LTM4609EVPBF datasheet(11/24 Pages) LINER

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LTM4609

4609fa

APPLICATIONS INFORMATION

ings are often based on temperature and hours of life. This

makes it advisable to properly derate the input capacitor,

or choose a capacitor rated at a higher temperature than

required. Always contact the capacitor manufacturer for

derating requirements.

Output Capacitors

In boost mode, the discontinuous current shifts from the

input to the output, so the output capacitor COUT must be

capable of reducing the output voltage ripple.

For boost and buck modes, the steady ripple due to charg-

ing and discharging the bulk capacitance is given by:

VRIPPLE,BOOST =

IOUT(MAX) •VOUT − VIN(MIN)

(

)

COUT •VOUT •ƒ

VRIPPLE,BUCK =

VOUT •VIN(MAX) − VOUT

(

)

8•L •COUT •VIN(MAX) •ƒ2

The steady ripple due to the voltage drop across the ESR

(effective series resistance) is given by:

VESR,BUCK =ΔIL(MAX) •ESR

VESR,BOOST =IL(MAX) •ESR

The LTM4609 is designed for low output voltage ripple.

The bulk output capacitors deﬁned as COUT are chosen

with low enough ESR to meet the output voltage ripple and

transient requirements. COUT can be the low ESR tantalum

capacitor, the low ESR polymer capacitor or the ceramic

capacitor. Multiple capacitors can be placed in parallel to

meet the ESR and RMS current handling requirements. The

typical capacitance is 300μF. Additional output ﬁltering may

be required by the system designer, if further reduction of

output ripple or dynamic transient spike is required. Table 3

shows a matrix of different output voltages and output

capacitors to minimize the voltage droop and overshoot

at a current transient.

Inductor Selection

The inductor is chieﬂy decided by the required ripple cur-

rent and the operating frequency. The inductor current

ripple

ΔIL is typically set to 20% to 40% of the maximum

inductor current. In the inductor design, the worst cases

in continuous mode are considered as follows:

LBOOST ≥

V2IN •VOUT(MAX) − VIN

(

)

V2OUT(MAX) •ƒ•IOUT(MAX) •Ripple%

LBUCK ≥

VOUT •VIN(MAX) − VOUT

(

)

VIN(MAX) •ƒ•IOUT(MAX) •Ripple%

where:

ƒ is operating frequency, Hz

Ripple% is allowable inductor current ripple, %

VOUT(MAX) is maximum output voltage, V

VIN(MAX) is maximum input voltage, V

VOUT is output voltage, V

IOUT(MAX) is maximum output load current, A

The inductor should have low DC resistance to reduce the

I2R losses, and must be able to handle the peak inductor

current without saturation. To minimize radiated noise,

use a toroid, pot core or shielded bobbin inductor. Please

refer to Table 3 for the recommended inductors for dif-

ferent cases.

RSENSE Selection and Maximum Output Current

RSENSE is chosen based on the required inductor current.

Since the maximum inductor valley current at buck mode

is much lower than the inductor peak current at boost

mode, different sensing resistors are suggested to use

in buck and boost modes.

The current comparator threshold sets the peak of the

inductor current in boost mode and the maximum inductor

valley current in buck mode. In boost mode, the allowed

maximum average load current is:

IOUT(MAX,BOOST) =

160mV

RSENSE

−

ΔIL

⎛

⎝⎜

⎞

⎠⎟

•

VIN

VOUT

where

ΔIL is peak-to-peak inductor ripple current.

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