LTM4625(RevC) Ver la hoja de datos (PDF) - Analog Devices

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LTM4625
(Rev.:RevC)

20VIN, 5A Step-Down DC/DC µModule Regulator

Analog Devices 

LTM4625

APPLICATIONS INFORMATION

Pre-Biased Output Start-Up

There may be situations that require the power supply to

start up with some charge on the output capacitors. The

LTM4625 can safely power up into a pre-biased output

without discharging it.

The LTM4625 accomplishes this by forcing discontinuous

mode (DCM) operation until the TRACK/SS pin voltage

reaches 0.6V reference voltage. This will prevent the BG

from turning on during the pre-biased output start-up

which would discharge the output.

Do not pre-bias LTM4625 with an output voltage higher

than INTVCC (3.3V) or a voltage higher than the output

voltage set by feedback resistor (RFB).

Overtemperature Protection

The internal overtemperature protection monitors the junc-

tion temperature of the module. If the junction temperature

reaches approximately 160°C, both power switches will be

turned off until the temperature drops about 15°C cooler.

Low Input Application

The LTM4625 module has a separate SVIN pin which

makes it suitable for low input voltage applications down

to 2.375V. The SVIN pin is the single input of the whole

control circuitry while the VIN pin is the power input which

directly connects to the drain of the top MOSFET. In most

applications where VIN is greater than 4V, connect SVIN

directly to VIN with a short trace. An optional filter, con-

sisting of a resistor (1Ω to 10Ω) between SVIN and VIN

along with a 0.1µF bypass capacitor between SVIN and

ground, can be placed for additional noise immunity. This

filter is not necessary in most cases if good PCB layout

practices are followed (see Figure 19). In a low input

voltage (2.375V to 4V) application, or to reduce power

dissipation by the internal bias LDO, connect SVIN to an

external voltage higher than 4V with a 1µF local bypass

capacitor. Figure 21 shows an example of a low input

voltage application. Please note the SVIN voltage cannot

go below the VOUT voltage.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD 51-12 and are intended for use with

finite element analysis (FEA) software modeling tools that

leverage the outcome of thermal modeling, simulation,

and correlation to hardware evaluation performed on a

µModule package mounted to a hardware test board.

The motivation for providing these thermal coefficients is

found in JESD 51-12 (Guidelines for Reporting and Using

Electronic Package Thermal Information).

Many designers may opt to use laboratory equipment

and a test vehicle such as the demo board to anticipate

the µModule regulator’s thermal performance in their ap-

plication at various electrical and environmental operating

conditions to compliment any FEA activities. Without FEA

software, the thermal resistances reported in the Pin Con-

figuration section are, in and of themselves, not relevant to

providing guidance of thermal performance; instead, the

derating curves provided in this data sheet can be used

in a manner that yields insight and guidance pertaining to

one’s application usage, and can be adapted to correlate

thermal performance to one’s own application.

The Pin Configuration section gives four thermal coeffi-

cients explicitly defined in JESD 51-12; these coefficients

are quoted or paraphrased below:

1. θJA, the thermal resistance from junction to ambient, is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. This environment is sometimes referred to as

“still air” although natural convection causes the air to

move. This value is determined with the part mounted

to a 95mm × 76mm PCB with four layers.

2. θJCbottom, the thermal resistance from junction to the

bottom of the product case, is determined with all of

the component power dissipation flowing through the

bottom of the package. In the typical µModule regulator,

the bulk of the heat flows out the bottom of the pack-

age, but there is always heat flow out into the ambient

environment. As a result, this thermal resistance value

Rev C

14

For more information www.analog.com

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