LTC3728LX(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC3728LX
(Rev.:RevA)

Dual, 550kHz, 2-Phase Synchronous Regulators

Linear Technology 

LTC3728L/LTC3728LX

APPLICATIO S I FOR ATIO

Figure 1 on the first page is a basic LTC3728L/LTC3728LX

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 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, ∆IL.

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 crite-

rion 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 architec-

ture with the frequency determined by an internal capaci-

tor. 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

2.5

2.0

1.5

1.0

0.5

0

200

300

400

500

600

OPERATING FREQUENCY (kHz)

3728 F05

Figure 5. PLLFLTR Pin Voltage vs 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:

∆IL

=

1

(f)(L)



VOUT  1–

VOUT

VIN





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.

3728lxfa

13

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