LTC1735 datasheet(13/32 Pages) LINER

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LTC3728L-1

3728l1fc

Figure 1 on the ﬁrst page is a basic LTC3728L-1 applica-

tion circuit. External component selection is driven by

the load requirement, and begins with the selection of

RSENSE and the inductor value. Next, the power MOSFETs

and D1 are selected. Finally, CIN and COUT are selected.

The circuit shown in Figure 1 can be conﬁgured for

operation up to an input voltage of 28V (limited by the

external MOSFETs).

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current.

The current comparator has a maximum threshold of

75mV/RSENSE and an input common mode range of SGND

to 1.1(INTVCC). The current comparator threshold sets the

peak of the inductor current, yielding a maximum average

output current IMAX equal to the peak value less half the

peak-to-peak ripple current, ΔIL.

Allowing a margin for variations in the IC and external

component values yields:

RSENSE =

50mV

IMAX

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to

the internal compensation required to meet stability criteria

for buck regulators operating at greater than 50% duty

factor. A curve is provided to estimate this reduction in

peak output current level depending upon the operating

duty factor.

Operating Frequency

The IC uses a constant frequency phase-lockable ar-

chitecture with the frequency determined by an internal

capacitor. This capacitor is charged by a ﬁxed current plus

an additional current which is proportional to the voltage

applied to the PLLFLTR pin. Refer to Phase-Locked Loop

and Frequency Synchronization in the Applications Infor-

mation section for additional information.

A graph for the voltage applied to the PLLFLTR pin vs

frequency is given in Figure 5. As the operating frequency

is increased the gate charge losses will be higher, reducing

APPLICATIONS INFORMATION

Figure 5. PLLFLTR Pin Voltage vs Frequency

efﬁciency (see Efﬁciency Considerations). The maximum

switching frequency is approximately 550kHz.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efﬁciency. A higher

frequency generally results in lower efﬁciency because

of MOSFET gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current and

low current operation must also be considered.

The inductor value has a direct effect on ripple current.

The inductor ripple current ΔIL decreases with higher

inductance or frequency and increases with higher VIN:

IL =

(f)(L)

VOUT 1–

VOUT

VIN

Accepting larger values of ΔIL allows the use of low in-

ductances, but results in higher output voltage ripple and

greater core losses. A reasonable starting point for setting

ripple current is ΔIL=0.3(IMAX). The maximum ΔIL occurs

at the maximum input voltage.

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

OPERATING FREQUENCY (kHz)

200

300

400

500

600

3728L1 F05

2.5

2.0

1.5

1.0

0.5

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