AN53 - Micropower High Side MOSFET Drivers

1 Application Note 53AN53-1 Micropower High side MOSFET DriversTim SkovmandFigure 2. Logic-Level and Standard N-channelMOSFET VGS RatingsAN53 TA02 RDS (ON) RATED AT VGS = 10V. MAX VGS = N- channel MOSFET SWITCHRDS (ON) RATED AT VGS = 4V. MAX VGS = N- channel MOSFET SWITCHP ortable electronic equipment, such as notebook andpalmtop computers, portable medical instruments andbattery powered tools, are increasingly dependent uponefficient power management to meet the challenge ofextracting more useful energy (time) from less batteryvolume and link in the power management chain which hasreceived increasing scrutiny, as power supply efficienciessoar above 90%, is the logic controlled power sections of a typical notebook computer system, forexample, are powered via topside or high side MOSFET switches.

2 These switches can become significant sourcesof power loss if not properly first glance, P- channel MOSFETs appear to be thenatural choice for high side switching. Unfortunately, theRDS(ON) exhibited by most P-channels is prohibitivelyhigh. (Mother Nature has decreed that electron mobilityshall exceed hole mobility in silicon by about times, sothat a P- channel MOSFET with the same RDS(ON) andvoltage rating as its N- channel counterpart is roughly 2 to3 times larger and more expensive.) Also, the gate drive fora P- channel switch is limited to the supply voltage whichmay not fully enhance the switch as the supply MOSFETs may seem less attractive becausethey require a gate voltage higher than the power supplyvoltage to become fully enhanced in high side switchingapplications. This limitation is eliminated by high sideMOSFET Drivers such as the LTC1155, which have built-in charge pumps to fully enhance N- channel LTC1155, dual Micropower MOSFET driver, gener-ates 12V from a 5V rail to fully enhance logic-level N- channel switches with no external components required(see Figure 1).

3 Further, the supply current is typically 85 Awith the switch fully enhanced and 8 A with the LTC1155in the standby mode (both inputs off). This combination oflow-drop N- channel MOSFET switch and micropowerFigure 1. High Efficiency Dual High side Switchdriver is the most efficient means of powering complexelectrical loads. Switch efficiencies in the 98% to 99%+range are easily attained with practical and economicN- channel SWITCH SELECTIONN- channel MOSFET switches fall into two main categories:logic-level and MOSFET SwitchesAlthough there is some variation among manufacturers,logic-level MOSFET switches are typically rated with VGS= with a maximum continuous VGS rating of (ON) and maximum VDS ratings are similar to standardMOSFETs and there is generally little price MOSFETs are frequently designated by an L and are usually available in surface mount 1993AN53 TA01DS1 VSDS2 LTC1155 GNDG1G2IN1IN25V LOAD5V LOADON/OFFON/OFFIRLR024 MTP3055EL5V(12V)(12V)Application Note 53AN53-2 Standard MOSFET SwitchesStandard N- channel MOSFET switches are rated with VGS= 10V and are generally restricted to a maximum of , there is some variation among MOSFET manufac-turers and individual data sheets should be consultedbefore making a final selection.

4 ( , MOSFETs with 30 Vmaximum VGS ratings can be used without VGS voltageclamps.) MOSFET /Driver Selector GuideTable 1 is a guide which simplifies the selection of aMOSFET switch and Micropower driver for a particularsupply voltage range. A family of Drivers , including asingle, dual and quad, are available for operation in to 18V range. A related device, the LTC1153, elec-tronic circuit breaker, also operates in the to LTC1157, dual Micropower MOSFET driver, isdesigned specifically for low voltage operation and Finally, the LTC1255, dual high side MOSFET driver, is designed to work in the 9V to 24V automotive andindustrial driver works with either logic-level or standardMOSFETs over a portion of the supply voltage range andis designed to work with 12V VGS Zener clamp diodeswhen the supply range exceeds 9V as shown in Figure Large Capacitive LoadsElectrical subsystems in portable battery powered equip-ment are typically bypassed with large filter capacitors toreduce supply transients and supply induced glitching.

5 Ifnot properly switched however, these capacitors maythemselves become the source of supply induced example, if a 100 F capacitor is powered through aP- channel switch as shown in Figure 4, and the slew rateof the switch is s, the current during start-up is:ISTART= C(dV/dt)= (100 x 10-6)(1 x 105)= 10 AObviously, this is too much current for the regulator tosupply and the output glitches by many volts!Table 1. MOSFET /Driver Selector GuideDEVICEDESCRIPTIONSUPPLY RANGEUSE LL FETUSE STD FETSTD FET & 12V CLAMPLTC1153 Electronic Circuit 9V9V 18 VLTC1154 Single Micropower MOSFET 9V9V 18 VLTC1155 Dual Micropower MOSFET 9V9V 18 VLTC1156 Quad Micropower MOSFET 9V9V 18 VLTC1157 Dual high side /Low side Industrial MOSFET Driver9V 24 VNANA9V 24 VFigure 4. Power Up Supply Glitch Produced by FastStarting a Large Capacitive LoadAN53 TA03DS1 VSDS2 LTC1155 GNDG1G2IN1IN2>9V LOAD>9V LOADON/OFFON/OFFSTANDARD N- channel MOSFET >9 VSTANDARD N- channel MOSFET12V*12V** 1N5242B (THROUGH HOLE) OR MMBZ5242B (SURFACE MOUNT) ZENERSF igure 3.

6 Adding 12V VGS Clamps when VS > 9V+++P- channel SWITCHOUTCLOAD 100 F1 F( )SUPPLY GLITCH> FLT1121-5AN53 TA04 Application Note 53AN53-3+++MTP3055 ELCLOAD 100 F1 F> FLT1121-5AN53 TA05 LTC1154 GSD+C1 FOUTDSVSSTATUSGNDENINR1 100kR2 1kFigure 5. Slew Rate Reduction Network for Powering Large Capacitive LoadsThe start-up current can be substantially reduced byreducing the slew rate at the gate of an N- channel switchas shown in Figure 5. The gate drive output of the LTC1154,single Micropower MOSFET driver, is passed through asimple RC network, R1 and C1, which substantially slowsthe slew rate of the MOSFET gate to approximately x10-4V/ s. Since the MOSFET is operating as a sourcefollower, the slew rate at the source is essentially the sameas that at the gate, reducing the start-up current toapproximately 15mA which is easily managed bythe system regulator.

7 R2 is required to eliminate thepossibility of parasitic MOSFET oscillations during switchtransitions. Also, it is good practice to isolate the gates ofFigure 7. Bidirectional Switch Using Two Back-to-Back MOSFETsC1 100 FVIN 6V-18 VAN53 TA07 LTC1154 GSDDSVSSTATUSGNDENIND2 1N5242 LOADSi9956 DYD3 1N41485,6,7,8Q1Q21342R4 100kR2 10kR1 1kR3 91k+D1 1N58174-10 CELLS+VIN 6V-18 VAN53 TA064-10 CELLS100 FS1++LOADF igure 6. Switched Battery Applicationparalleled MOSFETs with 1k resistors to decrease thepossibility of interaction between SwitchSometimes it is necessary to use back-to-back MOSFET switches to completely isolate the power source from theload, or from another power source, when the switch isturned off. This is the case when the supply voltage ishigher or lower than the load voltage when powered by asecondary switched battery application, as shown in Figure 6,illustrates a bidirectional (fully isolated) switch.

8 When thewall unit power supply is connected to VIN, the load isdisconnected from the battery by a fully isolated switchwhich allows the load voltage to fluctuate above or belowthe battery voltage without forcing current into the batteryor pulling current out of bidirectional battery switch shown in Figure 7 illus-trates how the LTC1154 drives two back-to-back lowApplication Note 53AN53-4 RDS(ON) N- channel MOSFETs, Q1 and Q2, to fully discon-nect the battery from the load immediately after the wallunit power supply is connected to VIN. The two bodydiodes in Q1 and Q2 are also connected back-to-back and therefore no current can flow through the switch whenthe gate drive is LTC1154 ENABLE input senses when the wall unitvoltage exceeds 3V and inverts the action of the switch sothat the two MOSFETs are turned off when the wall unitpower supply is connected.

9 The battery is subsequentlyreconnected immediately after the wall unit power supplyis disconnected. D2 and D3 are only required for batteryvoltages above 9V and limit the gate drive voltage to theMOSFET switches to 12V above the battery voltage. C1supplies load current during the short period of time (tensof microseconds) when the wall unit is disconnected andthe battery switch is turned back on. R1 acts as a bleedresistor to ensure that the VIN line is pulled down quicklyafter the the wall unit is 28V OperationAlthough designed for operation in the to 18V range,the LTC1154/LTC1155/LTC1156 family of Drivers can beoperated in the 18V to 28V range by clamping the supplypin to 18V as shown in Figure 8. These Drivers typicallyproduce 36V of gate drive from an 18V supply which fullyenhances an N- channel MOSFET switch operating from18V to 28V.

10 (12V Zener clamps should be added to ensurethat the maximum MOSFET VGS is never exceeded.)Bootstrapped OperationThe circuit shown in Figure 9 should be used if mi-cropower standby operation is required. The standbysupply current is reduced to < 30 A by increasing thevalue of R1 from 3k to 330k and adding a bootstrap network, R2 and D2, from each switch output to the supplypin. In this way, the extra supply current is provided onlywhen the switch is turned ON. The supply current dropsback to 30 A when the switch is turned OFF and theLTC1155 returned to the standby TA09DS1 VSDS2 LTC1155 GNDG1G2IN1IN218V-28V LOAD18V-28V LOADON/OFFON/OFFD218V-28V12V12V10 F18VR1 330k+R2 3kD2R2 3kFigure 9. Bootstrapping the SupplyMOSFET SWITCH PROTECTIONC ontrary to popular belief, power MOSFETs are not inde-structible.