LTC3565_1 Datasheet, PDF (11/22 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3565
Applications Information
A general LTC3565 application circuit is shown in
Figure 4. External component selection is driven by the load
requirement, and begins with the selection of the inductor
L1. Once L1 is chosen, C_IN and C_OUT can be selected.
Operating Frequency
Selection of the operating frequency is a trade-off 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 LTC3565 is determined
by an external resistor that is connected between the RT
pin and 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 = 1.21 × 10⁶ (f_O)^–1.2674 (kΩ)
where R_T is in kΩ and f_O is in kHz or can be selected
using Figure 1.
The maximum usable operating frequency is limited by
the minimum on-time and the 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
The operating frequency, f_O, has a direct effect on the
inductor value, which in turn influences the inductor ripple
current, ΔI_L:
ΔI_L
=
V_OUT
f_O • L
•
⎛⎝⎜_1−
V_OUT
V _IN
⎞
⎠⎟
The inductor ripple current decreases with larger induc-
tance or frequency, and increases with higher V_IN or V_OUT.
Accepting larger values of ΔI_L allows the use of lower
inductances, but results in higher output ripple voltage,
greater core loss and lower output capability.
A reasonable starting point for setting ripple current is
ΔI_L = 0.4 • I_OUT(MAX), where I_OUT(MAX) is 1.25A. The largest
ripple current 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 • ΔI_L
⎛
• _⎝⎜1−
V _I^V_NO(MUTAX)^{⎞⎠⎟}
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.
5000
T_A = 25°C
4500
4000
3500
3000
2500
2000
1500
1000
500
0
0 100 200 300 400 500 600
R_T (kΩ)
3565 F01
Figure 1. Frequency vs R_T
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI requirements
than on what the LTC3565 requires to operate. Table 1
3565fa
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