LTM4625
14
4625fa
For more information www.linear.com/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 INTV
CC
(3.3V) or a voltage higher than the output
voltage set by feedback resistor (R
FB
).
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 SV
IN
pin which
makes it suitable for low input voltage applications down
to 2.375V. The SV
IN
pin is the single input of the whole
control circuitry while the V
IN
pin is the power input which
directly connects to the drain of the top MOSFET. In most
applications where V
IN
is greater than 4V, connect SV
IN
directly to V
IN
with a short trace. An optional filter, con-
sisting of a resistor (1Ω to 10Ω) between SV
IN
and V
IN
along with a 0.1µF bypass capacitor between SV
IN
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 SV
IN
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 SV
IN
voltage cannot
go below the V
OUT
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. θ
JA
, 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. θ
JCbottom
, 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
