AN1157: Power Supply and DC to DC Converter Control using Intersil Digitally Controlled Potentiometers (XDCPs)

Power Supply and DC to DC Converter Control using
Intersil Digitally Controlled Potentiometers (XDCPs)
®
Application Note
October 1, 2008
AN1157.1
Author: Applications Staff
Introduction
Implementing Designs with XDCPs
The output voltage of power supplies and DC to DC
converters is regulated by dividing down the output,
comparing it with a stable reference, amplifying the error,
and then using this error signal to control the input to output
power transfer. Many applications use an expensive and
unreliable mechanical potentiometer in the divider to
precisely set the output voltage, as shown in Figure 1.
An example of a part that meets all of these requirements is
the X9CMME from Intersil, shown in Figure 2. This
electrically erasable, programmable nonvolatile
potentiometer greatly simplifies manufacturing and test, and
with a wiper position retention in excess of 100 years, can
significantly increase field reliability.
For fixed output supplies, manually adjusting the mechanical
pot is a costly and time consuming operation that is prone to
error. Physical access to the pot must be provided, which
often requires less than optimal PCB layout and additional
access holes in the chassis of enclosed units. Though initially
set during production, the mechanical pot is subject to shock
and vibration, incurring wiper position changes. Humidity and
moisture can result in resistive changes and the prying hands
of service technicians or end users can also is affect accuracy.
For variable output supplies, mechanical pots prevent the
option of automated or remote adjustment.
To overcome these problems, a potentiometer would have to
be electronically programmable in order to eliminate costly
and potentially inaccurate manual settings, nonvolatile so as
to power-up at the proper setting, and re-programmable for
testing in the factory or for adjustments in the field.
U/D
INC
CS
99
7-BIT
UP/DOWN
CONVERTER
VH
98
97
96
7-BIT
NONVOLATILE
MEMORY
ONE OF
ONE-HUNDRED
DECODER
Transfer Resistor
Gates
Array
3
2
VCC
GND
STORE AND
RECALL
CONTROL
CIRCUITRY
1
0
VL
VW
FIGURE 2.
Vin
Power
Stage &
Filters
VO
R1
Ve/a
VR
R2
VO = VR 1 +
R1
R2 + RP
RP
FIGURE 1.
1
Available in 8-pin DIP or SOIC packages, the 100 wiper
position X9CMME is easily implemented into a circuit.
Terminal connections are made in the same way as a
mechanical pot. The three control lines of the part can be
brought out through a connector to automate programming
and to test equipment, as shown in Figure 3.
INC, U/D, and CS pins control the setting of the wiper. Pulling
CS LOW enables the part. Each HIGH to LOW transition on
the INC line increases or decreases (depending on the state
of U/D) the resistance of the pot. After reaching the desired
output voltage, the final wiper setting can be stored in
nonvolatile memory by bringing CS HIGH while INC is HIGH.
This ensures that the X9CMME powers-up at the last setting.
It may appear that this is the only operating sequence for
power supply applications, but there is at least one important
scenario where wiper position storage is not required.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005, 2008. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 1157
V OU T
VR
R1
OUT
Ve /a
R2
U1
I NC
U/ D
CS
1
2
7
VH
VW
VL
I NC
U/ D
CS
3
5
6
X 9 C MME
FIGURE 3.
Overvoltage Testing
Overvoltage (OV) testing has always presented a challenge
to the manufacturing engineer. If a mechanical
potentiometer is available, it must be adjusted manually to
test the OV protection circuitry and then be readjusted to
reset the output voltage. If there is no potentiometer in the
system, or even if there is one that is not easily accessible,
an external power supply must be connected to the unit
under test (UUT). The voltage on the external supply is then
raised while monitoring the UUT, to determine the point at
which it shuts down. This often requires observation of
waveforms internal to the UUT that are also difficult to
access, compounded by the fact that the UUT control
circuitry may obscure the response of the OV protection
circuitry. This cumbersome procedure can be eliminated
through the use of XDCPs.
If the output voltage in Figure 3 is 5V, with a 2.5V reference,
and R1 is 10kΩ and R2 is 4.99kΩ, a good choice of XDCP
for voltage regulation and overvoltage testing would be the
X9C103 (10kΩ version of the X9CMME). Having adjusted
the output voltage to meet spec. at some nominal line and
load, and after storing this setting in nonvolatile memory by
deselecting the part with INC HIGH, the testing of the power
supply can begin. When the OV portion of the test is
reached, the output voltage is increased by toggling the INC
pin of the XDCP while holding U/D LOW. Upon reaching the
OV trip point and the supply shutting-down, the final value of
the output voltage can be automatically recorded and
compared to a maximum allowable value. The automatic test
equipment (ATE) could then generate a pass/fail response.
Provided CS does not transition from HIGH to LOW with INC
HIGH, the OV trip point wiper position will not be stored and
the XDCP will return to the previously calibrated regulation
voltage upon power-up. This approach provides an accurate,
automated means of overvoltage testing.
The X9CMME provides 100 tap positions for terminal
voltages of ±5V and is available in 1kΩ, 10kΩ, 50kΩ, and
100kΩ versions. The X9312 gives an extended 0 to 15V
terminal voltage capability and the X9313 provides an
inexpensive 32 tap alternative. All single XDCPs share the
same pinout and are available in DIP and SOIC packages.
Quad XDCP
For multiple output power supplies or in applications where
resolution greater than 100 wiper positions is required,
Intersil offers the X9241 quad XDCP, shown in Figure 4.
Designed to interface directly with a microcontroller, the
X9241 can be directly programmed with a wiper position or
incremented/decremented tap-by-tap like the X9CMME.
The X9241contains four 64 wiper position pots in a single 20
pin package. Each pot has a wiper counter register (WCR)
that controls the wiper position and four nonvolatile registers
that store wiper settings. The part provides four pins for
device addressing, allowing as many as 16 devices to share
the same bus, using an instruction oriented protocol.
Registers and wiper positions can be read and written by the
ATE using a two-wire bidirectional serial interface.
2
AN1157.1
October 1, 2008
Application Note 1157
R0 R1
WIPER
COUNTER
REGISTER
(WCR)
R2 R3
SCL
SDA
A0
A1
A2
A3
VH0
R0 R1
VL0
VW0
R2 R3
WIPER RESISTOR
COUNTER
ARRAY
REGISTER
POT 2
(WCR)
VH2
VL2
VW2
INTERFACE
AND
CONTROL
CIRCUITRY
DATA/8
R0 R1
R2 R3
VH1
WIPER
COUNTER RESISTOR
ARRAY
REGISTER
POT 1
(WCR)
R0 R1
VL1
VW1
R2 R3
WIPER
RESISTOR
COUNTER
ARRAY
REGISTER
POT 3
(WCR)
VH3
VL3
VW3
FIGURE 4.
Internal support is available for cascading pots together in
series or parallel to increase resolution and provide larger or
smaller potentiometer values. These cascading
configurations can be implemented in any number of ways
using two or more pots. For example, using the X9241M
(2kΩ, 10kΩ, 10kΩ, 50kΩ combo version), two pots to can be
used to give a fine and course adjustment for setting the
regulated output voltage and testing the overvoltage
circuitry. This configuration is shown in Figure 5. Pot 3 can
be used to provide course adjustment with 800W (50kΩ/63)
steps and pot 0 to provide fine adjustment with 32W
(2kΩ/63) steps. The wiper position is changed using the
increment/decrement or write WCR commands. Wiper
settings are stored in nonvolatile memory by directly writing
to data registers or transferring data from WCRs to data
registers. Each pots’ WCR is loaded with contents of data
register 0 upon power-up.
The X9241 is a ±5V terminal device available in 2kΩ, 10kΩ,
50kΩ, and combination versions, with either DIP or SOIC
package options. This digitally controlled IC greatly simplifies
and automates manufacturing and test. Like the X9CMME,
nonvolatile storage retention of the X9241 is at least 100
years, providing a significant increase in field reliability.
Conclusion
Nonvolatile XDCPs provide significant advantages over
mechanical potentiometers for power supplies in both
manufacturing and in the field. Automated assembly and
test provide labor savings, while increasing repeatability
and eliminating human error. Immune to shock and
vibration, and with superior resistance to environmental
stress, XDCPs increase long term reliability. In the never
ending struggle to cut costs and increase quality, Intersil
XDCPs are a major enhancement in the design of power
supplies and DC to DC converters.
OU T
U1
14
9
SCL
SDA
S CL
S DA
VH0
V W0
VL 0
VH1
V W1
VL 1
15
5
16
4
VH2
V W2
VL 2
A
A
A
A
3
2
1
0
VH3
V W3
VL 3
3
1
2
8
6
7
Ve/a
11
13
12
VR
17
19
18
R2
R1
V OU T
X9241
FIGURE 5.
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
3
AN1157.1
October 1, 2008
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