LM27222 High-Speed 4.5A Synchronous MOSFET Driver General Description Features The LM27222 is a dual N-channel MOSFET driver designed to drive MOSFETs in push-pull configurations as typically used in synchronous buck regulators. The LM27222 takes the PWM output from a controller and provides the proper timing and drive levels to the power stage MOSFETs. Adaptive shoot-through protection prevents damaging and efficiency reducing shoot-through currents, thus ensuring a robust design capable of being used with nearly any MOSFET. The adaptive shoot-through protection circuitry also reduces the dead time down to as low as 10ns, ensuring the highest operating efficiency. The peak sourcing and sinking current for each driver of the LM27222 is about 3A and 4.5Amps respectively with a Vgs of 5V. System performance is also enhanced by keeping propagation delays down to 8ns. Efficiency is once again improved at all load currents by supporting synchronous, non-synchronous, and diode emulation modes through the LEN pin. The minimum output pulse width realized at the output of the MOSFETs is as low as 30ns. This enables high operating frequencies at very high conversion ratios in buck regulator designs. To support low power states in notebook systems, the LM27222 draws only 5µA from the 5V rail when the IN and LEN inputs are low or floating. n n n n n n n n n n n Adaptive shoot-through protection 10ns dead time 8ns propagation delay 30ns minimum on-time 0.4Ω pull-down and 0.9Ω pull-up drivers 4.5A peak driving current MOSFET tolerant design 5µA quiescent current 30V maximum input voltage in buck configuration 4V to 6.85V operating voltage SO-8 and LLP packages Applications n n n n High Current Buck And Boost Voltage Converters Fast Transient DC/DC Power Supplies Single Ended Forward Output Rectification CPU And GPU Core Voltage Regulators Typical Application 20117902 FIGURE 1. © 2006 National Semiconductor Corporation DS201179 www.national.com LM27222 High-Speed 4.5A Synchronous MOSFET Driver March 2006 LM27222 Connection Diagram 20117901 Top View SO-8 (NS Package # M08A) θJA = 172˚C/W or LLP-8 (NS Package # SDC08A) θJA = 39˚C/W Ordering Information Order Number Size NSC Drawing # LM27222M SO-8 M08A LM27222MX LM27222SD LLP-8 SDC08A LM27222SDX Package Type Supplied As Rail 95 Units/Rail Tape and Reel 2500 Units/Reel Tape and Reel 1000 Units/Reel Tape and Reel 4500 Units/Reel Pin Descriptions Pin # Pin Name Pin Function 1 SW High-side driver return. Should be connected to the common node of high and low-side MOSFETs. 2 HG High-side gate drive output. Should be connected to the high-side MOSFET gate. Pulled down internally to SW with a 10K resistor to prevent spurious turn on of the high-side MOSFET when the driver is off. 3 CB Bootstrap. Accepts a bootstrap voltage for powering the high-side driver. 4 IN Accepts a PWM signal from a controller. Active High. Pulled down internally to GND with a 150K resistor to prevent spurious turn on of the high-side MOSFET when the controller is inactive. 5 LEN Low-side gate enable. Active High. Pulled down internally to GND with a 150K resistor to prevent spurious turn-on of the low-side MOSFET when the controller is inactive. 6 VCC Connect to +5V supply. 7 LG Low-side gate drive output. Should be connected to low-side MOSFET gate. Pulled down internally to GND with a 10K resistor to prevent spurious turn on of the low-side MOSFET when the driver is off. 8 GND www.national.com Ground. 2 LM27222 Block Diagram 20117903 3 www.national.com LM27222 Absolute Maximum Ratings (Note 1) Power Dissipation (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Storage Temperature VCC to GND -0.3V to 7V CB to GND -0.3V to 36V CB to SW -0.3V to 7V SW to GND (Note 2) -2V to 36V LEN, IN, LG to GND -0.3V to VCC + 0.3V ≤ 7V HG to GND −65˚ to 150˚C ESD Susceptibility Human Body Model 2kV Operating Ratings (Note 1) VCC 4V to 6.85V Junction Temperature Range −40˚ to 125˚C CB (max) -0.3V to 36V Junction Temperature 720mW 33V +150˚C Electrical Characteristics (Note 4) VCC = CB = 5V, SW = GND = 0V, unless otherwise specified. Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over the entire operating temperature range (-40˚C ≤ TJ ≤ 125˚C). Symbol Parameter Conditions Min Typ Max Units 5 15 µA POWER SUPPLY Iq_op Operating Quiescent Current IN = 0V, LEN = 0V 30 IN = 0V, LEN = 5V 500 540 650 µA 825 HIGH-SIDE DRIVER Peak Pull-up Current 3 A 2.5 Ω 1.5 Ω RH-pu Pull-up Rds_on RH-pd Pull-down Rds_on ISW = IHG = 0.3A 0.4 t4 Rise Time Timing Diagram, CLOAD = 3.3nF 17 t6 Fall Time Timing Diagram, CLOAD = 3.3nF 12 ns t3 Pull-up Dead Time Timing Diagram 9.5 ns t5 Pull-down Delay Timing Diagram 16.5 ns 30 ns 3.2 A ICB = IHG = 0.3A 0.9 Peak Pull-down Current ton_min 4.5 Minimum Positive Output Pulse Width A ns LOW-SIDE DRIVER Peak Pull-up Current RL-pu Pull-up Rds_on IVCC = ILG = 0.3A 0.9 Peak Pull-down Current RL-pd 2.5 Ω 1.5 Ω 4.5 A Pull-down Rds_on IGND = ILG = 0.3A 0.4 t8 Rise Time Timing Diagram, CLOAD = 3.3nF 17 t2 Fall Time Timing Diagram, CLOAD = 3.3nF 14 ns t7 Pull-up Dead Time Timing Diagram 11.5 ns t1 Pull-down Delay Timing Diagram 7.7 ns HG-SW Pull-down Resistance 10k Ω LG-GND Pull-down Resistance 10k Ω LEN-GND Pull-down Resistance 150K Ω IN-GND Pull-down Resistance 150K Ω IN = 0V, Source Current 50 nA IN = 5V, Sink Current 33 µA ns PULL-DOWN RESISTANCES LEAKAGE CURRENTS Ileak_IN IN pin Leakage Current www.national.com 4 (Continued) VCC = CB = 5V, SW = GND = 0V, unless otherwise specified. Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over the entire operating temperature range (-40˚C ≤ TJ ≤ 125˚C). Symbol Ileak_LEN Parameter LEN pin Leakage Current Conditions Min Typ Max Units LEN = 0V, Source Current 200 nA LEN = 5V, Sink Current 33 µA LOGIC VIH_LEN LEN Low to High Threshold VIL_LEN Low to High Transition LEN High to Low Threshold High to Low Transition VIH_IN IN Low to High Threshold Low to High Transition VIL_IN IN High to Low Threshold High to Low Transition Threshold Hysteresis 65 % of VCC 65 % of VCC 30 % of VCC 30 % of VCC 0.7 V Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating ratings are conditions under which the device operates correctly. Operating Ratings do not imply guaranteed performance limits. Note 2: The SW pin can have -2V to -0.5 volts applied for a maximum duty cycle of 10% with a maximum period of 1 second. There is no duty cycle or maximum period limitation for a SW pin voltage range of -0.5V to 30 Volts. Note 3: Maximum allowable power dissipation is a function of the maximum junction temperature, TJMAX, the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using: PMAX = (TJMAX-TA) / θJA. The junction-toambient thermal resistance, θJA, for the LM27222M, it is 165˚C/W. For a TJMAX of 150˚C and TA of 25˚C, the maximum allowable power dissipation is 0.76W. The θJA for the LM27222SD is 42˚C/W. For a TJMAX of 150˚C and TA of 25˚C, the maximum allowable power dissipation is 3W. Note 4: Min and Max limits are 100% production tested at 25˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). Timing Diagram 20117904 5 www.national.com LM27222 Electrical Characteristics (Note 4) LM27222 Typical Waveforms 20117908 20117907 FIGURE 3. PWM High-to-Low Transition at IN Input FIGURE 2. PWM Low-to-High Transition at IN Input 20117909 FIGURE 4. LEN Operation The typical waveforms are from a circuit similar to Figure 1 with: Q1: 2 x Si7390DP Q2: 2 x Si7356DP L1: 0.4 µH VIN: 12V www.national.com 6 LM27222 Application Information GENERAL The LM27222 is designed for high speed and high operating reliability. The driver can handle very narrow, down to zero, PWM pulses in a guaranteed, deterministic way. Therefore, the HG and LG outputs are always in predictable states. No latches are used in the HG and LG control logic so the drivers cannot get "stuck" in the wrong state. The driver design allows for powering up with a pre-biasing voltage being present at the regulator output. To reduce conduction losses in DC-DC converters with low duty factors the LM27222 driver can be powered from a 6.5V ± 5% power rail. It is recommended to use the same power rail for both the controller and driver. If two different power rails are used, never allow the PWM pulse magnitude at the IN input or the control voltage at the LEN input to be above the driver VCC voltage or unpredictable HG and LG outputs pulse widths may result. 20117906 MINIMUM PULSE WIDTH As the input pulse width to the IN pin is decreased, the pulse width of the high-side gate drive (HG-SW) also decreases. However, for input pulse widths 60ns and smaller, the HG-SW remains constant at 30ns. Thus the minimum pulse width of the driver output is 30ns. Figure 5 shows an input pulse at the IN pin 20ns wide, and the output of the driver, as measured between the nodes HG and SW is a 30ns wide pulse. Figure 6 shows the variation of the SW node pulse width vs IN pulse width. At the IN pin, if a falling edge is followed by a rising edge within 5ns, the HG may ignore the rising edge and remain low until the IN pin toggles again. If a rising edge is followed by a falling edge within 5ns, the pulse may be completely ignored. FIGURE 6. ADAPTIVE SHOOT-THROUGH PROTECTION The LM27222 prevents shoot-through power loss by ensuring that both the high- and low-side MOSFETs are not conducting at the same time. When the IN signal rises, LG is first pulled down. The adaptive shoot-through protection circuit waits for LG to reach 0.9V before turning on HG. Similarly, when IN goes low, HG is pulled down first, and the circuit turns LG on only after the voltage difference between the high-side gate and the switch node, i.e. HG-SW, has fallen to 0.9V. It is possible in some applications that at power-up the driver’s SW pin is above 3V in either buck or boost comverter applications. For instance, in a buck configuration a pre-biasing voltage can be either a voltage from anothert power rail connected to the load, or a leakage voltage through the load, or it can be an output capacitor precharged above 3V while no significant load is present. In a boost application it can be an input voltage rail above 3V. In the case of insufficient initial CB-SW voltage (less than 2V) such as when the output rail is pre-biased, the shootthrough protection circuit holds LG low for about 170ns, beginning from the instant when IN goes high. After the 170ns delay, the status of LG is dictated by LEN and IN. Once LG goes high and SW goes low, the bootstrap capacitor will be charged up (assuming SW is grounded for long enough time). As a result, CB-SW will be close to 5V and the LM27222 will now fully support synchronous operation. The dead-time between the high- and low-side pulses is kept as small as possible to minimize conduction through the body diode of the low-side MOSFET(s). 20117905 FIGURE 5. Min On Time 7 www.national.com LM27222 Application Information 4. The high-current loop between the high-side and lowside MOSFETs and the input capacitors should be as small as possible. 5. There should be enough copper area near the MOSFETs and the inductor for heat dissipation. Vias may also be added to carry the heat to other layers. (Continued) POWER DISSIPATION The power dissipated in the driver IC when switching synchronously can be calculated as follows: TYPICAL APPLICATION CIRCUIT DESCRIPITON The Application Example on the following page shows the LM27222 being used with National’s LM27212, a 2-phase hysteretic current mode controller. Although this circuit is capable of operating from 5V to 28V, the components are optimized for an input voltage range of 9V to 28V. The high-side FET is selected for low gate charge to reduce switching losses. For low duty cycles, the average current through the high-side FET is relatively small and thus we trade off higher conduction losses for lower switching losses. The low-side FET is selected solely on RDS_ON to minimize conduction losses. If the input voltage range were 4V to 6V, the MOSFET selection should be changed. First, much lower voltage FETs can be used, and secondly, high-side FET RDS_ON becomes a larger loss factor than the switching losses. Of course with a lower input voltage, the input capacitor voltage rating can be reduced and the inductor value can be reduced as well. For a 4V to 6V application, the inductor can be reduced to 200nH to 300nH. The switching frequency of the LM27212 is determined by the allowed ripple current in the inductor. This circuit is set for approximately 300kHz. At lower input voltages, higher frequencies are possible without suffering a significant efficiency loss. Although the LM27222 can support operating frequencies up to 2MHz in many applications, the LM27212 should be limited to about 1MHz. The control architecture of the LM27212 and the low propagation times of the LM27222 potentially gives this solution the fastest transient response in the industry. where fSW = switching frequency VCC = voltage at the VCC pin, QG_H = total gate charge of the (parallel combination of the) high-side MOSFET(s) QG_L = total gate charge of the (parallel combination of the) low-side MOSFET(s) RG_H = gate resistance of the (parallel combination of the) high-side MOSFET(s) RG_L = gate resistance of the (parallel combination of the) low-side MOSFET(S) RH_pu = pull-up RDS_ON of the high-side driver RH_pd = pull-down RDS_ON of the high-side driver RL_pu = pull-up RDS_ON of the low-side driver RL_pd = pull-down RDS_ON of the low-side driver PC BOARD LAYOUT GUIDELINES 1. Place the driver as close to the MOSFETs as possible. 2. HG, SW, LG, GND: Run short, thick traces between the driver and the MOSFETs. To minimize parasitics, the traces for HG and SW should run parallel and close to each other. The same is true for LG and GND. 3. Driver VCC: Place the decoupling capacitor close to the VCC and GND pins. www.national.com 8 * Q1, Q3: 2 x Si7390DP ** Q2, Q4: 2 x Si7356DP Application Example 20117920 LM27222 9 www.national.com LM27222 Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead Small Outline Package Order Number: LM27222M, LM27222MX NS Package Number M08A 8-Lead LLP Package Order Number: LM27222SD, LM27222SDX NS Package Number SDC08A www.national.com 10 LM27222 High-Speed 4.5A Synchronous MOSFET Driver Notes National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. 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