LTM4607 datasheet(17/28 Pages) LINER

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LTC3780

3780fe

APPLICATIONS INFORMATION

where

∆IL is peak-to-peak inductor ripple current. In buck

mode, the maximum average load current is:

IOUT(MAX,BUCK) =

130mV

RSENSE

+ Δ

Figure 8 shows how the load current (IMAXLOAD • RSENSE)

varies with input and output voltage

The maximum current sensing RSENSE value for the boost

mode is:

RSENSE(MAX) =

2 160mV VIN(MIN)

2 IOUT(MAX,BOOST) VOUT +ΔIL,BOOST VIN(MIN)

The maximum current sensing RSENSE value for the buck

mode is:

RSENSE(MAX) =

2 130mV

2 IOUT(MAX,BUCK) – ΔIL,BUCK

The ﬁnal RSENSE value should be lower than the calculated

RSENSE(MAX) in both the boost and buck modes. A 20% to

30% margin is usually recommended.

CIN and COUT Selection

In boost mode, input current is continuous. In buck mode,

input current is discontinuous. In buck mode, the selection

of input capacitor CIN is driven by the need to ﬁlter the

input square wave current. Use a low ESR capacitor sized

to handle the maximum RMS current. For buck operation,

the input RMS current is given by:

IRMS ≈ IOUT(MAX) •

VOUT

VIN

•

VIN

VOUT

–1

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT(MAX)/2. This simple worst-case condition

is commonly used for design because even signiﬁcant

deviations do not offer much relief. Note that ripple cur-

rent ratings from capacitor manufacturers are often based

on only 2000 hours of life which makes it advisable to

derate the capacitor.

In boost mode, the discontinuous current shifts from the

input to the output, so COUT must be capable of reducing

the output voltage ripple. The effects of ESR (equivalent

series resistance) and the bulk capacitance must be

considered when choosing the right capacitor for a given

output ripple voltage. The steady ripple due to charging

and discharging the bulk capacitance is given by:

Ripple (Boost,Cap)

IOUT(MAX) •VOUT –VIN(MIN)

(

)

COUT •VOUT •f

Ripple (Buck,Cap)

IOUT(MAX) •VIN(MAX) –VOUT

(

)

COUT •VIN(MAX) •f

where COUT is the output ﬁlter capacitor.

The steady ripple due to the voltage drop across the ESR

is given by:

∆VBOOST,ESR = IL(MAX,BOOST) • ESR

∆VBUCK,ESR = IL(MAX,BUCK) • ESR

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Ceramic capacitors have excellent low ESR

characteristics but can have a high voltage coefﬁcient.

Capacitors are now available with low ESR and high ripple

current ratings, such as OS-CON and POSCAP.

VIN/VOUT (V)

0.1

100

110

120

130

140

160

110

3780 F08

150

Figure 8. Load Current vs VIN/VOUT

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