LTM4605 Просмотр технического описания (PDF) - Linear Technology

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LTM4605

High Efficiency Buck-Boost DC/DC uModule

Linear Technology 

LTM4605

APPLICATIONS INFORMATION

For a buck converter, the switching duty-cycle can be

estimated as:

D = VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS)

=

IOUT(MAX)

η

•

D • (1− D)

In the above equation, η is the estimated efficiency of the

power module. CIN can be a switcher-rated electrolytic

aluminum capacitor, OS-CON capacitor or high volume

ceramic capacitors. Note the capacitor ripple current rat-

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) •

COUT

VOUT − VIN(MIN)

• VOUT • f

( ) VRIPPLE,BUCK

=

VOUT

8 •L •

• VIN(MAX) − VOUT

COUT • VIN(MAX) • f2

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 LTM4605 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 a low ESR tanta-

lum capacitor, a low ESR polymer capacitor or a 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

≥

VIN •

VOUT(MAX)

VOUT(MAX) −

• f •IOUT(MAX)

VIN

• Ripple%

( ) LBUCK

≥

VOUT •

VIN(MAX) • f

VIN(MAX) −

• IOUT(MAX)

VOUT

• Ripple%

where:

f 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.

4605fa

11

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