LTC1872 datasheet(7/12 Pages) LINER

7 / 12 page

LTC1872

APPLICATIONS INFORMATION

Inductor Selection

When selecting the inductor, keep in mind that inductor

saturation current has to be greater than the current limit

set by the current sense resistor. Also, keep in mind that

the DC resistance of the inductor will affect the efficiency.

Off the shelf inductors are available from Murata, Coilcraft,

Toko, Panasonic, Coiltronics and many other suppliers.

Power MOSFET Selection

The main selection criteria for the power MOSFET are the

threshold voltage VGS(TH), the “on” resistance RDS(ON),

reverse transfer capacitance CRSS and total gate charge.

Since the LTC1872 is designed for operation down to low

input voltages, a logic level threshold MOSFET (RDS(ON)

guaranteed at VGS = 2.5V) is required for applications that

work close to this voltage. When these MOSFETs are used,

make sure that the input supply to the LTC1872 is less than

the absolute maximum VGS rating, typically 8V.

The required minimum RDS(ON) of the MOSFET is gov-

erned by its allowable power dissipation given by:

DC I

DS ON

() ≅ () +

()

where PP is the allowable power dissipation and δp is the

temperature dependency of RDS(ON). (1 + δp) is generally

given for a MOSFET in the form of a normalized RDS(ON) vs

temperature curve, but

δp = 0.005/°C can be used as an

approximation for low voltage MOSFETs. DC is the maxi-

mum operating duty cycle of the LTC1872.

Output Diode Selection

Under normal load conditions, the average current con-

ducted by the diode in a boost converter is equal to the

output load current:

D avg

OUT

() =

It is important to adequately specify the diode peak current

and average power dissipation so as not to exceed the

diode ratings.

Schottky diodes are recommended for low forward drop

and fast switching times. Remember to keep lead length

short and observe proper grounding (see Board Layout

Checklist) to avoid ringing and increased dissipation.

CIN and COUT Selection

To prevent large input voltage ripple, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current for a boost

converter is approximately equal to:

RIPPLE

Required IRMS ≈

()

where IRIPPLE is as defined in the Inductor Value Calcula-

tion section.

Note that capacitor manufacturer’s ripple current ratings

are often based on 2000 hours of life. This makes it

advisable to further derate the capacitor, or to choose a

capacitor rated at a higher temperature than required.

Several capacitors may be paralleled to meet the size or

height requirements in the design. Due to the high operat-

ing frequency of the LTC1872, ceramic capacitors can also

be used for CIN. Always consult the manufacturer if there

is any question.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple (

∆VOUT) is approximated by:

∆VI

ESR

OUT

RIPPLE

OUT

≈•









•

























LTC1872 Datasheet(PDF) 7 Page - Linear Technology

LTC1872 Datasheet(HTML) 7 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12

Datasheet Download

Link URL