_OUT2 can be selected.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f_O, of the LTC3417A-2 is deter-
mined by pulling the FREQ pin to V_IN for 1.5MHz opera-
tion or by connecting an external resistor from FREQ to
ground. The value of the resistor sets the ramp current
that is used to charge and discharge an internal timing
capacitor within the oscillator and can be calculated by
using the following equation:
R_T
≈
1.61• 10¹¹
f_O
(Ω)
–
16.586kΩ
for 0.6MHz ≤ f_O ≤ 4MHz. Alternatively, use Figure 1 to
select the value for R_T.
The maximum operating frequency is also constrained
by the minimum on-time and duty cycle. This can be
calculated as:
( ) f_O(MAX)



6.67 _

V_OUT
V_IN(MAX)



MHz
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R_T.
160
140
120
100
80
60
40
20
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
FREQUENCY (MHz)
3417A-2 F01
Figure 1. Frequency vs R_T
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current, ΔI_L, decreases with
higher inductance and increases with higher V_IN or V_OUT.
I_L
=
V_OUT
f_O • L


1–
V_OUT
V_IN




Accepting larger values of ΔI_L allows the use of low induc-
tances, but results in higher output voltage ripple, greater
core losses and lower output current capability.
A reasonable starting point for setting ripple current is
ΔI_L = 0.35I_LOAD(MAX), where I_LOAD(MAX) is the maximum
current output. The largest ripple, ΔI_L, occurs at the maxi-
mum 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_OUT
f_O •
I_L



1–
V_OUT
V_IN(MAX)



_
3417a2fa
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