LTC1775CS datasheet(9/24 Pages) LINER

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LTC1775

The basic LTC1775 application circuit is shown in Figure 1.

External component selection is primarily determined by

the maximum load current and begins with the selection of

the sense resistance for the desired current level. Since the

LTC1775 senses current using the on-resistance of the

power MOSFET, the maximum application current prima-

rily determines the choice of MOSFET. The operating

frequency and the inductor are chosen based largely on

the desired amount of ripple current. Finally, CIN is se-

lected for its ability to handle the RMS current into the

converter and COUT is chosen with low enough ESR to

meet the output voltage ripple specification.

Power MOSFET Selection

The LTC1775 requires two external N-channel power

MOSFETs, one for the top (main) switch and one for the

bottom (synchronous) switch. Important parameters for

the power MOSFETs are the breakdown voltage V(BR)DSS,

threshold voltage VGS(TH), on-resistance RDS(ON), reverse

transfer capacitance CRSS and maximum current ID(MAX).

The gate drive voltage is set by the 5.2V INTVCC supply.

Consequently, logic level threshold MOSFETs must be

used in LTC1775 applications. If low input voltage opera-

tion is expected (VIN < 5V), then sub-logic level threshold

MOSFETs should be used. Pay close attention to the

V(BR)DSS specification for the MOSFETs as well; many of

the logic level MOSFETs are limited to 30V or less.

The MOSFET on-resistance is chosen based on the

required load current. The maximum average output cur-

rent IO(MAX) is equal to the peak inductor current less half

the peak-to-peak ripple current

∆IL. The peak inductor

current is inherently limited in a current mode controller

by the current threshold ITH range. The corresponding

maximum VDS sense voltage is about 300mV under nor-

mal conditions. The LTC1775 will not allow peak inductor

current to exceed 300mV/RDS(ON)(TOP). The following

equation is a good guide for determining the required

RDS(ON)(MAX) at 25°C (manufacturer’s specification), al-

lowing some margin for ripple current, current limit and

variations in the LTC1775 and external component values:

DS ON MAX

O MAX

()(

)

()

≅ ()()

240

The

ρT is a normalized term accounting for the significant

variation in RDS(ON) with temperature, typically about

0.4%/

°C as shown in Figure 2. Junction to ambient tem-

perature TJA is around 20°C in most applications. For a

maximum ambient temperature of 70

°C, using ρ90°C≅1.3

in the above equation is a reasonable choice. This equation

is plotted in Figure 3 to illustrate the dependence of

maximum output current on RDS(ON). Some popular

MOSFETs are shown as data points.

Figure 2. RDS(ON) vs Temperature

Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V

The 300mV maximum sense voltage of the LTC1775

allows a large current to be obtained from power MOSFET

switches. It also causes a significant amount of power

dissipation in those switches and careful attention must be

JUNCTION TEMPERATURE (

°C)

–50

1.0

1.5

150

1775 F02

0.5

100

2.0

RDS(ON) (Ω)

0.08

1775 F03

0.02

0.04

0.06

0.10

IRL3803

SUD50N03-10

FDS8936A

Si9936

APPLICATIO S I FOR ATIO

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