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Keywords:
Sequencing, Power Supplies, ADC, LDO, Bipolar
APPLICATION NOTE 6369
BIPOLAR WORK AROUND FOR THE MAXREFDES74#
By:
John Greene, Associate Member Technical Staff, Applications
Abstract: The MAXREFDES74# allows for both unipolar and bipolar modes of operation from a single +15V to +20V DC source. The unipolar
mode operates as expected; however, the bipolar mode contains a potential power sequencing issue upon startup if J6 is in position 1-2. This app
note discusses the cause of the problem and several work arounds to get the board functioning upon startup.
Introduction
The
MAXREFDES74# is a high-speed, 18-bit precision data-acquisition system featuring the MAX11156 ADC and MAX5318 DAC. The reference
design allows for both unipolar and bipolar modes of operation from a single +15V to +20V DC source. The unipolar mode operates as expected;
however, the bipolar mode contains a potential power sequencing issue upon startup if J6 is in position 1-2. This only applies to boards using a
100kΩ resistor for R31.
The reference design uses a single +15V to +20V power source, an on-board transformer to create power isolation, and relies on several
regulators to generate the power rails necessary for board operation. When operating in bipolar mode, J6 connects the
MAX664 regulator output
to AVSS of the MAX5318.
Figure 1 provides a block diagram of the connections between the MAX5318 and MAX664 along with the power
supplies to each chip.
Figure 1. MAXREFDES74# schematic for DAC AVSS generation.
On the MAXREFDES74#, the +5V supply to the MAX5318 always powers on prior to the -1.3V supply. Before the MAX664 can provide a
regulated output, a ~0.65V appears on the AVSS pin of the MAX5318. Based on the schematic connections, this positive voltage appears on the
V
, V , V , and SENSE pins on the MAX664, raising these pins above their absolute maximum value of +0.3V as specified in the
MAX664 data sheet.
Figure 2 shows an oscilloscope capture of the power-on sequencing when conditioned for bipolar operation. The positive rail
powers on first and causes a positive voltage to appear on AVSS of the MAX5318.
OUT1 OUT2 SET
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