LTM4648EY#PBF Datasheet LTM4648IY#PBF, LTM4648EY#PBF | P16-P18

LTM4648

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Run Enable

The RUN pin has an enable threshold of 1.40V maximum,

typically 1.25V with 150mV of hysteresis. It controls the

turn-on of the µModule. The RUN pin can be pulled up to

for 5V operation, or a 5V Zener diode can be placed

on the pin and a 10k to 100k resistor can be placed up to

higher than 5V input for enabling the µModule. The RUN

pin can also be used for output voltage sequencing.

In parallel operation the RUN pins can be tied together and

controlled from a single control. See the Typical Applica

tion circuits in Figures 20 and 21. The RUN pin can also

be left floating. The RUN pin has a 1µA pull-up current

source that increases to 4.5µA during ramp-up.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is provided

in the LTM4648 to sense low output voltages accurately

at the remote load points. This is especially true for high

current loads. It is very important that the DIFFP and

DIFFN are connected properly at the output, and DIFFOUT

is connected to V

OUT_LCL

. Review the parallel schematics

in Figures 20 and 21.

SW Pins

The SW pin is generally for testing purposes by monitor

ing the pin. The SW pin can also be used to dampen out

switch node ringing caused by LC parasitic in the switched

current path.

Usually a series R-C combination is used

called a snubber circuit. The resistor will dampen the

resonance and the capacitor is chosen to only affect the

high frequency ringing across the resistor.

If the stray inductance or capacitance can be measured or

approximated then a somewhat analytical technique can

be used to select the snubber values. The inductance is

usually easier to predict. It combines the power path board

inductance in combination with the MOSFET interconnect

bond wire inductance.

First the SW pin can be monitored with a wide bandwidth

scope with a high frequency scope probe. The ring fre

quency can be measured for its value. The impedance Z

can be calculated

= 2π • f • L

where f is the resonant frequency of the ring, and L is the

total parasitic inductance in the switch path. If a resistor

is selected that is equal to Z, then the ringing should be

dampened. The snubber capacitor value is chosen so that

its impedance is equal to the resistor at the ring frequency.

Calculated by:

2π • f •C

These values are a good place to start with. Modification

to these components should be made to attenuate the

ringing with the least amount the power loss.

Temperature Monitoring

Measuring the absolute temperature of a diode is pos

sible due to the relationship between current, voltage

and temperature described by the classic diode equation

= I

• e

η • V

⎛

⎝

⎜

⎞

⎠

⎟

= η • V

• ln

where I

is the diode current, V

is the diode voltage, η is

the ideality factor (typically close to 1.0) and I

(satura-

tion current) is a process dependent parameter. V

can

be broken out to:

k • T

where T is the diode junction temperature in Kelvin, q is

the electron charge and k is Boltzmann’s constant. V

approximately 26mV at room temperature (298K) and

scales linearly with Kelvin temperature. It is this linear

temperature relationship that makes diodes suitable

temperature sensors. The I

term in the equation above

is the extrapolated current through a diode junction when

the diode has zero volts across the terminals. The I

term

varies from process to process, varies with temperature,

and by definition must always be less than I

. Combining

all of the constants into one term:

• k

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where K

= 8.62

−5

, and knowing ln(I

) is always posi-

tive because I

is always greater than I

, leaves us with

the equation that:

= T(KELVIN) • K

• ln

where V

appears to increase with temperature. It is com-

mon knowledge that a silicon diode biased with a current

source has an approximately

–2mV/°C temperature rela-

tionship (

Figure 7

), which is at odds with the equation. In

fact, the I

term increases with temperature, reducing the

ln(I

) absolute value yielding an approximately –2mV/°C

composite diode voltage slope.

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 JESD51-9 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—also defined

by JESD51-9 (“Test Boards for Area Array Surface Mount

Package Thermal Measurements”). The motivation for

providing these thermal coefficients in found in JESD51-12

(“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 the 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 typically gives four thermal

coefficients explicitly defined in JESD51-12; these coef

ficients are quoted or paraphrased below:

: 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 JESD51-9 defined test board, which does not reflect

an actual application or viable operating condition.

4648 F07

TEMPERATURE (°C)

–173 –73 27 127

DIODE VOLTAGE (V)

0.4

0.6

0.8

1.0

∆V

= 100µA

Figure 7. Diode Voltage, V

, vs Temperature T (°C)

for Different Bias Currents

An external diode connected PNP transistor can be pulled

up to V

with a resistor to set the current to 100µA for

using this diode connected transistor as a general tem-

perature monitor by monitoring the diode voltage drop

with temperature

or a specific temperature monitor can

be used that injects two currents that are at a 10:1 ratio

for very accurate temperature monitoring. See Figure 22

for an example.

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2. θ

JCbottom

: the thermal resistance from junction to ambi-

ent, is the natural convection junction-to-ambient air

thermal resistance

measured in a one cubic foot sealed

enclosure. 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 JESD51-9 defined test board, which does not reflect

an actual application or viable operating condition.

3. θ

JCtop

: the thermal resistance from junction to top of

the product case, is determined with nearly all of the

component power dissipation flowing through the top

of the package. As the electrical connections of the

typical µModule are on the bottom of the package, it

is rare for an application to operate such that most of

the heat flows from the junction to the top of the part.

As in the case of θ

JCbottom

, this value may be useful

for comparing packages but the test conditions don’t

generally match the user’s application.

4. θ

: the thermal resistance from junction to the printed

circuit board, is the junction-to-board thermal resistance

where almost all of the heat flows through the bottom of

the µModule and into the board, and is really the sum of

the θ

JCbottom

and the thermal resistance of the bottom

of the part through the solder joints and through a por-

tion of the board. The board temperature is measured a

specified distance from the package

using a two sided,

two layer board. This board is described in JESD51-9.

A graphical representation of the aforementioned ther

mal resistances is given in Figure 8; blue resistances are

contained within the μ

Module regulator, whereas green

resistances are external to the µModule.

As a practical matter, it should be clear to the reader that

no individual or sub-group of the four thermal resistance

parameters defined by JESD51-12 or provided in the Pin

Configuration section replicates or conveys normal op

erating conditions of a μModule. For example, in normal

board-mounted applications,

never does 100% of the

device’s total power loss (heat) thermally conduct exclu

sively through the top or exclusively through bottom of the

µModule—as the standard defines for θ

JCtop

and θ

JCbottom

respectively. In practice, power loss is thermally dissipated

in both directions away from the package—granted, in the

absence of a heat sink and airflow, a majority of the heat

flow is into the board.

Within a SIP (system-in-package) module, be aware there

are multiple power devices and components dissipating

power, with a consequence that the thermal resistances

relative to different junctions of components or die are not

exactly linear with respect to total package power loss. To

reconcile this complication without sacrificing modeling

simplicity—but also, not ignoring practical realities—an

approach has been taken using FEA software modeling

along with laboratory testing in a controlled-environment

chamber to reasonably define and correlate the thermal

4648 F08

µMODULE DEVICE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 8. Graphical Representation of JESD51-12 Thermal Coefficients

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P28

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Manufacturer:

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Description:

Switching Voltage Regulators Low VIN, 10A Step-Down DC/DC Module Regulator

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