LTM4643 datasheet(15/32 Pages) LINER

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LTM4643

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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

LTM4643 can safely power up into a pre-biased output

without discharging it.

The LTM4643 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 LTM4643 with an output voltage higher

than INTVCC (3.3V).

Overtemperature Protection

Theinternalovertemperatureprotectionmonitorsthejunc-

tiontemperatureofthemodule.Ifthejunctiontemperature

reachesapproximately160°C,bothpowerswitcheswillbe

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

Low Input Application

The LTM4643 module has a separate SVIN pin for each

regulator channel which makes it compatible with opera-

tion from an input voltage as low as 2.375V. The SVIN pin

is the signal input of the regulator control circuitry while

the VIN pin is the power input which directly connected

to the drain of the top MOSFET. In most application with

input voltage ranges from 4V to 20V, connect the SVIN

pin directly to the VIN pin of each regulator channel. An

optionalfilter,consistingofaresistor(1Ωto10Ω)between

SVIN and VIN 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 28).

In a low input voltage (2.375V to 4V) application, or to

reducepowerdissipationbytheinternalbiasLDO,connect

SVIN to an external voltage higher than 4V with a 0.1µF

local bypass capacitor. Figure 30 shows an example of a

low input voltage application. Please note, SVIN voltage

cannot go below VOUT voltage.

Temperature Monitoring

A diode connected PNP transistor is used for the TEMP

monitor function by monitoring its voltage over tempera-

ture. The temperature dependence of this diode voltage

can be understood in the equation:

VD = nVT ln

















where VT is the thermal voltage (kT/q), and n, the ideality

factor, is 1 for the diode connected PNP transistor be-

ing used in the LTM4643. IS is expressed by the typical

empirical equation:

IS =I0 exp

–VG0

















where I0 is a process and geometry dependent current, (I0

is typically around 20k orders of magnitude larger than IS

at room temperature) and VG0 is the band gap voltage of

1.2V extrapolated to absolute zero or –273°C.

If we take the IS equation and substitute into the VD equa-

tion, then we get:

VD = VG0 –















 ln















 , VT =

The expression shows that the diode voltage decreases

(linearly if I0 were constant) with increasing temperature

and constant diode current. Figure 6 shows a plot of VD

vs Temperature over the operating temperature range of

the LTM4643.

If we take this equation and differentiate it with respect to

temperature T, then:

dVD

= –

VG0 – VD

This dVD/dT term is the temperature coefficient equal to

about –2mV/K or –2mV/°C. The equation is simplified for

the first order derivation.

Solving for T, T = –(VG0 – VD)/(dVD/dT) provides the

temperature.

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