LTC3546 Datasheet, PDF (14/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
APPLICATIONS INFORMATION
A general LTC3546 application circuit is shown in Figure 7.
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.
Assuming a worst-case minimum on-time of 150ns, this
can be calculated as:
( ) ^f_O(MAX)



6.67_
V_OUT
V_IN(MAX)


MHz
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 LTC3546 is determined
by pulling the FREQ pin to V_IN, for 2.25MHz operation, by
connecting an external resistor from FREQ to ground, or
by driving an external clock signal into SYNC/MODE.
When using an external resistor to set the oscillator fre-
quency use the following equation:
R_T
=
2.51•10¹¹
f_O
(^Ω)
–
20kΩ
for 0.75MHz ≤ f_O ≤ 4MHz. Or use Figure 1 to select the
value for R_T.
The maximum operating frequency is also constrained
by the minimum on-time (typically 70ns) and duty cycle,
especially when forced continuous mode is selected.
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 operat-
ing 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 output
current. 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)


500
450
400
350
300
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
FREQUENCY (MHz)
3546 F01
Figure 1. Frequency vs R_T
3546fb
14