LTM4609EVPBF datasheet(15/24 Pages) LINER

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LTM4609

4609fa

APPLICATIONS INFORMATION

For the input capacitor, only minimum capacitors are

needed to handle the maximum RMS current, since it

is a continuous input current at boost mode. A 100μF

capacitor is only needed if the input source impedance is

compromised by long inductive leads or traces.

Since the output capacitors at boost mode need to ﬁlter

the square wave current, more capacitors are expected

to achieve the same output ripples as the buck mode. If

assuming that the ESR dominates the output ripple, the

output ripple is as follows:

ΔVOUT(P-P) =ESR •IL(MAX)

If a total low ESR about 5mΩ is chosen for output capaci-

tors, the maximum output ripple of 70mV occurs at the input

voltage of 5V with the peak inductor current at 14A.

An RC snubber is recommended on SW1 to obtain low

switching noise, as shown in Figure 17.

Wide Input Mode Operation

If a wide input range is required from 5V to 36V, the module

will work in different operation modes. If input voltage

VIN = 5V to 36V, VOUT = 12V and ƒ = 400kHz, the design

needs to consider the worst case in buck or boost mode

design. Therefore, the maximum output power is limited

to 60W. The sensing resistor is chosen at 8mΩ, the input

capacitor is the same as the buck mode design and the

output capacitor uses the boost mode design. Since the

maximum output ripple normally occurs at boost mode

in the wide input mode design, more inductor ripple cur-

rent, up to 150% of the inductor current, is allowed at

buck mode to meet the ripple design requirement. Thus,

a 3.3μH inductor is chosen at the wide input mode. The

maximum output ripple voltage is still 70mV if the total

ESR is about 5mΩ.

Additionally, the current limit may become very high when

the module runs at buck mode due to the low sensing

resistor used in the wide input mode operation.

Safety Considerations

The LTM4609 modules do not provide isolation from VIN to

VOUT. There is no internal fuse. If required, a slow blow fuse

with a rating twice the maximum input current needs to be

provided to protect each unit from catastrophic failure.

Figure 4. Current Ripple Ratio at Different Inputs for Boost Mode

INPUT VOLTAGE VIN (V)

0.8

0.6

0.4

0.2

4609 F04

911

1.5μH

2.5μH

3.3μH

4.7μH

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