Designing with Si9976DY N-Channel Half-Bridge Driver LITTLE FOOTR Dual
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AN709
Designing with Si9976DY N-Channel Half-Bridge Driver LITTLE FOOTR Dual MOSFETs
Wharton McDaniel
INTRODUCTION
Si9976DY fully integrated half-bridge driver which designed work with LITTLE FOOT family power MOSFET products 40-V systems. Si9976DY provides gate drive both low- high-side MOSFETs while Si9945 (SO-8, Si4946EY (SO-8, dual n-channel LITTLE FOOT MOSFETs provide power handling capability without need heatsink. these devices supplied surface-mount packages. combination Si9976DY dual n-channel MOSFETs creates powerful flexible solution power switching motor drives.
provide logic signal compatibility hysteresis noise immunity. impedance outputs provided drive both low- high-side MOSFETs half-bridge. addition bootstrap capacitor allows internal circuitry level shift both power supply logic signals that required high-side n-channel MOSFET gate drive. charge pump been included replace leakage current high-side driver, which allows static (dc) operation. separate voltage input, VCC, powers FAULT output allow easy interfacing user's system. Protection circuits include undervoltage lockout assure safe gate-drive levels, timing delays prevent cross-conduction, monitor short circuits half-bridge output (S1). internal voltage regulator drops input voltage (V+) nominal low-side circuitry, which allows Si9976DY operate over input voltage range device specified over industrial temperature range (-40_ +85_C).
SI9976DY OVERVIEW
Si9976DY integrated driver n-channel MOSFET half-bridge (see Figure Schmitt trigger inputs
Voltage Regulator Under Voltage Lockout
Bootstrap Regulator
Under Voltage Lockout
CBoot
Charge Pump
Half-Bridge Output LITTLE FOOT
Short Detect
FAULT
0.01
Delay
Delay
Substrate
Enable Latch
Figure Si9976DY Functional Block Diagram
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INPUT VOLTAGE REQUIREMENTS
Si9976DY operates from single supply voltage This voltage feeds both bootstrap low-voltage regulators. bootstrap voltage regulator charges bootstrap capacitor, while low-voltage regulator drops input voltage nominal low-side logic output drive low-side MOSFET. FAULT output used, separate voltage (4.5 must applied pin. This guarantees compatibility with logic levels motor controller. charge while charge pump provides leakage current allow static operation. Because bootstrap supply used, bootstrap capacitor must charged immediately after power then recharged after every high-side turn Likewise, low-side MOSFET must turned complete charging circuit bootstrap capacitor. Some drive schemes toggle between bottom MOSFETs, which accomplishes required charge recharge bootstrap capacitor automatically. important understand that charge pump operates only when high-side turned bootstrap capacitor provides charge that turns high-side MOSFET. This capacitor should sized such that will hold times charge required turn MOSFET fully (i.e., typical capacitor value calculated using equation CBOOT (Qg/VGS). value taken from gate charge curve MOSFET being driven Using this method capacitor selection, bootstrap voltage will drop approximately when MOSFET turned 0.018-mF capacitor works well Si9945DY, which requires 15-nC charge turn with certain minimum recharge time required bootstrap capacitor after each high-side turn-on. recharge time function amount charge which been used turn high-side MOSFET, size bootstrap capacitor, drain current bootstrap transistor Si9976DY. case Si9976DY, recharge time decreases increases. Part this decrease contribution charge pump recharging bootstrap capacitor. increases, charge pump contribution increases. some cases, charge pump becomes only source charge required recharge bootstrap capacitor.
OUTPUT DRIVE DETAILS
unique feature Si9976DY integral high-side drive circuitry. This includes logic-signal level shifting, bootstrap power supply, charge pump, undervoltage lockout, 40-mA output driver. bootstrap supply charge pump comprise high-side power supply, utilize benefits each technique. itself, bootstrap supply provides sufficient charge MOSFET turn-on. However, drawbacks when used alone. First, bootstrap capacitor must recharged after every MOSFET turn-on. Second, bootstrap supply cannot sustain MOSFET state indefinitely because gate leakage current continues deplete charge bootstrap capacitor. charge pump meanwhile, provide continuous source charge, fully integrated form cannot provide sufficient charge MOSFET turn-on typical modulation frequencies. Combining techniques solves these problems. bootstrap supply provides turn-on
Bootstrap Regulator
Under Voltage Lockout
Charge Pump High-Side Logic From High-Side Logic
Figure High-Side Drive
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TABLE RECOMMENDED VALUES
Part Number
Si4946EY Si9945
High Side Logic
rDS(on)
0.055 0.10
(nC)
Minimum Recommended CBOOT (mF)
0.039 0.018
Delay
Delay
Table shows selected bootstrap capacitor each MOSFET selected using method described, with switching frequency kHz.
Side Logic
shorter recharge time required, external signal diode added from positive side bootstrap capacitor (CAP). This increases charging current, especially lower values Also, value capacitor should increased, since this source additional charging current. low-side drive circuitry operates directly from does have recharge requirements. capacitor connected supplies charge required turn low-side MOSFET. must sized ensure that does drop below which would trigger undervoltage condition. case bootstrap capacitor, bypass capacitor should sized such that will hold times charge required MOSFET Qg/VGS). Si9945 requires 15-nC charge turn with Therefore, 0.018 capacitor will work well. Since requirements value selection same bootstrap capacitor, recommended values Table also apply bypass capacitor. external bootstrap diode used reduce bootstrap capacitor recharge time, value bypass capacitor should doubled. This compensates additional load recharging bootstrap capacitor prevents occurrence undervoltage condition.
Figure Cross Conduction Protection
UNDERVOLTAGE LOCKOUT
During power both MOSFETs held until internal power supply, VDD, within approximately final value, which nominally After power low-side undervoltage lockout circuitry, UVL2, continues monitor VDD. undervoltage condition occurs, both high-side low-side MOSFETs will turned off, FAULT output will high. When undervoltage condition longer exists, FAULT output will cleared normal function will resume. separate undervoltage lockout circuit, UVL1, monitors bootstrap voltage. undervoltage condition exists when line switched high, this circuit will prevent high-side MOSFET from turning addition, following conditions will exist. high result inductive flyback current through high-side MOSFET's body-drain diode short from V+), high-side MOSFET will allowed turn soon undervoltage condition been removed. low, high-side MOSFET will allowed turn only after undervoltage condition been removed line been toggled back high.
CROSS CONDUCTION PROTECTION
Turn-on delays have been incorporated prevent cross conduction half-bridge MOSFETs (Figure high-side MOSFET turned only after 250-ns time delay, which initiated low-side output, switching ground. low-side MOSFET turned only after 300-ns delay which initiated high-side control logic. These delays prevent half-bridge MOSFET from turning before other completely turned off. difference method generating delays occurs because high-side output, level shifted with respect
SHORT CIRCUIT PROTECTION
load voltage, does make intended transition through either ground before specified time, Si9976DY sees this output short circuit (Figure transition should take place less than transition transition ground. Detection short circuit condition latches both outputs fault line high. outputs re-enabled rising edge enable line,
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Short Circuit Detect Transition Window (300
Window Window
Transition Window (200
Figure Short Circuit Protection
FAULT OUTPUT
FAULT output goes high whenever Si9976DY detects output short circuit undervoltage condition. detection short circuit inhibits operation sets fault latch which cleared rising edge enable line, undervoltage condition inhibits operation indicates fault nonlatching.
duty cycle defines zero full speed direction, duty cycle zero speed, 100% duty cycle defines zero full speed opposite direction. This approach ensures that bootstrap capacitor always charged, since H-bridge continuously switching. basic hook-up anti-phase H-bridge very simple. half-bridge driven directly with signal, other half-bridge driven with inverse signal (see Figure
TABLE FAULT OUTPUT TRUTH TABLE
Condition
Normal Operation Normal Operation Disabled Load Shorted Load Shorted Ground Undervoltage CBOOT Undervoltage CBOOT Undervoltage
FAULT Output
High
High High
Si9976
Dual LITTLE FOOT MOSFET Dual LITTLE FOOT MOSFET
Si9976
Figure Anti-Phase Control
system-logic supply voltage 16.5 applied facilitate interfacing FAULT output user's system. supplied, there will signal FAULT output. However, fault protection circuitry will continue function described.
Sign-Magnitude Control secondary function, Si9976 used sign-magnitude controls. this approach, direction rotation determined diagonal pair MOSFETs that turned speed controlled pulse width modulation active diagonal pair. logic required control H-bridge more complex need steer pulse width modulation signal active MOSFET pair. circuit Figure applies signal only low-side active MOSFET.
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CIRCUITS BRIDGES
Anti-Phase Control Si9976 designed used anti-phase control strategy. This approach unique that signal controls both speed direction with duty cycle alone. Zero
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Si9976
Dual LITTLE FOOT MOSFET Dual LITTLE FOOT MOSFET
Si9976
Figure Sign-Magnitude Control
Si9976
Dual LITTLE FOOT MOSFET Dual LITTLE FOOT MOSFET
Si9976
Figure Sign-Magnitude Control Low-Side MOSFET
There couple things aware this mode operation. Application signal input when input held will create erroneous Fault signal which inverse signal. This eliminated applying inverse signal input shown Figure Secondly, care must taken ensure that bootstrap capacitor been charged prior high-side turn low-side on-times decrease, this becomes greater concern. Minimum low-side on-times must observed ensure that high-side will turn Remember that this minimum time reduced adding external bootstrap diode (see Figure When this done, increases load therefore decoupling capacitor. value decoupling capacitor should doubled prevent undervoltage condition from occurring.
CURRENT SENSING
current sensing required, fractional resistor inserted between low-side MOSFET source connection ground. External amps comparators then used implement current limit some other current control. Schottky diode must connected from half-bridge output ground protect output from negative voltage spikes. addition causing potential damage Si9976, negative spikes cause erroneous latching FAULT. sensing resistor provides small amount isolation MOSFET decoupling capacitors from ground. Make sure that decoupling capacitors MOSFETs connected directly across MOSFET pair, high-side drain low-side source maximize their effectiveness reducing noise (see Figure
CBOOT
BRAKING
Braking accomplished turning both upper both lower MOSFETs H-bridge motor windings shorted together. upper MOSFETs used this function, certain that bootstrap capacitors charged prior turning them
IN4148 Equivalent
Si9976
Figure External Bootstrap Diode
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Si9976
Si9976
Current Sense Circuitry
Figure Current Sensing
0.01 Si9945DY Si9945DY
0.01
Si9976DY
Si9976DY
FAULT
FAULT
0.01
0.01
0.01 0.01
Figure Full-Bridge Configuration with Si9976DY Si9945DY
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FULL BRIDGE APPLICATION
Figure shows basic implementation Si9976DY Si9945DY full-bridge configuration. Each half-bridge made Si9976DY driver Si9945DY LITTLE FOOT dual n-channel MOSFET, bootstrap capacitor, filter capacitor VDD, decoupling capacitors each This configuration yields full-bridge circuit with continuous current rating without heatsinking. Si9945DY Si4946EY yields current ratings respectively. circuit which generates signals with fast rise fall times generate noise. This noise, dealt with, affect operation circuit. Proper board layout techniques device decoupling will take care these problems. signal ground trace from Si9976DY trace from low-side MOSFET source should separately common ground point. This prevents noise generated fast MOSFET transitions from modulating signal ground Si9976DY. Similarly, trace input Si9976DY trace drain high-side MOSFET should connected separately supply bypass capacitor. addition layout considerations, decoupling capacitors required deal with noise. Adding capacitors across power supply lines, VDD, VCC, provides impedance ground switching noise serves local energy reservoir when there demand surge current. capacitor provides surge current required turn low-side MOSFET. addition basic decoupling, capacitors added across half-bridge itself minimize surge current power supply traces, therefore reduce generated noise. Although single capacitor, typically 0.01 works well decouple single pin, advisable apply several decades capacitance across input power, GND, handle broad spectrum noise that present. high-frequency (lower value) capacitors should located close possible device being decoupled, while larger capacitors located farther away bypass only power supply. Figure shows typical layout Si9976DY with LITTLE FOOT dual n-channel MOSFETs. surface-mount packages allows automated assembly entire motor drive circuit, without need separate heatsink associated material assembly costs.
SUMMARY
Si9976DY provides both low- high-side gate drive, high-side level shifting, bootstrap/charge pump high-side power supply, protection undervoltage short circuit conditions single surface-mount Si4946EY, Si9945DY, Si9945DY surface-mount MOSFETs power switching over broad current range require heatsinking. surface-mount packages allows automated assembly entire drive system while minimizing board space. Si9976DY, when used with dual n-channel LITTLE FOOT power MOSFETs, provides very flexible approach power switching motor drives.
FAULT
FAULT
Figure Typical Board Layout (Scale 1:1)
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