LTC3417_15 Datasheet, PDF (9/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
APPLICATIONS INFORMATION
A general LTC3417 application circuit is shown in Figure 4.
External component selection is driven by the load require-
ment, 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 LTC3417 is determined
by pulling the FREQ pin to V_IN for 1.5MHz operation 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
(
)
–
16.586k

for 0.6MHz ≤ f_O ≤ 4MHz. Alternatively, use Figure 1 to
select the value for 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)
3417 F01
Figure 1. Frequency vs 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.
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 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_OUT
f_O •
I_L

 ^1–
V_OUT
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 inductor
values will cause the burst frequency to increase.
3417fd
9