AN90017

Load switches for mobile and computing applications
Rev. 1.0 — 2 September 2020

application note

Document information
Information

ContentKeywords

Load switch, MOSFET, mobile, computing

Abstract

This application note presents simple load switches in mobile and computing applications.

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Nexperia

Load switches for mobile and computing applications

1. Types of load switches
There are several reasons why a circuit or subsystem is required to be disconnected from thepower supply using a load switch. A very simple and very common reason is that it helps saving
power. An unpowered subsystem can eliminate power consumption due to leakage or standby
currents. In portable electronic devices, load switches can be used to prevent damage fromelectrical surges, incorrect battery insertion and other damaging events that can enter through thepower source.
Before diving deeper into the key parameters, let’s look at the different types of load switches. A
high-side load switch connects or disconnects a power source to a load. The switch is controlled by
an external enable signal. High-side switches source current to a load. Low-side switches connector disconnect the load to ground, thus sinking current from the load.
A load switch can be easily implemented with a MOSFET pass transistor. The MOSFET passes
current from the power source to the load and is turned on or off via a control signal. Providing thecontrol signal to the MOSFET, a gate-drive circuit connects to the MOSFET’s gate to switch theMOSFET on or off.

2. P-channel MOSFET load switch
Using a P-channel MOSFET a high-side load switch can be implemented by connecting the input
voltage to the MOSFET source and the load to the MOSFET drain; see Fig. 1. Pulling the gate low
will enable current to flow into the load.

P-channel MOSFET
VinenableLoadaaa-032106

Fig. 1.

P-channel MOSFET load switch

3. N-channel MOSFET load switches
N-channel MOSFETs have lower on-state resistance values than P-channel devices of thesame size. However, to achieve the lower resistance values, a high voltage is needed to drive aMOSFET’s gate in the implementation of a high-side load switch using an N-channel MOSFET,
see Fig. 2. A voltage higher than the input voltage Vin has to be provided for correct gate drive,otherwise the MOSFET would not turn on fully and will operate in linear mode with high powerlosses.

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Load switches for mobile and computing applications

N-channel MOSFET
Input
Load

HV control line

aaa-032107

Fig. 2.

N-channel MOSFET load switch with high voltage control line

If a high enough voltage to drive the gate is not available, a charge-pump circuit can be used toincrease the drive voltage applied to the MOSFET’s gate, see Fig. 3. While this increases thecomplexity of the circuit, it is rewarded with the lower ON resistance of the N-channel MOSFET.
However a charge-pump circuit will disipate some power so that in very energy critical applications
that may stay most of the time in stand-by mode, a P-channel topology can be more efficient.

N-channel MOSFETVinChargepumpLoadenableaaa-032108

Fig. 3.

N-channel MOSFET load switch with charge pump

Without high voltage or additional circuitry, the N-channel MOSFET can be used in a low-side loadswitch. This implementation of a low-side load switch is shown in Fig. 4. A downside of the lowside load switch is the slight lifting of the ground potential of the load. This needs to be consideredespecially if the load has communication lines with external components.

VinLoadenableN-channel MOSFET

aaa-032109

Fig. 4.

N-channel MOSFET load switch low-side load switch

More advanced load switch features include reverse voltage protection, reverse current protection.
These can be implemented with MOSFETs in common-drain or common-source configuration asshown in the next section.

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Load switches for mobile and computing applications

4. Reverse current protected load switches
Load switches based on a single MOSFET topology can block current in one direction only. AsMOSFETs have an inherent body diode they act like a diode in off-state if driven reverse. In someapplications it is required to be able to block current flow in both directions.
Applications, that need blocking of reverse currents are e.g. battery driven applications, where
discharge of the battery shall be prevented for fault conditions like a short at the charger connector
side or in case of electrical failure causing leakage in connected cables and AC adapters.
A common way to block reverse currents is by applying diodes. However, this function can berealized far more efficiently with MOSFET load switches. In order to achieve blocking for bothcurrent directions, two MOSFETs have to be put in series with reverse polarity to each other. In
this case one of the body diodes can block current flow if not both MOSFETs are turned on. Thisapproach allows to create two alternative topologies of back-drive protected load switches. Either
the drain contacts or the source contacts can be connected.

P-channel MOSFETs
VinLoadenableaaa-032110

Fig. 5.

Reverse current protected common drain load-switch

Control of the back-drive protected load switches can be done with separate gate drive orconnected gates. The common drain topology, (see Fig. 5), has the advantage for a semiconductor
component that the drain contact is located towards the substrate side. So, the common drain
can be attached directly to the lead frame with a good thermal path into the PCB. For mobile
applications often Chip-Scale Packages (CSP) can be found. Here the source and gate contacts
are facing down to the PCB whereas a drain contact needs an additional down contact. If there isno need to make the drain accessible necessarily. In this case a CSP design becomes very space
efficient because no chip area needs to be sacrificed for down connections of drain.
Back Metal

RDSon

RBDSonRBDDSD

Chip 145 µm

Solder ball
aaa-032114

Fig. 6.

Turn-on resistance (RDSon) of a single Chip-scale Package MOSFET
ISSon

S1

Back Metal
Chip 100 -200 µm
Polyimide
S2
Bump Cu/Sn
aaa-032115

Fig. 7.

AN90017application noteTurn-on resistance (RSSon) of a common-drain Chip-scale Package MOSFET

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Load switches for mobile and computing applications
The common source topology requires an access to the source connection between the MOSFETs.
The gate has to be discharged after a turn on, see Fig. 8. This does not work without access to thisnode as the common sources might float.

P-channel MOSFETs
VinLoadenableaaa-032111

Fig. 8.

Reverse current protected common source load-switch

5. Reverse polarity protection and supply OR-ing
In cases where no load switching is needed a single MOSFET can be used for reverse polarity
protection only, see Fig. 9. The MOSFET can replace simple diodes e.g. in supply OR-ing
applications this has the advantage of a lower voltage drop across the MOSFET compared to thevoltage drop across the diode.

P-channel MOSFET
VinLoadaaa-032112

Fig. 9.

Reverse current and reverse polarity protection with a P-channel MOSFET

Another important application of back-drive load-switches is power OR-ing, see Fig. 10. If a circuit
is able to be powered by one of several power supplies then current flow back into a non-selectedsupply has to be avoided.

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Load switches for mobile and computing applications

P-channel MOSFETs
Vin1

internal supply

Vin2

Or-ing / supply
priority circuit
aaa-032113

Fig. 10. Power supply OR-ing with P-channel MOSFETs

6. Important parameters
Important key parameters of a load switch are the on-state resistance (RDSon) of the MOSFET thatconnects between the voltage input and voltage output pins, the maximum current (ID(max)) that
the MOSFET can conduct and the maximum voltage (VDS(max)) that the circuit can withstand. Thelower the on-state resistance, the lower the power dissipation of the transistor and also the lower
the voltage drop from input to output.
While RDSon, ID(max), and VDS(max) are parameters of the MOSFET, the maximum voltage drop andthe maximum power dissipation of a load switch can be calculated for a given current I with theformulas below:

Vdrop = I × RDSon(max)(1)

P = I × Vdrop = I 2 × RDSon(max)(2)

Today’s MOSFETs typically have on-state resistance values in the tens of milliohms, so, forexample, if the load switch has an ON resistance of 50 mΩ and controls a 200 mA load, theMOSFET dissipates just 2 mW when ON, and has an input-to-output voltage drop of 10 mV. Evena peak current of 1 A would only cause a voltage drop of 50 mV and a peak power dissipation of50 mW.
As load switch circuits are active whenever the power is on they are designed to have low leakage
currents, see Nexperia application note AN90009.

7. Revision history
Table 1. Revision history
Revision
Date
numberDescription

1.0

Initial version.

20200902

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Load switches for mobile and computing applications8. Legal information
Definitions
Draft — The document is a draft version only. The content is still underinternal review and subject to formal approval, which may result in
modifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

Disclaimers
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to be accurate and reliable. However, Nexperia does not give any
representations or warranties, expressed or implied, as to the accuracy
or completeness of such information and shall have no liability for theconsequences of use of such information. Nexperia takes no responsibility
for the content in this document if provided by an information source outsideof Nexperia.
In no event shall Nexperia be liable for any indirect, incidental, punitive,
special or consequential damages (including - without limitation - lostprofits, lost savings, business interruption, costs related to the removalor replacement of any products or rework charges) whether or not suchdamages are based on tort (including negligence), warranty, breach ofcontract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, Nexperia’s aggregate and cumulative liability towards customer
for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
Right to make changes — Nexperia reserves the right to make changes
to information published in this document, including without limitation
specifications and product descriptions, at any time and without notice. This
document supersedes and replaces all information supplied prior to thepublication hereof.
Suitability for use — Nexperia products are not designed, authorized or
warranted to be suitable for use in life support, life-critical or safety-criticalsystems or equipment, nor in applications where failure or malfunctionof an Nexperia product can reasonably be expected to result in personal
injury, death or severe property or environmental damage. Nexperia and its
suppliers accept no liability for inclusion and/or use of Nexperia products insuch equipment or applications and therefore such inclusion and/or use is at
the customer’s own risk.
Applications — Applications that are described herein for any of theseproducts are for illustrative purposes only. Nexperia makes no representation
or warranty that such applications will be suitable for the specified use
without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using Nexperia products, and Nexperia accepts no liability forany assistance with applications or customer product design. It is customer’ssole responsibility to determine whether the Nexperia product is suitableand fit for the customer’s applications and products planned, as well as
for the planned application and use of customer’s third party customer(s).
Customers should provide appropriate design and operating safeguards tominimize the risks associated with their applications and products.
Nexperia does not accept any liability related to any default, damage, costs
or problem which is based on any weakness or default in the customer’s
applications or products, or the application or use by customer’s third party
customer(s). Customer is responsible for doing all necessary testing for thecustomer’s applications and products using Nexperia products in order toavoid a default of the applications and the products or of the application or
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Trademarks
Notice: All referenced brands, product names, service names andtrademarks are the property of their respective owners.

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Load switches for mobile and computing applications

List of Tables
Table 1. Revision history......................................................6

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Load switches for mobile and computing applications

List of Figures
Fig. 1. P-channel MOSFET load switch............................... 2Fig. 2. N-channel MOSFET load switch with highvoltage control line............................................................... 3
Fig. 3. N-channel MOSFET load switch with charge pump.. 3
Fig. 4. N-channel MOSFET load switch low-side load
switch....................................................................................3Fig. 5. Reverse current protected common drain loadswitch....................................................................................4Fig. 6. Turn-on resistance (RDSon) of a single Chipscale Package MOSFET......................................................4Fig. 7. Turn-on resistance (RSSon) of a common-drain
Chip-scale Package MOSFET............................................. 4Fig. 8. Reverse current protected common sourceload-switch............................................................................5Fig. 9. Reverse current and reverse polarity protection
with a P-channel MOSFET.................................................. 5
Fig. 10. Power supply OR-ing with P-channel MOSFETs.... 6

AN90017application note

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Rev. 1.0 — 2 September 2020

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Load switches for mobile and computing applications

Contents
1. Types of load switches................................................ 2
2. P-channel MOSFET load switch.................................. 2
3. N-channel MOSFET load switches..............................2
4. Reverse current protected load switches...................4
5. Reverse polarity protection and supply OR-ing.........5
6. Important parameters...................................................6
7. Revision history............................................................68. Legal information......................................................... 7© Nexperia B.V. 2020. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 2 September 2020

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