LTM4605 datasheet(14/24 Pages) LINER

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LTM4605

4605fa

APPLICATIONS INFORMATION

To set the output voltage at 12V, the resistor RFB from VFB

pin to ground should be chosen as:

OUT

−

≈

0 8

100

0 8

715

.•

To choose a proper inductor, we need to know the current

ripples at different input voltages. The inductor should

be chosen by considering the worst case in the practical

operating region. If the maximum output power P is 150W

at buck mode, we can get the current ripple ratio of the

current ripple

ΔIL to the maximum inductor current IL as

follows:

ΔI

VL f P

OUT

(–

) •

•• •

Figure 3 shows the current ripple ratio at different input

voltages based on the inductor values: 1.5μH, 2.5μH,

3.3μH and 4.7μH. If we need 30% ripple current ratio at

all inputs, the 3.3μH inductor can be selected.

Consider the safety margin about 30%, we can choose

the sensing resistor as 8m

Ω.

For the input capacitor, use a low ESR sized capacitor to

handle the maximum RMS current. Input capacitors are

required to be placed adjacent to the module. In Figure 14,

the 10μF ceramic input capacitors are selected for their

ability to handle the large RMS current into the converter.

The 100μF bulk capacitor is only needed if the input source

impedance is compromised by long inductive leads or

traces.

For the output capacitor, the output voltage ripple and

transient requirements require low ESR capacitors. If

assuming that the ESR dominates the output ripple, the

output ripple is as follows:

ΔΔ

VESR

OUT P P

()

•

If a total low ESR of about 5m

Ω is chosen for output ca-

pacitors, the maximum output ripple of 17.5mV occurs at

the input voltage of 20V with the current ripple at 3.5A.

Boost Mode Operation

For boost mode operation, use input voltage VIN = 5V to

12V, VOUT = 12V and f = 400kHz.

Set the PLLFLTR pin and RFB as in buck mode.

If the maximum output power P is 60W at boost mode

and the module efﬁciency

η is about 95%, we can get

the current ripple ratio of the current ripple

ΔIL to the

maximum inductor current IL as follows:

ΔI

VL f P

OUT

−

() •

•• •

2η

Figure 4. shows the current ripple ratio at different input

voltages based on the inductor values: 1.5μH, 2.5μH,

3.3μH and 4.7μH. If we need 30% ripple current ratio at

all inputs, the 3.3μH inductor can be selected.

Figure 3. Current Ripple Ratio at Different Inputs for Buck Mode

At buck mode, sensing resistor selection is based on

the maximum output current and the allowed maximum

sensing threshold 130mV.

SENSE

OUT

−

2 130

•

•( /

)

INPUT VOLTAGE VIN (V)

0.8

0.6

1.5μH

2.5μH

3.3μH

4.7μH

0.4

0.2

4605 F03

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