AN2833
Application note
Dual step-down controller with auxiliary voltages
for industrial system power
Introduction
The PM6681A is a dual step-down controller with adjustable output voltages that can be
used in industrial power systems and this demonstration kit represents a typical application
circuit. The PM6681A demonstration kit allows testing all the device's functions and provides
two switching sections, with (typically) 3.3 V (OUT1) and 1.8 V (OUT2) outputs from 5.5 V to
36 V input voltage. The typical operating switching frequency of the two available sections is
200 kHz/300 kHz, respectively. Each switching section delivers more than 2.5 A output
current. An internal linear regulator provides a fixed 5 V output voltage. Another internal
linear regulator provides an adjustable output voltage (default 2.5 V). Both linear regulators
can deliver up to 100 mA peak current.
Figure 1.
PM6681A demonstration kit
AM01436v1
February 2009
Rev 2
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Contents
AN2833
Contents
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2
3
4
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6
7
8
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10
11
Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Demonstration kit schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Demonstration kit layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Quick start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Jumper settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Feedback output connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Test setup and performance summary . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Power-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Soft-start and shutdown waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3 V and 1.8 V output efficiency vs. load current . . . . . . . . . . . . . . . . . . 22
Power consumption analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Switching frequency vs. load current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Linear regulator output voltages vs. output current . . . . . . . . . . . . . . . . . 26
Load transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
12
13
Representative waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
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AN2833
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
PM6681A demonstration kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Demonstration kit schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
PM6681A demonstration board layout - top layer (PGND plane and component side) . . . 10
PM6681A demonstration board layout - inner layer 1 (SGND layer and VIN plane) . . . . . 10
PM6681A demonstration board layout - inner layer 2 (SGND layer and signals). . . . . . . . 11
PM6681A demonstration board layout - bottom layer (PM6681A and component side) . . 11
Setup connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
REF, LDO5 and LDO power-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Section 1 soft-start waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Section 2 soft-start waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Section 1 shutdown waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Section 2 shutdown waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 V SMPS efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.8 V SMPS efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Input current vs. input voltage in PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Input current vs. input voltage in skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Input current vs. input voltage in non-audible skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Device current consumption vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Device current consumption vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3 V output switching frequency vs. load current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.8 V output switching frequency vs. load current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
LDO5 output vs. load current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
ADJ_LDO load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
SMPS 1.5 V load transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
SMPS 1.05 V load transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
SMPS pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
SMPS non-audible skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
SMPS PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
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Main features
AN2833
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Main features
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5.5 V to 36 V input voltage range
Adjustable output voltages
0.9 - 3.3 V adjustable LDO delivers 100 mA peak current
5 V LDO delivers 100 mA peak current
1.237 V ±1% reference voltage available
Lossless current sensing using low-side MOSFET R
DS(ON)
Negative current limit
Soft-start internally fixed at 2 ms
Soft output discharge
Latched UVP
Non-latched OVP
Selectable pulse-skipping at light loads
Selectable minimum frequency (33 kHz) in pulse-skip mode
4 mW maximum quiescent power
Independent power-good signals
Output voltage ripple compensation
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AN2833
Applications
2
Applications
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Embedded computer systems
FPGA system power
Industrial applications at 24 V
High-performance and high-density DC/DC modules
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