LTM4609EVPBF datasheet(13/24 Pages) LINER

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LTM4609

4609fa

APPLICATIONS INFORMATION

pin voltage <1.6V). In this mode, the onboard 6V linear

regulator can provide power to keep-alive functions such

as a keyboard controller.

INTVCC and EXTVCC

An internal P-channel low dropout regulator produces 6V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the control chip and internal circuitry within the module.

The LTM4609 also provides the external supply voltage pin

EXTVCC. When the voltage applied to EXTVCC rises above

5.7V, the internal regulator is turned off and an internal

switch connects the EXTVCC pin to the INTVCC pin thereby

supplying internal power. The switch remains close as long

as the voltage applied to EXTVCC remains above 5.5V. This

allows the MOSFET driver and control power to be derived

from the output when (5.7V < VOUT < 7V) and from the

internal regulator when the output is out of regulation (start-

up, short-circuit). If more current is required through the

EXTVCC switch than is speciﬁed, an external Schottky diode

can be interposed between the EXTVCC and INTVCC pins.

Ensure that EXTVCC ≤ VIN.

The following list summarizes the three possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 6V regulator at the cost

of a small efﬁciency penalty.

2. EXTVCC connected directly to VOUT (5.7V < VOUT < 7V).

This is the normal connection for a 6V regulator and

provides the highest efﬁciency.

3. EXTVCC connected to an external supply. If an external

supply is available in the 5.5V to 7V range, it may be

used to power EXTVCC provided it is compatible with

the MOSFET gate drive requirements.

Thermal Considerations and Output Current Derating

In different applications, LTM4609 operates in a variety

of thermal environments. The maximum output current is

limited by the environmental thermal condition. Sufﬁcient

cooling should be provided to ensure reliable operation.

When the cooling is limited, proper output current de-

rating is necessary, considering ambient temperature,

airﬂow, input/output condition, and the need for increased

reliability.

The power loss curves in Figures 5 and 6 can be used

in coordination with the load current derating curves in

Figures 7 to 14 for calculating an approximate

θJA for

the module. Column designation delineates between no

heat sink, and a BGA heat sink. Each of the load current

derating curves will lower the maximum load current as

a function of the increased ambient temperature to keep

the maximum junction temperature of the power module

at 115°C allowing a safe margin for the maximum operat-

ing temperature below 125°C. Each of the derating curves

and the power loss curve that corresponds to the correct

output voltage can be used to solve for the approximate

θJAofthecondition.Acompleteexplanationofthethermal

characteristics is provided in the thermal application note

for the LTM4609.

DESIGN EXAMPLES

Buck Mode Operation

As a design example, use input voltage VIN = 12V to 36V,

VOUT = 12V and ƒ = 400kHz.

Set the PLLFLTR pin at 2.4V or more for 400kHz frequency

and connect FCB to ground for continuous current mode

operation. If a divider is used to set the frequency as shown

in Figure 16, the bottom resistor R3 is recommended not

to exceed 1kΩ.

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

pin to ground should be chosen as:

RFB =

0.8V •100k

VOUT − 0.8V

≈ 7.15k

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

ripple 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 120W

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