LMZ14203_2 Datasheet, PDF (1/6 Pages) National Semiconductor (TI) – Inverting Application for the SIMPLE SWITCHER® Power Module

Inverting Application for the
LMZ14203 SIMPLE
SWITCHER® Power Module
National Semiconductor
Application Note 2027
Alan Martin
March 24, 2010
Introduction
This application note illustrates how to apply the LMZ14203
integrated buck module into the buck-boost configuration
such that a positive input voltage can be used to create a
regulated negative output voltage. It is also shown how the
conventional non-inverting demonstration and evaluation
boards for the device can be connected into the inverting
configuration without the need to acquire a new PCB to eval-
uate the application. Refer to AN-2024, AN-2031, and
AN-2032 for detailed descriptions on the PCB assemblies in
the conventional buck topology.
Figure 1 illustrates the method of reassigning the terminals of
the evaluation board (or demo board) for the inverting appli-
cation. Careful labeling of leads is suggested to avoid confu-
sion since the terminals formerly at ground potential are now
connected to –V_OUT, and the connection formerly assigned to
+Vout is now connected to ground. The 3 connections to the
power path are straightforward. But there may be additional
circuitry required for the enable signal to function as desired
in the target system as the precision enable reference voltage
is referred to ground while disabled and to –V_OUT once en-
abled. This adds a large and possibly undesirable amount of
hysteresis to the simplest form of enable. To alleviate this sit-
uation several level shift methods are discussed in the fol-
lowing text.
It should also be noted that the maximum output current ob-
tained from the module is decreased from that obtained in the
conventional buck configuration. (See graphs) Further, since
the ground terminal of the module is connected to –V_OUT the
maximum positive input voltage into the inverting application
is decreased by the amplitude of the output voltage.Thus for
a –5V output application the maximum input voltage will be
37V.
This configuration can be applied to the whole family of
LMZ1420x and LMZ1200x modules so long as the input volt-
age and output current limits are observed. Be aware that
efficiency is lower in the inverting configuration resulting in
higher dissipation for a given output power and that thermal
derating may need to be observed when operating at maxi-
mum output current.
FIGURE 1. Evaluation board connections for inverting application
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