LTC1625IGN datasheet(9/24 Pages) LINER

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LTC1625

APPLICATIONS INFORMATION

The basic LTC1625 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 and power MOSFETs. Because the

LTC1625 uses MOSFET VDS sensing, the sense resistance

is the RDS(ON) of the MOSFETs. The operating frequency

and the inductor are chosen based largely on the desired

amount of ripple current. Finally, CIN is selected for its

ability to handle the large RMS current into the converter

and COUT is chosen with low enough ESR to meet the

output voltage ripple specification.

Power MOSFET Selection

The LTC1625 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 LTC1625 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 150mV under nor-

mal conditions. The LTC1625 will not allow peak inductor

current to exceed 150mV/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 LTC1625 and external component values:

DS ON MAX

O MAX

()(

)

()

≅ ()( )

120

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 case tempera-

ture TJC is around 10°C in most applications. For a

maximum ambient temperature of 70

°C, using ρ80°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 from Siliconix are shown as data points.

JUNCTION TEMPERATURE (

°C)

–50

1.0

1.5

150

1625 F02

0.5

100

2.0

Figure 2. RDS(ON) vs Temperature

RDS(ON) (Ω)

0.08

1625 F03

0.02

0.04

0.06

0.10

Si4420

Si4410

Si4412

Si9936

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

The power dissipated by the top and bottom MOSFETs

strongly depends upon their respective duty cycles and

the load current. When the LTC1625 is operating in con-

tinuous mode, the duty cycles for the MOSFETs are:

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