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LTC3546

3546fb

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.

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, 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:

T =

2.51•10

() –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.

Assuming a worst-case minimum on-time of 150ns, this

can be calculated as:

fO(MAX) 6.67

VOUT

VIN(MAX)

MHz

()

The minimum frequency is limited by leakage and noise

coupling due to the large resistance of RT.

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 ΔIL decreases

with higher inductance and increases with higher VIN or

VOUT.

IL =

VOUT

fO •L

VOUT

VIN

Accepting larger values of ΔIL 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 ΔIL

= 0.35ILOAD(MAX),whereILOAD(MAX)isthemaximumoutput

current. The largest ripple ΔIL 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:

VOUT

fO • IL

VOUT

VIN(MAX)

Figure 1. Frequency vs RT

FREQUENCY (MHz)

500

450

350

250

150

400

300

200

100

2.5

1.5

3.5

3546 F01

4.5

2.0

4.0

1.0

3.0

0.5

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