Interfacing the ispPAC-POWR1208
with Modular DC-to-DC Converters
January 2003
Application Note AN6046
Introduction
The ispPAC®-POWR1208 is a single-chip, fully integrated solution to supervisory and control problems encoun-
tered when implementing on-board power conversion and distribution systems. The ispPAC-POWR1208 provides
several types of programmable on-chip resources which can be used to meet the requirements of these applica-
tions. In addition to providing four high-voltage FET driver outputs, four general purpose open-drain outputs are
also provided which can be used to control modular DC-to-DC converters. This application note describes several
issues associated with controlling modular DC-to-DC converters, and presents several example circuits for interfac-
ing the ispPAC-POWR1208’s digital outputs to these devices.
Characteristics of Modular DC-to-DC Converters
The use of modular DC-to-DC converters is becoming increasingly common as power conversion systems are
being implemented in a distributed manner, with final conversions to end-use voltages being performed on circuit
cards. Many systems and even individual integrated circuits require power supplies to be sequenced at startup and
shutdown, or otherwise switched on and off when changing operating modes. For this reason, an externally con-
trollable ENABLE signal is often provided to switch a DC-to-DC converter on and off (Figure 1). Both positive logic
(a HIGH signal turns converter on) and negative logic (a LOW signal turns the converter on) are common types of
ENABLE inputs.
Figure 1. DC-to-DC Converter with ENABLE Input
VIN+
VOUT+
DC-to-DC
Converter
ENABLE
VIN-
VOUT-
While the ENABLE inputs of some converter modules accept standard (5V or 3.3V) logic levels, many other electri-
cal interfaces are common as well. The purpose of this applications note is to describe how to interface the logic
outputs of Lattice Semiconductor’s ispPAC-POWR1208 to some of these non-standard interfaces.
Most converters with non-standard-logic interfaces are turned on by shorting their ENABLE pins to either their pos-
itive or negative input power supplies, or leaving them floating (open). Table 1 summarizes a few of the possible
operating modes.
Table 1. Common Converter ENABLE Modes
ENABLE Input Condition
Input Type
Open
ON
OFF
OFF
ON
Short to Vin- Short to Vin+
Positive logic with pull-up
Negative logic with pull-up
Positive logic with pull-down
Negative logic with pull-down
OFF
ON
ON1
OFF1
ON
OFF1
ON1
OFF
1. These conditions may not be tolerated by a given model of DC-to-DC converter
The first major distinction in operating modes is whether the converter turns on with a positive signal (positive logic)
or a negative one (negative logic). The second distinction is whether the input is internally pulled-up or pulled-down
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