Datasheet

LTC3546
14
3546fc
For more information www.linear.com/3546
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.
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•10
11
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.
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
( )
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
.
ΔI
L
=
V
OUT
f
O
•L
1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Accepting larger values of ΔI
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 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
• ΔI
L
1−
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
Figure 1. Frequency vs R
T
FREQUENCY (MHz)
0
R
T
(kΩ)
500
50
450
350
250
150
400
300
200
100
0
2.51.5 3.5
3546 F01
4.5
2.0 4.01.0 3.00.5