Page 23 - EngineerIT May 2022

ELECTRONICS

The level shifter circuit is large, creating a challenge for the Challenge 2: Inefficiency
designers. In addition, legacy negative DC-DC converter solutions The legacy negative output DC-DC solution is inefficient. The
are complex and inefficient, imposing yet another challenge. extra heat generated due to inefficiency creates another
challenge for designers, who now have the extra burden of
Challenge 1: level shifter removing that heat out of the system. Figure 5 is a simplified
Figure 3 illustrates a typical level shifter circuit. Its purpose circuit schematic of an example of such a system.
is to shift the ground reference of a signal to match that of This topology faces two inefficiency issues. First, it
the system micro-controller. It is used here to translate the employs non-synchronous switching where the output
ON command from the system micro-controller to turn on/ rectifying diode, D1, dissipates more power compared to a
off the DC-DC converter. This level shifter consists of nine synchronous solution. Second, it has an extra power inductor,
components. Its operation is straightforward: when ON is L1, and an extra capacitor, C1, which also dissipates more
driven high by the system controller, Q1 turns on, which in power. Figure 6 shows the efficiency curve for this converter,
turn biases Q2 on and drives EN high to enable the DC-DC measured at 12V input and -15V output. Its peak efficiency is
converter. When ON is driven low, both Q1 and Q2 are off, EN only 83% while dissipating approximately 460mW at 150mA
is driven low to disable the converter. output current.
Figure 4 describes a common level-shifter circuit variation.
It is used here to translate the PGOOD signal from the DC-DC Smaller, more efficient negative output dc-dc solution
converter so that the system micro-converter can read. When The MAX17577 and MAX17578 were developed to meet
PGOOD is driven high (open drained) by the DC-DC converter, growing requirements for smaller-solution-size and lower-heat
Q3 turns on, which in turn biases on Q4 and drives RESET high, generating devices in factory automation, building automation,
taking the system micro-controller out of reset. and communications systems. The devices integrate level
These two level shifters require 18 external components, shifting circuitry to reduce component cost and count and
presenting a challenge to the designers trying to fit the employ synchronous rectification for best efficiency. These
solution to the ever-shrinking equipment and board space.

Figure 5: Example of a simplified schematic of a non-synchronous,
dual-inductor inverting output DC-DC converter.

Figure 3: The typical level shifter circuit translates the ON command
from the system controller.

Figure 4: A level shifter translates the PGOOD signal from the DC-DC Figure 6: The power loss curve shows the efficiency of a non-
converter. synchronous, dual-inductor inverting output DC-DC converter.

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