LTC3728 datasheet(13/32 Pages) LINER

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LTC3728

3728fb

Figure 1 on the first page is a basic LTC3728 application

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 LTC3728 current comparator has a maximum thresh-

old 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,

∆IL.

Allowing a margin for variations in the LTC3728 and

external component values yields:

SENSE

MAX

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 crite-

rion for buck regulators operating at greater than 50%

duty factor. A curve is provided to estimate this reducton

in peak output current level depending upon the operating

duty factor.

Operating Frequency

The LTC3728 uses a constant frequency phase-lockable

architecture 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

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

∆IL decreases with higher induc-

tance or frequency and increases with higher VIN:

∆I

OUT

⎛

⎝⎜

⎞

⎠⎟

()( )

–

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL=0.3(IMAX). The maximum ∆IL

occurs at the maximum input voltage.

APPLICATIO S I FOR ATIO

Figure 5. PLLFLTR Pin Voltage vs Frequency

OPERATING FREQUENCY (kHz)

200

250

300

350

550

400

450

500

3728 F05

2.5

2.0

1.5

1.0

0.5

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