Dec 2003 Voltage Margining Made Easy

DESIGN FEATURES
Voltage Margining Made Easy
by Jeff Heath
Introduction
Linear Technology has introduced two
devices that simplify supply margin
testing and are particularly well suited
for multiple power supply applications.
The LTC2920-1 is a single power supply margining controller packaged in
a 5-lead SOT-23. The LTC2920-2 is a
dual power supply margining controller packaged in an 8-lead MSOP. Both
parts provide an easy and accurate way
to accomplish onboard power supply
margining with a minimum of design
time and board space.
testing is often called “supply margining” or “voltage margining.” The testing
is typically accomplished by forcing
the power supply modules or DC/DC
converters in the system to ±5% of
their nominal voltage. Once the supply
voltage has settled at the margined
voltage, the system performance can
be evaluated. Figure 1 shows typical
rise and fall times for voltage margining waveforms.
Typical Applications
Figure 2 shows how the 5-lead SOT23 LTC2920-1 controls a LTC1435A
switching regulator by sourcing or
sinking current at the feedback pin.
This margining solution takes fewer
components, less board space, and
less design time than the traditional
approach of using resistors, switches,
and level shifters. Figure 3 shows the
LTC2920-1 controlling a DC/DC converter module. In this case, current is
forced into or out of the trim pin on the
module, causing it to raise or lower its
output voltage accordingly.
What is Voltage Margining?
High performance and high reliability
systems typically require end-of-line
testing, or include automated self-testing, to assure rated performance with
the supply voltage at the upper and
lower limits of its regulation band. This
5%
+VOUT
NOM
trim pin of the power supply to be
margined is connected to the IM pin.
The margining current is selected with
a single resistor, RSET, connected to the
RS pin. The RSET resistor can program
the margin current over a 400:1 range
from 5µA to 2mA. The control input is
a three-state signal: LOW to margin
down, HIGH to margin UP, and FLOAT
to allow the power supply to regulate at
its nominal voltage. The only other pins
are power and ground. The LTC2920-2
has an additional independent margining channel, adding a second set
of IM, RS, and IN pins.
The LTC2920-1 uses less than
14mm2 of application board space,
and the LTC2920-2 uses less than
27mm2 (Figure 4). These numbers
include the current setting resistors
and the LTC2920’s power supply bypass capacitor.
How It Works
Feedback Circuits
The LTC2920 provides symmetric
power supply margining by taking
advantage of the architecture common
to most power supplies. Most regulated power supplies rely on feedback
and gain to maintain power supply
output voltages. Even complicated
multi-phase switching power supplies
can typically be modeled as a simple
amplifier with a voltage reference and
two feedback resistors (Figure 5). The
Easy to Use, Easy to Connect
–5%
LOGIC HI
The LTC2920-1 uses only three pins
to setup and control the margining of
a power supply: the IM current pin,
the current selecting pin RS, and the
control pin IN. The feedback node or
IN1
LOGIC FLOAT
LOGIC LOW
1ms/DIV
2920-1/2 TA01a
Figure 1. Typical voltage margining waveforms
VIN
4.5V TO 28V
COSC
68pF
51pF
CSS
0.1µF
CC
150pF
RC
10k
COSC
VIN
RUN/SS
TG
ITH
SW
DB
CMDSH-3
LTC1435A
CB
0.1µF
INTVCC
SGND
BOOST
100pF
BG
VOSENSE
SENSE –
+
M1
Si4412DY
+
L1
4.7µH
D1
MBRS140T3
4.7µF
M2
Si4412DY
CIN
22µF
35V
× 2 RSENSE
0.025Ω
VCC
R1
3.57k
R2
2k
+
PGND
SENSE +
VCC
LTC2920
IM
1000pF
CBYP
0.1µF
RB
499Ω
COUT
100µF
6.3V
×2
VOUT
3.3V
4.5A
GND
SYSTEM
CONTROLLER
IN
THREE-STATE
RS
21.5k
GND
2920-1/2 TA03
Figure 2. Voltage margining a switching power supply
Linear Technology Magazine • December 2003
31
DESIGN FEATURES
1
33µF
+VIN
+VOUT
POWER ONE
I5S013ZE-A
+
5
+
3.3V
AT 4A
150Ω
–48V
–VOUT
–VIN
SYSTEM
CONTROLLER
VCC
IN1
IM1 LTC2920-1
R
S1
GND
TRIM
2
0.1µF
2µF
6
THREE-STATE
RSET1
10k
1%
Figure 4. Dual power supply voltage margin
controller occupies less than 27mm2
7
2920-1/2 TA01
Figure 3. DC to DC converter voltage margining
LTC2920 creates a delta voltage across
the feedback resistor RF by sourcing
or sinking current from the feedback
node. Here’s how it works:
Knowing the value of the resistors RF
and RG, and the voltage of VREF, VPSOUT
can be calculated from the basic op
amp feedback equation:
VPSOUT = VREF • [1 + (RF/RG)]
Since the op amp keeps its inverting terminal equal to the noninverting
terminal, the voltage at the inverting
terminal between RF and RG is VREF.
There is no significant current flowing
into or out of the op amp inputs, so:
IF = IG = VREF/RG
Knowing that this current flows in
the feedback resistor network, VPSOUT
can be alternately calculated by:
VPSOUT = VREF + (IFB • RF)
This equation is in a form which
is helpful for understanding how the
LTC2920 changes the power supply
output voltage. Figure 6 shows the
simplified model with the LTC2920
added. As before, the op amp keeps
the voltage at its inverting input at
VREF. If we add or subtract current at
this node, the delta current is always
added to or subtracted from IFB, and
never IRG. Because of this, the voltage
across RF is:
VRF = (IFB(NOM) ±IMARGIN) • RF.
or
VRF = (IFB(NOM) • RF) ±(IMARGIN • RF)
When added to the nominal voltage
at the feedback pin VREF, the power
supply output becomes:
VPOSOUT =
VREF + (IFB(NOM) • RF) ±(IMARGIN • RF)
32
This is the nominal power supply
output voltage, plus or minus the
margining current, IMARGIN, supplied
by the LTC2920 times the feedback
resistor RF:
VPSOUT = VPOSOUT(NOM) ±(IMARGIN • RF)
Note that the delta voltage VMARGIN
depends only on IMARGIN and RF, and
not RG or VREF.
Simple Design:
Calculate the Value of One Resistor
There is one resistor value to calculate
when designing in the LTC2920 for
voltage margining applications: the
current setting resistor RSET. To calculate this value, there are only two
things the designer needs to know:
the amount of voltage to margin the
power supply, VMARGIN, and the value of
the feedback resistor of the margined
power supply, RF.
The value of the current setting
resistor can then be determined by
calculating the desired IMARGIN current
sourced or sunk by the LTC2920:
IMARGIN = VMARGIN / RF
The value of the current setting resistor is then:
RSET = 1V/IMARGIN
No Access to the Feedback Pin?
No Problem
DC to DC power supply modules
typically do not provide access to the
feedback pin directly, but DC to DC
power modules often have a trim pin. In
this application, the LTC2920 operates
by sourcing and sinking current out
of the trim pin on the power supply
being margined. Calculation of the RSET
resistor is slightly more complicated
than the case described above, requiring a bench measurement to be made
on the DC to DC power supply module.
See the LTC2920 data sheet for details
on the operational details and how to
calculate the RSET resistor.
Conclusion
The LTC2920-1 and LTC2920-2 Power
Supply Margining Controllers provide
a cost effective way to add self-testing capability to high performance
and high reliability systems. They
are easy to design with easy to interface to. Both parts have the ability
to control power supplies with either
feed back nodes, or trim pins. With
printed circuit footprints of less than
14mm2 and 27mm2, the LTC2920-1
and LTC2920-2 respectively provide
versatile power supply margining
with a minimum impact on PCB
space.
And in its simplest form:
RSET = RF / VMARGIN
IMARGIN
LTC2920
RS
IFB
RG
–
VPSOUT
VREF
+
–
RF
IM
RF
+
RSET
IFB
RG
–
IRG
VREF
VPSOUT
+
–
+
2920-1/2 F02
2920-1/2 F01
Figure 5. Simplified power supply model
Figure 6. Simplified power supply
model with voltage margining
Linear Technology Magazine • December 2003
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