LTC2920-1/LTC2920-2 - Single/Dual Power Supply Margining

LTC2920-1/LTC2920-2 - Single/Dual Power Supply Margining
LTC2920-1/LTC2920-2
Single/Dual Power Supply
Margining Controller
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FEATURES
DESCRIPTIO
■
The LTC®2920 allows power supplies and power supply
module output voltages to be precisely adjusted both up
and down for automated PCB testing. The power supply
output voltage is changed by sourcing or sinking current
into the feedback node or voltage adjust pin of the power
supply. This allows a system to test the correct operation
of electrical components at the upper and/or lower power
supply voltage limits specified for a given design (Power
Supply “Margining”).
■
■
■
■
■
■
■
■
Margin Voltage Precision <0.4%
400:1 Current Programming Range
Symmetric/Asymmetric High and Low Voltage
Margining
Single Control Pin per Supply—High, Float, Low
Single Current Setting Resistor per Supply
Wide VCC Compliance 2.3V < VCC < 6V
Wide Output Compliance
0.6V < VMARGIN < (VCC – 0.6V)
Single in 5-Pin ThinSOTTM (LTC2920-1)
Dual in 8-Pin MSOP (LTC2920-2)
The LTC2920 uses a single resistor to set the voltage
margining current. The margining current is adjustable
over a 400:1 range. Precision margin currents can be
supplied to within 0.6V of ground or VCC.
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APPLICATIO S
■
■
■
The LTC2920-1 is a single margining controller. The
LTC2920-2 has two independently controllable margining
channels. Each channel has its own control pin and current
setting resistor. The LTC2920-2 can be used to symmetrically margin two power supplies, or asymmetrically margin a single power supply.
Automated PCB Production Testing
Automated Preventative Maintenance Testing
DC/DC Converter Module Margining
Both the LTC2920-1 and LTC2920-2 feature a trimmed onboard voltage reference. Typical power supply margining
accuracy is better than 0.4%.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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TYPICAL APPLICATIO
3.3V Quarter Brick with ±5% Voltage Margining
1
5
+VIN
POWER ONE
I5S013ZE-A
+
2
–VIN
SYSTEM
CONTROLLER
VCC
IN1
IM1 LTC2920-1
R
S1
GND
6
5%
0.1μF
2μF
TRIM
–VOUT
150Ω
+
33μF
–48V
3.3V
AT 4A
+VOUT
+VOUT
NOM
THREE-STATE
–5%
LOGIC HI
RSET1
10k
1%
IN1
LOGIC FLOAT
LOGIC LOW
7
2920-1/2 TA01
1ms/DIV
2920-1/2 TA01a
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LTC2920-1/LTC2920-2
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ABSOLUTE
AXI U RATI GS
(Note 1)
Supply Voltage (VCC) ................................– 0.3V to 6.5V
Input Voltages
(IN1, IN2, RS1, RS2)................. – 0.3V to (VCC + 0.3V)
Output Voltages (IM1, IM2) ........... – 0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC2920-1C/LTC2920-2C ....................... 0°C to 70°C
LTC2920-1I/LTC2920-2I .................... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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PACKAGE/ORDER I FOR ATIO
ORDER PART NUMBER
TOP VIEW
VCC 1
LTC2920-1CS5
LTC2920-1IS5
5 IN1
GND 2
IM1 3
4 RS1
S5 PART MARKING
S5 PACKAGE
5-LEAD PLASTIC SOT-23
TJMAX = 125°C, θJA = 250°C/W
LTD7
LTD8
ORDER PART NUMBER
LTC2920-2CMS8
LTC2920-2IMS8
TOP VIEW
RS2
IN2
IN1
RS1
1
2
3
4
8
7
6
5
VCC
IM2
GND
IM1
MS8 PART MARKING
MS8 PACKAGE
8-LEAD PLASTIC MSOP
LTB6
LTA4
TJMAX = 125°C, θJA = 200°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, CRS1 = CRS2 = 20pF, otherwise specifications are at TA = 25°C.
SYMBOL
PARAMETER
CONDITIONS
MIN
VCC
Supply Operating Range
(Note 2)
●
ICC(SOURCE)
Supply Current while Sourcing Max IIM
RSET1 = RSET2 = 15k,
IN1 = IN2 < VIL
●
ICC(Q)
Quiescent Supply Current
RSET1 = RSET2 = 200k,
IN1 = IN2 ≤ VIL
●
TYP
MAX
UNITS
Supplies
2.3
0.23
6
V
6
mA
1
mA
Current Margining Outputs IM1, IM2
IIMLOW
Low Range IMARGIN Current—
Sourcing or Sinking
RSET1, RSET2 Tied to GND,
IN1, IN2 > VIH or IN1, IN2 < VIL, (Note 4)
●
5
167
μA
IIMHIGH
High Range IMARGIN Current—
Sourcing or Sinking
RSET1, RSET2 Tied to VCC,
IN1, IN2 > VIH or IN1, IN2 < VIL, (Note 4)
●
0.15
2
mA
VM
IM1, IM2 Output Voltage Compliance
(Note 3)
●
0.55
VCC – 0.55
V
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LTC2920-1/LTC2920-2
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
SYMBOL
PARAMETER
CONDITIONS
IIMACCURACY
Low Range Current Accuracy
100μA ≤ ⏐IM⏐ ≤ 167μA, (Note 6)
C-Grade
I-Grade
High Range Current Accuracy
IOZ
IM1, IM2 Leakage Current
CIM
Equivalent Capacitance At IM1, IM2
MIN
TYP
MAX
UNITS
●
●
3
3
7.5
13
%
%
30μA ≤ ⏐IM⏐ < 100μA, (Note 6)
C-Grade
I-Grade
●
●
5
5
11
15
%
%
5μA ≤ ⏐IM⏐ < 30μA, (Note 6)
C-Grade
I-Grade
●
●
5
5
20
25
%
%
1.5mA ≤ ⏐IM⏐ ≤ 2mA, (Note 7)
C-Grade
I-Grade
●
●
3
3
7.5
11
%
%
600μA ≤ ⏐IM⏐ ≤ 1.5mA, (Note 7)
C-Grade
I-Grade
●
●
5
5
11
15
%
%
150μA ≤ ⏐IM⏐ ≤ 600μA, (Note 7)
C-Grade
I-Grade
●
●
5
5
15
20
%
%
100
nA
●
VIN = VOFF, (Note 5)
VIN = VIL, High Range, (Note 5)
VIN = VIL, Low Range, (Note 5)
10
2
30
pF
nF
pF
Control Inputs IN1, IN2
●
●
VIH
Control Voltage for IM Current Sinking
VCC < 2.5V
VCC ≥ 2.5V
2.1
2.4
VIL
Control Voltage for IM Current Sourcing
●
VOFF
Control Voltage for IM Current Off
●
VOZ
Control Voltage when Left Floating
RIN
IN1, IN2 Input Resistance
●
5
IFLT
Maximum Allowed Leakage at IN1, IN2
for IM Current Off
●
–10
V
V
0.6
1.1
V
1.4
V
1.2
12
V
20
kΩ
10
μA
Switching Characteristics
VIN(DELAYON)
IM1, IM2 Turn-On Time
VIN Transitions from VOFF to
VIH or VIL
●
15
100
μs
VIN(DELAYOFF)
IM1, IM2 Turn-Off Time
VIN Transitions from
VIH or VIL to VOFF
●
15
100
μs
IM(ON)
IM1 Rise Time
⏐IM⏐ 5% to 95%, (Note 5)
5
μs
IM(OFF)
IM1 Fall Time
⏐IM⏐ 95% to 5%, (Note 5)
0.3
μs
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: VCC must always be above the maximum of IM1 and IM2 less 0.2V.
See Preventing Potential Power Supply Overvoltages in the Applications
Information section.
Note 3: VM compliance is the voltage range within which IM1 and IM2 are
guaranteed to be sourcing or sinking current. IM accuracy will vary within
this range.
Note 4: Consult LTC Marketing for parts specified with wider IM current
limits.
Note 5: Determined by design, not production tested.
Note 6: ⏐1 – (IM – RS)⏐ • 100%; VCC ≤ 4V: 0.58 ≤ VM ≤ (VCC – 1.1);
VCC > 4V: 0.58 ≤ VM ≤ (VCC – 1.4); CRS ≤ 20pF
Note 7: ⏐1 – (IM • RS / 30)⏐ • 100%; 0.79 ≤ VM ≤ (VCC – 0.6);
CRS ≤ 20pF
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LTC2920-1/LTC2920-2
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PI FU CTIO S
(S5 Package/MS8 Package)
VCC (Pin 1/Pin 8): Power Supply Input. All internal circuits
are powered from this pin. VCC should be connected to a
low noise power supply voltage between 2.3V and 6V and
should be bypassed with at least a 0.1μF capacitor to the
GND pin in close proximity to the LTC2920. Current
sourced out of the IM pins comes from the VCC pin. Note
that VCC must come up no later than the time the
controlled power supply turns on or damage to the load
may result. See Preventing Potential Power Supply Overvoltages in the Applications Information section for power
sequencing considerations. In certain applications, it may
be necessary to further isolate VCC by adding a resistor in
series with its power source. See VCC Power Filtering in the
Applications Information section.
GND (Pin 2/Pin 6): Ground. All internal circuits are returned to the GND pin. Connect this ground pin to the
ground of the power supply(s) being margined. Current
sunk into the IM pins of the LTC2920 is returned to ground
through this pin.
RS1 (Pin 4/Pin 4): IM1 Current Set Input. The RS1 pin is
used to set the margining current which is sourced out of
or sunk into the IM1 pin. The RS1 pin must be connected to
either VCC or ground with an external resistor RSET with a
value between 6k and 200k. Connecting RSET to ground
sets the current at the IM1 pin with a multiplier of 1.
Connecting RSET to VCC sets the current at the IM1 pin with
a multiplier of 30. If RSET is connected to ground, ≈1V will
appear at the RS1 pin. If RSET is connected to VCC, ≈(VCC –
1V) will appear at the RS1 pin. In either case, the current
through RSET will be ≈1V/RSET.
IM1 (Pin 3/Pin 5): IM1 Current Output. This pin should be
connected to the power supply feedback pin or voltage
adjust pin. (See the Applications Information section for
further details.) Current is either sourced out of or sunk
into this pin. The direction of the current is controlled by
the IN1 pin. The amount of current flowing into or out of
the IM1 pin is controlled by the RS1 pin.
IN1 (Pin 5/Pin 3): IM1 Control Pin. This pin is a 3-level input
pin which controls the IM1 pin. If the IN1 pin is pulled above
VIH, current is sunk into the IM1 pin. If the IN1 pin is pulled
below VIL, current is sourced from the IM1 pin. If the IN1
pin is left floating, or held between 1.1V and 1.4V, the IM1
pin is a high impedance output. Internally, the IN1 pin is
connected to a 1.2V voltage source by an internal ~10k
resistor. The LTC2920 has an internal RC circuit to suppress noise entering from this pin.
LTC2920-2 Only
RS2 (NA/Pin 1): IM2 Current Set Input. Sets the current for
IM2. See RS1.
IM2 (NA/Pin 7): IM2 Current Output. This pin is the second
margin current output for the LTC2920. See IM1.
IN2 (NA/Pin 2): IM2 Control Pin. This pin controls the
current at the IM2 pin. See IN1.
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LTC2920-1/LTC2920-2
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TYPICAL PERFOR A CE CHARACTERISTICS
ICC vs IMARGIN High Range
Sourcing Current
ICC vs IMARGIN Low Range
Sourcing Current
5.0
1800
4.5
1600
2.0
1000
1 CHANNEL
800
1 CHANNEL
600
1.5
1.5
1.0
200
0.5
0
0
1.5
IMARGIN (mA)
2
0
0
2.5
20
40
0
60 80 100 120 140 160 180
IMARGIN (μA)
2920-1/2 G01
IMARGIN Error vs VMARGIN
IMARGIN Error vs VMARGIN
(mA)
4.0
50
100
3
166.7
1.0
50
100
3
166.7
2
2
1.5
1
1
VCC = 2.5V
LOW RANGE
VCC = 5V
LOW RANGE
0.5
0
0
0
0
0.5
1
1.5
VMARGIN (V)
2
2.5
0
0.5
1
1.5
2 2.5 3 3.5
VMARGIN (V)
4
2920-1/2 G04
4.5
SOURCE
1
1.5
VMARGIN (V)
2
2.5
2920-1/2 G06
100%
SOURCE
VIN(DELAYON)
ENDS
LOW
RANGE
0.5
IMARGIN Fall Time
100%
0%
0
5
2920-1/2 G05
IMARGIN Rise Time
HIGH
RANGE
5
20
ERROR (%)
ERROR (%)
1
2
2.0
4.5
4
4
0.5
4
(μA)
5
5
20
2.5
2 2.5 3 3.5
VMARGIN (V)
IMARGIN Error vs VMARGIN
(μA)
5
5
0.3
3.0
1.5
6
0.15
3.5
1
2920-1/2 G03
6
VCC = 2.5V
HIGH RANGE
0.5
2920-1/2 G02
5.0
4.5
1
2
2.0
0.5
1
0.5
2.5
400
0.5
0.3
3.0
1.0
0
ERROR (%)
ERROR (%)
ICC (μA)
2.5
0.15
3.5
1200
3.0
(mA)
4.0
1400
2 CHANNELS
3.5
VCC = 5V
HIGH RANGE
4.5
2 CHANNELS
4.0
ICC (mA)
IMARGIN Error vs VMARGIN
5.0
VIN(DELAYOFF)
ENDS
RSET = 20k
LOW RANGE
HIGH RANGE
0%
RSET = 20k
LOW RANGE
HIGH RANGE
100%
SINK
HIGH
RANGE
1μs/DIV
2920-1/2 G07
100%
SINK
100ns/DIV
2920-1/2 G08
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LTC2920-1/LTC2920-2
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FU CTIO AL BLOCK DIAGRA
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UVLO
IN1
THERMAL SHUTDOWN
INPUT DETECTION
CURRENT
SETTING
VCC
LOW
RANGE
CONNECT TO VCC
FOR HIGH RANGE OR
TO GND FOR LOW RANGE
RSET1
VOLTAGE REFERENCE
RS1
SOURCE
OFF
SINK
IM1
IPROGRAM
OUTPUT
CONTROL
HIGH
RANGE
IN2
RS2
IRNG
VMOK
RANGE DETECTION
VM COMPLIANCE
LTC2920-2 ONLY
IM2
2920-1/2 BD
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APPLICATIO S I FOR ATIO
OVERVIEW
POWER SUPPLY VOLTAGE MARGINING
In high reliability PCB manufacturing and test, it is desirable to test system functionality and performance at the
upper and/or lower power supply voltage limits allowed
for a given design (known as “power supply margining”).
Doing so can greatly improve the lifetime reliability of a
system.
The LTC2920 provides a means of power supply voltage
margin testing which is:
• Flexible
• Easy to design
voltage above the nominal power supply voltage and a
different voltage below the nominal, the LTC2920-2 can be
used. One channel is used for margining above the nominal power supply voltage, and the other channel is used to
margin below the nominal voltage.
VOLTAGE MARGINING POWER SUPPLIES USING A
FEEDBACK PIN
One common power supply architecture supported by the
LTC2920 is a power supply with a feedback pin and two
feedback resistors. Even complicated switching power
supplies can be typically modeled as a simple amplifier
with a reference voltage and a two resistor feedback
network (Figure 1).
• Requires very little PCB board space
Symmetric/Asymmetric Power Supply Margining
Any one LTC2920 channel requires only a single external
resistor to symmetrically margin both above and below
the nominal power supply voltage. The LTC2920-2 can be
used to symmetrically margin two different power supplies. In cases where the design calls for margining one
RF
IFB
RG
–
VPSOUT
+
VREF
+
–
2920-1/2 F01
Figure 1
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LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
Knowing the value of the resistors RF and RG, and the
voltage of VREF, VPSOUT can be calculated by:
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. Knowing the current flowing
in the feedback resistor network, VPSOUT can be also
calculated by:
VPSOUT = VREF + (IFB • RF)
This is the voltage on one side of RF, plus the voltage
across RF. This equation is helpful in understanding how
the LTC2920 changes the power supply output voltage.
Figure 2 shows the simplified model with the LTC2920
added.
IMARGIN
RF
IM
LTC2920
RS
RSET
IFB
RG
–
IRG
VPSOUT
POWER SUPPLY MODULE VOLTAGE MARGINING
Another method of accomplishing voltage margining is
useful for power supply “brick” modules with voltage
adjust pins. Typically, the power supply manufacturer will
design the power supply to be adjusted up or down, using
external resistors connected to the trim pin. The values of
these resistors are usually calculated by the design engineer using two different equations supplied by the manufacturer. There is usually one equation for trimming the
voltage up, and another equation for trimming the voltage
down. In most cases, the power supply module is treated
like a “black box” and very little information is given on
how the trimming is accomplished from an internal circuit
standpoint.
Traditionally such power supply modules are margined by
calculating the two resistors, and alternately connecting
each to VCC or ground with analog switches or relays.
Figure 3 shows how the LTC2920 can be used in these
applications as well. Using the LTC2920 for these applications can save a significant amount of PCB real estate
and cost.
+
VREF
+
–
POWER MODULE
SENSE +
2920-1/2 F02
Figure 2. Simplified Power Supply Model
Again in this circuit, the op amp will keep the voltage at its
inverting input at VREF. If we add or subtract current at this
node, the delta current will always be added or subtracted
from IFB, and never IRG. (“±IMARGIN” is used rather than a
signed IMARGIN value to emphasize the fact that current is
added or subtracted at the feedback pin.) Because of this,
the voltage across RF will be:
VRF = (IFBNOM ± IMARGIN) • RF
or
VRF = (IFBNOM • RF) ± (IMARGIN • RF)
and finally
VPSOUT = VREF + (IFBNOM • RF) ± (IMARGIN • RF)
Note that the delta voltage VMARGIN depends only on
IMARGIN and RF, not RG or VREF.
VIN+
VO+
TRIM
VIN–
VO–
–
RSYSTEM
VPSOUT
LTC2920
IMARGIN
IM
RS
RSET
VO–
SENSE
2920-1/2 F03
Figure 3. Margining a Power Supply Module
Power Supply Module Design Considerations
There are usually practical limits to VO+. For instance, VO+
usually has upper and lower voltage limits specified by the
power module manufacturer. A common value is 10%
above and 20% below the rated output voltage of the power
supply module. This limit includes VMARGIN plus any voltage drop across RSYSTEM. See the manufacturer’s power
supply module specifications for details. See the “Selecting The RSET Resistor” section of this datasheet for instructions on how to choose RSET in module applications.
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LTC2920-1/LTC2920-2
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Since ΔVPSOUT will appear on RF as noted in the Overview
section, margin current IMARGIN can be calculated by:
SELECTING THE RSET RESISTOR
Selecting RSET with an Existing Power Supply
Containing a Feedback Pin and Two Feedback
Resistors
IMARGIN = ΔVPSOUT/RF
Example: If ΔVPSOUT = 0.165V and RF = 10k:
Calculating the value of the current setting resistor, RSET,
for a power supply with a feedback pin is straight forward.
When the LTC2920 is being added to an existing power
supply design, the power supply feedback resistors RF and
RG have already been selected. By knowing RF, the power
supply output voltage, VPSOUT, and the amount to margin,
%change, RSET can be calculated.
IMARGIN
LTC2920
IFB
RG
VPSOUT
+
VREF
RSET = 1V/IMARGIN = 1V/16.5μA = 60.6k
In this case, RSET would be connected between the RS pin
and ground.
RSET = 1V/(IMARGIN/30)
–
RSET
If IMARGIN is between 5μA and 167μA, use the LTC2920’s
low current range. RSET is then calculated by:
If IMARGIN is between 150μA and 2mA, use the LTC2920’s
high current range. RSET is then calculated by:
RF
IM
RS
IMARGIN = 0.165/10k = 16.5μA
or simply:
RSET = 30V/IMARGIN
+
–
IMARGIN = 330μA
2920-1/2 F04
VCC
Figure 4. Simplified Power Supply Model
First, the margining voltage ΔVPSOUT can be calculated by
knowing the percentage of the power supply voltage
VPSOUT change desired.
ΔVPSOUT = %Change • VPSOUT
ΔVPSOUT = 0.05 • 3.3V = 0.165V
LTC2920
RG = 5.76k
IFB = 4.2mA
–
VPSOUT = 3.3V
+
+
–
2920-1/2 F06
Figure 6. 3.3V Supply with 5% Margining (High Range)
ΔVPSOUT = 0.05 • 3.3V = 0.165V
IFB = 210μA
IMARGIN = 0.165V/500Ω = 330μA
VPSOUT = 3.3V
+
VREF = 1.2V
RG = 286Ω
RF = 10k
–
RSET = 60.6k
LTC2920
RS
Example: If the value of the feedback resistor RF is 500Ω
in the example above then:
IM
RS
RSET = 90k
VREF = 1.2V
Example: If a 3.3V power supply is to be margined by 5%,
then:
IMARGIN = 16.5μA
RF = 500Ω
IM
RSET = 30V/IMARGIN = 30V/330μA = 90.1k
In this case, RSET would be connected between the RS pin
and VCC.
+
–
2920-1/2 F05
Figure 5. 3.3V Supply with 5% Margining (Low Range)
If IMARGIN is less than 5μA, or greater than 2mA, it will be
necessary to adjust both power supply feedback resistors
RF and RG. Again, this is usually a simple process. It is easy
to calculate the magnitude of the change by dividing the
IMARGIN current calculated above by the desired new
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LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
IMARGIN current. Select a new IMARGIN current that is within
one of the two LTC2920’s IMARGIN ranges, then calculate
the scaling factor:
IFACTOR = IMARGIN(OLD)/IMARGIN(NEW)
The new feedback resistors would then be:
RF(NEW) = RF(OLD) • IFACTOR
RG(NEW) = RG(OLD) • IFACTOR
And RSET can then be calculated as descibed above.
WARNING
In some cases, adjusting the feedback resistors on a
switching supply might require recompensating the power
supply. Please refer to the applications information supplied with the power supply for further information.
POWER MODULE
VIN–
VO–
VPSOUT
LTC2920
IMARGIN
IM
VO–
RS
RSET
SENSE –
TRIM DOWN RESISTANCE (Ω)
VO+
TRIM
1M
100k
SENSE +
VIN+
between the trim pin and the power supply positive voltage
output or the trim pin and the negative power supply
output (ground). The polarity of the voltage trim and trim
resistor configuration are chosen by the manufacturer.
The equations describing the resistor values versus the
desired output voltage changes are typically not linear.
Fortunately, the relationship between trim pin current and
output voltage change is typically linear. The current trim
equation is usually the same (in magnitude) for changing
the output voltage up or down. Once the equation for trim
current is determined, it is much easier to use than trim
resistors. To illustrate this, Figure 8 shows a typical
resistor trim down curve for a power module. Figure 9
shows a typical current trim down curve for the same
power module.
10k
1k
100
2920-1/2 F07
Figure 7. Using a Power Module Trim Pin for Voltage Margining
10
1
0
0.1
Selecting the RSET Resistor Using Voltage Trim Pins
with ‘Brick’ Type Power Supply Modules
The amount of current necessary to adjust the output
voltage of the power supply module is not normally given
directly by the manufacturer. However, by using information that is supplied by the manufacturer, a measurement
can be made to determine a simple equation that is useful
for power supply module voltage margining.
Typically, the manufacturer will supply two different equations for selecting trim resistors: one for trimming the
output voltage up and a different one for trimming the
output voltage down. Trim resistors are nominally placed
0.4
0.5
2920-1/2 F08
Figure 8. Typical Trim Voltage vs Trim Resistor Curve
300
250
TRIM CURRENT (μA)
‘Brick’ power supply modules often have a trim pin which
can be used for voltage margining. Figure 7 shows a
typical connection using the LTC2920 for voltage margining a power supply module.
0.2
0.3
TRIM VOLTAGE (V)
200
150
100
50
0
0
0.1
0.2
0.3
TRIM VOLTAGE (V)
0.4
0.5
2920-1/2 F09
Figure 9. Typical Trim Voltage vs Trim Current Curve
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LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
Even though the manufacturer does not directly supply the
equation for the trim current, a simple measurement can
be made to calculate an equation for VTRIM as a function
of ITRIM.
For any desired VMARGIN:
To do this, select the trim resistor configuration which
places the trim resistor between the trim pin and ground
(see Figure 10).
For 5μA ≤ ITRIM ≤ 167μA:
With the trim resistor connected to ground, note the
direction of the power module output voltage change. This
is the direction that the power module output voltage will
change when the LTC2920 IN control pin is HIGH, above
VIH. Remember that the direction of the voltage trim for
this configuration can vary among power modules, even
among power modules from the same manufacturer.
Calculate a resistor value from the manufacturer’s equation, or select it from a chart (if a chart is supplied by the
manufacturer). Pick a value near the middle of the trim
resistor range. Obtain and measure the selected resistor
with an ohmmeter or use a precision 0.1% resistor.
Knowing the correct value of this resistance is critical to
obtaining good results. Make provisions to connect and
disconnect this test resistor between the trim pin and the
power supply module’s negative output pin. (Figure 10.)
Carefully follow all other manufacturer’s application notes
regarding power supply input voltage, minimum and
maximum output voltages, sense pin connections (if any),
minimum and maximum current loads, etc. Failure to do
so may permanently damage the power supply module!
ITRIM = VMARGIN/KTRIM
RSET can now be calculated for the LTC2920.
RSET = 1V/ITRIM
Connect RSET between the RS pin and the LTC2920 ground
pin.
For 167μA < ITRIM ≤ 2mA:
RSET = 1V/(ITRIM/30)
Connect RSET between the RS pin and the LTC2920 VCC
pin.
If ITRIM falls outside of this range, the LTC2920 cannot be
used for this application.
The LTC2920 can source or sink current only when the
voltage at the IM pin is between 0.6 and (VCC – 0.6) volts.
In order to be sure that the LTC2920 will operate correctly
in this application, ensure that the VT node will stay within
these limits. To do this, calculate the effective output
resistance of the power supply module’s trim output pin,
RVT (refer to Figure 10). Using the measurements taken
above, the open circuit voltage is:
VREF = VTNOM
To calculate RVT, subtract the untrimmed VTNOM and
trimmed VTTRIM voltages measured above:
VTDELTA = VTNOM – VTTRIM
Apply the specified input voltage to the power supply
module. Measure the power supply output voltage VPS
and the VT voltages before and after connecting the trim
resistor.
The effective TRIM pin source resistance can then be
calculated by:
Subtract the untrimmed (VPSNOM) and trimmed (VPSTRIM)
power supply output voltages to obtain the trim voltage
(VDELTA):
The voltage at the LTC2920 IMARGIN pin for any ITRIM can
now calculated for both voltage margin directions. Refering
to Figure 10:
VDELTA = VPSNOM – VPSTRIM
and the trim current:
ITRIM = VTRIM/RTRIM
Calculate the linear current trim constant KTRIM:
RVT = VTDELTA/ITRIM
VTSINK = VREF – (RVT • ITRIM)
VTSOURCE = VREF + (RVT • ITRIM)
Note: be sure to use these equations to verify that VTSINK
and VTSOURCE are within LTC2920 VM voltages specified in
KTRIM = VDELTA/ITRIM
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10
LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
SENSE +
VIN+
VO +
RVT
VREF
+
–
VIN–
TRIM
VPS
VT
RTRIM
VO –
ITRIM
VO –
SENSE –
2920-1/2 F10
Figure 10. Power Module ITRIM Model
Accuracy of Power Supply Voltages when Margining
The accuracy of margined power supply voltages depends
on several factors. Figure 11 shows the magnitude of the
errors discussed in detail below as a function of power
supply margining percentage.
for this example). The second error is the power supply
initial set point accuracy. In this example the RF resistor
has a 1% accuracy error causing a 0.6% initial set point
error in the power supply. Because the margined power
supply voltage is the change in the voltage, VMARGIN, from
the power supply initial set point voltage, this error shows
up in the margined power supply voltage. When these two
errors are combined, the error is:
Error = ⏐1 – (3.4043/3.3825)⏐ • 100 = 0.65%
The error caused by a 1% inaccuracy in RG will be similar
since the dominate error source is the power supply initial
set point voltage.
Errors caused by RF and RG can be a major contributor to
voltage margin errors. Using 0.1% resistors for both RF
and RG is often the best choice for improving both voltage
margin accuracy and power supply initial accuracy.
POWER SUPPLY MARGINED VOLTAGE ERROR
⏐1 – ACTUAL VOLTAGE/EXPECTED VOLTAGE⏐ • 100 (%)
the IMACCURACY specification. If VT does not fall within this
range, the LTC2920 cannot be used for this application.
In a typical feedback model (Figure 12), the delta voltage
is a function of the margin current, IMARGIN, and the
feedback resistor, RF.
VMARGIN = IMARGIN • RF
Errors in VMARGIN are directly proportional to errors in
IMARGIN and errors in RF. A 5% error in IMARGIN will cause
a 5% error in VMARGIN. In this example, a 3.3V power
supply is margined by 2.5%, or 0.0825V to 3.3825V. With
a 5% VMARGIN error, the actual margin voltage is 0.0866V
and the actual power supply voltage is 3.3866V. The error
in the expected voltage is then:
Error = ⏐1 – (3.3866/3.3825)⏐ • 100 = 0.12%
Similarly, a 1% inaccuracy in the RSET resistor would
cause only 0.024% error in the expected power supply
margined voltage. In effect, IMARGIN errors caused by the
RSET resistor or the LTC2920 are attenuated by the voltage
margining percentage.
The accuracy of the RF resistor introduces two errors in the
margined supply voltage. The first is the error in VMARGIN
(IMARGIN • RF). This error is similar in magnitude to the
errors described above and is generally quite small (0.024%
0.7
0.6
1% FEEDBACK
RESISTOR INACCURACY
0.5
1% RSET
RESISTOR
INACCURACY
0.4
0.3
5% LTC2920
IMARGIN INACCURACY
0.2
0.1
0
0
1
2
3
4
5
6
POWER SUPPLY VOLTAGE MARGINING (%)
2920-1/2 F11
Figure 11. Sources of Power Supply Margined Voltage Errors
IMARGIN = ± 50μA
RF = 1.65k
IM
LTC2920
RS
RG = 944k
IFB = 1.27mA
–
RSET = 20k
VPSOUT = 3.3V
+
VREF = 1.2V
+
–
2920-1/2 F12
Figure 12. Power Supply Voltage Margin Model
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11
LTC2920-1/LTC2920-2
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PREVENTING POTENTIAL POWER SUPPLY
OVERVOLTAGES
Care must be taken when selecting the power source for
the LTC2920. If VCC on the LTC2920 is not powered, and
the power supply being margined is on, undesired IM fault
current can flow into the IM pin of the LTC2920. This can
cause the margined power supply to create an overvoltage
condition causing serious damage to power supply and its
load. The best solution is to connect the LTC2920 to a
power source that is guaranteed to be on when the power
supply being margined is on. Often this is the input or
output voltage of the power supply being margined. See
the design guidelines below for the best solution for your
application. Be sure to follow all other LTC2920 design
specifications.
At a minimum, the voltage at the VCC pin of the LTC2920
must be maintained above 0.2V below the highest voltage
present at the IM1 and IM2 pins. This will keep the IM fault
current below 5μA. The voltage at the IM1 and IM2 pins is
normally the voltage at the feedback node of the power
supply. See the power supply manufacturer’s data sheet
for this voltage.
PREVENTING IM FAULT CURRENT IN THE LTC2920-1
Connecting VCC to the Power Supply VIN or VOUT of the
Supply Being Margined
Connecting the LTC2920-1 VCC to VIN or VOUT is the best
choice and should be used when conditions permit. It
requires no external components and provides the best
protection from power supply overvoltage.
If the power supply being margined has a VIN voltage that
is within the LTC2920’s VCC range, connect the LTC2920-1
VCC pin to the power supplies VIN (Figure 13).
If the power supply being margined has a VOUT voltage that
is within the LTC2920’s VCC range, connect the LTC2920-1
VCC pin to the power supplies VOUT (Figure 14). Make sure
the power supply voltage is within the LTC2920’s VCC
specification when the power supply is being margined!
VIN
2.3V TO 6V
VCC
VO
0.1μF
VCC
VOUT
LTC2920-1
IM
FB
GND
2920-1/2 F13
Figure 13. Connecting LTC2920-1 to VIN
VOUT 2.3V TO 6V
VIN
VIN
VO
0.1μF
VCC
VOUT
LTC2920-1
IM
FB
GND
2920-1/2 F14
Figure 14. Connecting LTC2920-1 to VOUT
Connecting VCC to Power Sources Other than the
Supply Being Margined
If it is not practical to power the LTC2920-1 from the VIN
or VOUT of the power supply being margined, connect the
VCC pin of the LTC2920-1 using a Schottky diode (Figure 15). This solution works with power supply feedback
voltages of less than 1.5V and IMARGIN currents >30μA. Be
sure to account for the diode drop across all temperatures
to ensure the LTC2920-1 VCC and VMARGIN specifications
are met.
VPOWER
BAT54C
SCHOTTKY DIODE
VIN
VIN
VO
VCC
VOUT
0.1μF
LTC2920-1
FB
<1.5V
IM
GND
2920-1/2 F15
Figure 15. Diode Connected VCC
292012fa
12
LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
PREVENTING IM FAULT CURRENT IN THE LTC2920-2
Connecting VCC to a Common VIN
Connecting the LTC2920-2 VCC to VIN is the best choice
and should be used when conditions permit. It requires no
external components and provides the best protection
from power supply overvoltage (Figure 16).
VIN
VIN
OUT
FB
IM2
VCC
LTC2920-2
IM1
GND
2920-1/2 F16
VCC Power Supply Filtering
Figure 16. Connecting VCC to VIN
Connecting VCC to Diode OR’d Supplies
If the margined power supplies derive their VIN from
different sources, or if a common VIN cannot supply power
to the LTC2920-2, power the LTC2920-2 using a diode
OR’d connection (Figure 17). Note that in this example,
Power Supply 2 has only a 1.8V output. Power Supply 1
will supply the LTC2920-2 under normal operation conditions. If Power Supply 1 fails, or if it is sequenced up after
POWER SUPPLY 1
VOUT1
3.3V
OUT
FB
POWER SUPPLY 2
VOUT2
1.8V
OUT
Connecting VCC to Power Sources Other than the
Supplies Being Margined
If it is not practical to power the LTC2920-2 from the VINs
and/or VOUTs of the power supplies being margined,
connect the VCC pin of the LTC2920-2 using a Schottky
diode (Figure 18). This solution works with power supply
feedback voltages less that 1.5V and IMARGIN currents
>30μA. Be sure to account for the diode drop across all
temperatures to ensure the LTC2920-2 VCC and VMARGIN
specifications are met.
OUT
FB
VIN
Power Supply 2, Power Supply 2 supplies enough voltage
to keep the LTC2920 from sinking fault current into the IM1
and IM2 pins. The LTC2920-2 will not operate normally
under these conditions but it will not cause overvoltage to
occur.
FB
If the LTC2920 is both powered by and margins a power
supply that is marginally stable, oscillations can occur. In
these cases, it may be necessary to provide an additional
filtering resistor between the LTC2920 and the power
supply being margined (see Figure 19). The oscillation is
most likely to occur when the LTC2920 is sourcing current
from the IMARGING pin. The RBYP resistor in combination
with the CBYP capacitor form a lowpass filter. The value of
the filter resistor RBYP can be calculated by deciding how
much voltage drop across the resistor the application can
tolerate and how much current the LTC2920 will sink
under worst-case conditions. In the LTC2920 low current
range, a safe value for the LTC2920 ICC current is the
maximum LTC2920 quiescent current plus 4 times the
IMARGIN current. In the high current range, a safe value for
the LTC2920 ICC current is the maximum LTC2920 quiescent current plus 1.2 times the IMARGIN current.
Example: If the IMARGIN current is 100μA, then:
ICCMAX = IQ + (4 • IMARGIN)
= 1mA + (4 • 100μA ) = 1.4mA
VCC
IM2
LTC2920-2
IM1
BAT54C
GND
2920-1/2 F17
In this example, the power supply voltage is 3.3V. Dropping 0.5V across RBYP will provide a VCC at the LTC2920
of 2.8V. This is well above the LTC2920’s minimum VCC
Figure 17. Dual Diode Connected VCC
292012fa
13
LTC2920-1/LTC2920-2
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APPLICATIO S I FOR ATIO
voltage. The value of the RBYP resistor can then be calculated by:
RBYP = VRB/ICCMAX = 0.5V/1.4mA = 360Ω
With CBYP = 0.1μF, this will provide a pole at 2870Hz. If
additional filtering is necessary, the value of CBYP can be
increased. In this example, if CBYP is increased from 0.1μF
to 1μF, the pole would now be at 287Hz.
POWER SUPPLY 1
OUT
FB
POWER SUPPLY 2
OUT
FB
to Figure 20, Slowing Down VMARGIN, a capacitor (CS) and
a resistor (RS) have been added to the power supply model
described in previous applications sections. To choose
RS, the voltage at the feedback pin of the power supply
must be known. Refer to the power supply manufacturer’s
data sheet for this voltage. The voltage at the IM pin must
be within specified limits of the LTC2920, including the
voltage drop across RS. In the example below, the power
supply feedback pin voltage is 1.21V, IMARGIN is 100μA
and VCC is 3.3V. To maintain LTC2920 current accuracy,
the voltage at the IM pin must be between 0.58V and
(VCC – 1) or 2.3V (in the low current range). A reasonable
value for the voltage drop across RS is 0.5V. The value of
RS is then:
RS = VRS/IMARGIN = 0.5V/100μA = 5k
IM2
VCC
LTC2920-2
IM1
Assuming the desired RC time constant is 1ms, CS is
calculated by:
VPOWER
BAT54C
SCHOTTKY
DIODE
CS = TRC/RS = 1ms/5k = 0.2μF
2920-1/2 F18
Figure 18. Diode Connected to VCC
Controlling IMARGIN Turn On and Turn Off Times
Designers of power supply voltage margining circuits
often need to ensure that power supply voltages do not
overshoot or undershoot (the desired margining voltage)
when the margining current is enabled or disabled. The
LTC2920 IMARGIN current sourced or sinked at the IM
pin(s) is reasonably well behaved (see the Typical Performance Characteristics curves). The differences in speed
between the various curves is caused by the relative
impedance differences within the LTC2920.
Note: When CS and RS are used, an additional pole and a
zero are added to the power supply feedback loop. It is
beyond the scope of this data sheet to predict the behavior
of all power supplies but, in general, as long as the smaller
of the two feedback resistors is no larger than 2 • RS, the
effect on the power supply stability should be minimal. The
larger RS is with respect to the two feedback resistors, the
less effect it will have.
3.3V
VCC
LTC2920
IM
GND
If slower turn on and turn off times are desired, a resistorcapacitor network can be used at the IM pin(s). Referring
IMARGIN
5k
RS
5k
CS
0.2μF
–
+
+
–
VREF
1.21V
1.5k
2920-1/2 F20
RBYP
360Ω
VPSOUT = 3.3V
CBYP
0.1μF
VCC
LTC2920
RS
RSET
IMARGIN =
100μA
Figure 20. Slowing Down VMARGIN
Thermal Shutdown
RF
IM
GND
–
+
+
–
VREF = 1.2V
RG
2920-1/2 F19
Figure 19. VCC Power Filtering
This IC includes overtemperature protection that is intended to protect the device during momentary overload
conditions. Junction temperature will exceed 125°C when
overtemperature protection is active. Continuous operation above the specified maximum operating junction
temperature may result in device degradation or failure.
292012fa
14
LTC2920-1/LTC2920-2
U
PACKAGE DESCRIPTIO
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.62
MAX
0.95
REF
2.90 BSC
(NOTE 4)
1.22 REF
2.80 BSC
1.4 MIN
3.85 MAX 2.62 REF
1.50 – 1.75
(NOTE 4)
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
1.90 BSC
S5 TSOT-23 0302
MS8 Package
8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
0.254
(.010)
0.42 ± 0.038
(.0165 ± .0015)
TYP
0.65
(.0256)
BSC
8
7 6 5
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.52
(.0205)
REF
0° – 6° TYP
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
RECOMMENDED SOLDER PAD LAYOUT
DETAIL “A”
1
2 3
1.10
(.043)
MAX
4
0.86
(.034)
REF
0.18
(.007)
SEATING
NOTE:
PLANE
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.22 – 0.38
(.009 – .015)
TYP
0.65
(.0256)
BSC
0.127 ± 0.076
(.005 ± .003)
MSOP (MS8) 0204
292012fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC2920-1/LTC2920-2
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TYPICAL APPLICATIO S
12V Supply with 5% Margining
L1
10μH
VIN
5V
C1
2.2μF
5
1
VIN
SW
VCC
R1
113k
C3*
10pF
LT1930
4
SHDN
SHDN
VOUT
12V
300mA
MARGIN ±5%
D1
FB
LTC2920-1
3
GND
R2
13.3k
2
RS
113k
RB
1k
IM1
CS
0.01μF
GND
RS1
SYSTEM
CONTROLLER
CB
0.1μF
5
IIN1
C2
4.7μF
3
1
THREE-STATE
4
RSET
188.3k
GND
2
C1: TAIYO YUDEN X5R LMK212BJ225MG
C2: TAIYO YUDEN X5R EMK316BJ475ML
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-100
*OPTIONAL
2920-1/2 TA02
3.3V Supply with ±0.165V (5%) Voltage Margining
VIN
4.5V TO 28V
COSC
68pF
CSS
0.1μF
COSC
VIN
RUN/SS
TG
M1
Si4412DY
ITH
CC
150pF
SW
DB
CMDSH-3
LTC1435A
51pF
CIN
22μF
35V
×2
+
RC
10k
SGND
BOOST
L1
4.7μH
CB
0.1μF
INTVCC
BG
VOSENSE
SENSE –
VCC
R1
3.57k
+
4.7μF
100pF
RSENSE
0.025Ω
M2
Si4412DY
D1
MBRS140T3
R2
2k
VOUT
3.3V
4.5A
CBYP
0.1μF
RB
500Ω
OUT
+ C100μF
6.3V
×2
VCC
SYSTEM
CONTROLLER
LTC2920
IN
PGND
SENSE +
IM
1000pF
THREE-STATE
RS
GND
21.5k
GND
2920-1/2 TA03
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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LTC2900-1/LTC2900-2
Quad Voltage Monitors in MSOP
16 User-Selectable Combinations, ±1.5% Threshold Accuracy
LTC2901-1/LTC2901-2
Quad Voltage Monitors with Watchdog
16 User-Selectable Combinations, Adjustable RST and Watchdog Timers
LTC2902-1/LTC2902-2
Quad Voltage Monitors with RST Disable
16 Selectable Combinations, RST Disable for Margining, Tolerance Select
LTC2921/LTC2922
Power Supply Tracker with Remote Sensing
Three (LTC2921) or Five (LTC2922) Remote Sense Switches
LTC2923
Power Supply Tracking Controller
Controls Two Supplies without Series FETs or a Third Supply with a Series FET
292012fa
16
Linear Technology Corporation
LT/LT 0505 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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