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LTC3546 Datasheet, PDF (14/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator | |||
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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, CIN, COUT1, and COUT2
can be selected.
Assuming a worst-case minimum on-time of 150ns, this
can be calculated as:
( ) fO(MAX)
6.67
VOUT
VIN(MAX)
MHz
Operating Frequency
Selection of the operating frequency is a trade-off between
efï¬ciency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efï¬ciency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, fO, of the LTC3546 is determined
by pulling the FREQ pin to VIN, 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:
RT
=
2.51â¢1011
fO
(Ω)
â
20kΩ
for 0.75MHz ⤠fO ⤠4MHz. Or use Figure 1 to select the
value for RT.
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 RT.
Inductor Selection
Although the inductor does not inï¬uence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ÎIL decreases
with higher inductance and increases with higher VIN or
VOUT.
IL
=
VOUT
fO ⢠L
1
VOUT
VIN
Accepting larger values of ÎIL 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 ÎIL
= 0.35ILOAD(MAX), where ILOAD(MAX) is the maximum output
current. The largest ripple ÎIL occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a speciï¬ed maximum, the inductor value should be
chosen according to the following equation:
L
VOUT
fO ⢠IL
1
VOUT
VIN(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 RT
3546fb
14
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