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Texas Instruments Use of an LDO as a Load Switch for Space Applications Application notes
Application Report
SLVA894 – August 2018
Use of an LDO as a Load Switch for Space Applications
Hollis Liu, Hector Torres and Javier Valle
ABSTRACT
This application note highlights the similarities and differences of function and architecture between load
switches and low-dropout (LDO) regulators. It analyzes and provides a specific configuration to use the
TPS7H1101A-SP space rated low dropout linear regulator as a load switch and presents data using this
configuration.
1
2
3
4
5
6
Contents
Introduction ...................................................................................................................
Setup ..........................................................................................................................
Results ........................................................................................................................
Current Limit ..................................................................................................................
Conclusions...................................................................................................................
References ...................................................................................................................
2
3
3
7
8
9
List of Figures
1
Typical Load Switch (left) and LDO (right) Block Diagrams............................................................ 2
2
EVM Modifications Used to Test the TPS7H1101A-SP as a Load Switch ........................................... 3
3
Dropout Voltage Measurements Across Load and Temperature...................................................... 4
4
On-State Resistance Across Load and Temperature ................................................................... 4
5
3-A Step Response .......................................................................................................... 5
6
Soft Start Behavior With No Load ......................................................................................... 6
7
Soft Start Behavior With a 3-A Load ...................................................................................... 6
8
Lack of Soft Start Behavior at 3 A Load with FB Pin Grounded ....................................................... 7
9
Current Limit Test. The Device Was Programmed With a Current Limit of About 4 A. Current Foldback
Feature Can Be Observed .................................................................................................. 8
List of Tables
Trademarks
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Use of an LDO as a Load Switch for Space Applications
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1
Introduction
1
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Introduction
The load switch is usually used in the power path to control whether the power supply and load be
connected by an external enable or disable signal. When the device is enabled via the enable pin, the
pass FET turns on, thereby allowing current to flow from the input pin to the output pin, and power is
passed to the downstream circuitry. Essentially, most of the load switches have the same structure, a
pass FET plus a driver circuit connected to the FET’s gate to turn on or turn off the pass FET.
The pass FET is the main component of the load switch, which determines the maximum input voltage
and maximum load current the load switch can handle. The on-resistance of the load switch is a
characteristic of the pass FET and will be used in calculating the power dissipated by the load switch [1].
The type of the pass FET determines the architecture of the load switch. A NMOS load switch requires a
charge pump to bring the gate voltage above the source voltage which is usually the Vin, while the PMOS
based architecture doesn’t need that. However, the NMOS has a better performance when the input
voltage is low.
In some advanced load switches, there are some other features, such as soft start to avoid inrush current,
over-temperature shutdown, current limiting to protect device or programmable fault timer for automatic
restart of the device after a fault.
A load switch has some similarities with a LDO as both make usage of a large pass transistor. However,
the LDO provides regulation of the output voltage while the load switch does not. In a standard LDO
application, the input voltage is larger than the regulated output voltage. In this condition, the LDO
regulates normally due to the feedback loop. Even though an LDO is not originally intended or optimized
to be used as a load switch, due to their similarities, the LDO could be used as a load switch. However,
the LDO has to be set out of regulation mode, in other words, set it in dropout mode [4].
Figure 1 shows a basic topology comparison between an LDO and a load switch. From this figure we
notice that the main difference is the feedback loop part of the LDO. When the LDO regulates, the
feedback loop adjusts the Vgs value in response to changes in the output voltage. However, in this
application where an LDO is being used as a load switch, the LDO is forced to be in dropout mode so that
the feedback loop is not active (no regulation) and only the gate drive is present.
VOUT
VIN
VIN
Feedback Loop
VOUT
VREF
+
±
GATE
DRIVER
GND
GATE
DRIVER
Figure 1. Typical Load Switch (left) and LDO (right) Block Diagrams
In this application note, we focus on the TPS7H1101A-SP, a space rated LDO, used as a load switch. The
TPS7H1101A-SP is a very low minimum input voltage (1.5 – 7 VIN), 3-A linear regulator. Some of the
features of this device are enable pin, soft start, power good, thermal shutdown, current limit and output
current sensing. All these features are also applicable and desired in load switch applications.
2
Use of an LDO as a Load Switch for Space Applications
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Setup
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2
Setup
The TPS7H1101SPEVM was used to test this application. Figure 2 shows a schematic of the
configuration.
TPS7H1101A-SP
Power
VIN
Good
VIN=4.943 V
CIN
Rcs
COMP
EN
RTOP
76.8 kŸ
PCL
Soft
Start
RPCL
VOUT
VOUT
CS
GND
COUT
Feed
Back
CSS
39 nF
RBOTTOM
10.5 kŸ
Figure 2. EVM Modifications Used to Test the TPS7H1101A-SP as a Load Switch
In normal operation of LDO, the desired output voltage is calculated using Equation 1 where the typical
value for VFB = 0.605 V. In this case, VIN – VOUT > VDROP_MAX allows the LDO to regulate normally.
‚{•€_upqmx =
:~€{| + ~n{€€{y ; × ‚rn
~n{€€{y
(1)
As mentioned earlier, to be used as a load switch, the LDO needs to be set in dropout mode and there are
2 options for this. The first option as mentioned in the E2E post [4] is to connect the feedback pin to
ground. The downside of this option is that the soft start feature of this device is no longer present as the
feedback is not able to track the reference voltage. The second option is to set the ideal output of the LDO
slightly larger than the input voltage to stop the feedback loop from taking over but still allowing the
feedback to track the reference at startup or when the LDO is enabled. A typical dropout value for the
TPS7H1101A-SP is 210 mV at a 3-A load and therefore, setting the input voltage 50 mV lower than the
output voltage would work for this purpose.
For this particular test, the feedback resistors shown in Figure 2 provide a VOUT = 5.03 V theoretically,
while the measured VOUT value is actually 4.993 V. Therefore, in order to set the LDO in dropout mode,
the input voltage is set to 4.943 V (50 mV below the measured output voltage). This way the LDO will
operate as a load switch without feedback loop.
3
Results
Due to their typical different applications, load switches usually have lower FET on-resistance than LDOs.
For this specific application of the TPS7H1101A-SP, the dropout and load step measurements can help us
evaluate the on-resistance. As mentioned above, the soft start function is high desired in load switches to
minimize inrush current so it will be tested for this specific application as well. The results of these tests
follow.
3.1
Dropout Test
The curves in Figure 3 show the dropout voltage of the pass MOSFET as a function of the load current
across temperature. The slope of the curves is the RDS_ON as shown in Figure 4. It can be observed how
the maximum RDS_ON is 90 mΩ under worst conditions. A thermal stream was used for the temperature
measurements and an electronic load was used for the dropout voltage measurements.
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3
Results
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Dropout Voltage vs Load Current
275
250
Dropout Voltage (mV)
225
200
175
150
125
100
75
50
-55C
25C
125C
25
0
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
Load current (A)
D001
Figure 3. Dropout Voltage Measurements Across Load and Temperature
Rds_on vs Load Current
95
90
85
Rds_on (m:)
80
75
70
65
60
55
50
-55C
25C
125C
45
40
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
Load Current (A)
3
D002
Figure 4. On-State Resistance Across Load and Temperature
From Figure 4, it can be concluded that the fairly linear behavior of the MOSFET RDS_ON indicates how the
device is behaving like a resistor as intended.
3.2
Load Step Response
A resistive load was used to perform load step testing. Figure 5 shows a 3-A load step response
performed at room temperature. It can be observed that the dropout is about 200 mV agreeing with the
25°C curve shown in Figure 5.
4
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Results
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VIN
VOUT
IOUT
3A
0A
Figure 5. 3-A Step Response
3.3
Soft Start
A very important feature in load switches is the soft start capability to avoid large inrush currents. Figure 6
and Figure 7 show the soft start function of the LDO as load switch at room temperature. The soft start
capacitor size is 39 nF, which corresponds to a theoretical soft start time of about 9.4 ms based on
Equation 2 where the typical values of ISS and VFB are 2.5 µA and 0.605 V.
=
ñ ×÷
ôð
(2)
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Results
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VIN
EN
VOUT
IOUT
Figure 6. Soft Start Behavior With No Load
VIN
EN
VOUT
IOUT
Figure 7. Soft Start Behavior With a 3-A Load
As mentioned earlier, grounding the feedback pin would not allow the LDO as a load switch to use the soft
start feature. Figure 8 below shows how the soft start of about 7.2 ms in Figure 7 is no longer present if
the feedback pin is grounded.
6
Use of an LDO as a Load Switch for Space Applications
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Current Limit
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VIN
EN
VOUT
IOUT
Figure 8. Lack of Soft Start Behavior at 3 A Load with FB Pin Grounded
4
Current Limit
Another important feature of load switches is the current limit feature. The TPS7H1101A-SP offers this
feature and when used as load switch the feature can be used. In addition to current limit, the
TPS7H1101A-SP offers current foldback. This feature takes place once current limit is triggered and
remains there while the output starts drooping. After the output voltage falls below an internal threshold
value, the output current will foldback to approximately half the programmed current limit as described in
the TPS7H1101A-SP data sheet. At this point, the output of the LDO, or load switch in this case, will
collapse to a few hundred mV. Figure 9 below shows this application with a programmed current limit of
about 4 A. The current foldback (about 2 A) and the output reaching ground can be observed as well. This
is a very important protection feature of the LDO as a load switch.
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Conclusions
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VIN
IOUT
VOUT
Figure 9. Current Limit Test. The Device Was Programmed With a Current Limit of About 4 A. Current
Foldback Feature Can Be Observed
5
Conclusions
This application note has successfully demonstrated how to use the TPS7H1101A-SP as a load switch
and presented testing data for critical parameters and features of a typical load switch. Ideally, a device
designed merely for load switch applications would be able to provide lower RDS_ON as well as additional
features. However, this application note intends to highlight an additional usage of the TPS7H1101A-SP
that could be useful for certain applications.
8
Use of an LDO as a Load Switch for Space Applications
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References
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6
References
(1) Basics of Load Switches, SLVA652
(2) Understanding Low Drop Out (LDO) Regulators, SLUP239
(3) TPS7H1101A-SP Data Sheet, SLVSDW6
(4) "How to use an LDO as a load switch", E2E Post
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