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LM3477

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SNVS141K –OCTOBER 2000–REVISED MARCH 2013

LM3477 High Efficiency High-Side N-Channel Controller for Switching Regulator

Check for Samples: LM3477

FEATURES

DESCRIPTION

The LM3477/A is a high-side N-channel MOSFET

• 500kHz Switching Frequency

switching regulator controller. It can be used in

• Adjustable Current Limit

topologies requiring a high side MOSFET such as

• 1.5% Reference

buck, inverting (buck-boost) and zeta regulators. The

LM3477/A's internal push pull driver allows

• Thermal Shutdown

compatibility with a wide range of MOSFETs. This,

• Frequency Compensation Optimized with a

the wide input voltage range, use of discrete power

Single Capacitor and Resistor

components and adjustable current limit allows the

• Internal Softstart

LM3477/A to be optimized for a wide variety of

applications.

• Current Mode Operation

• Undervoltage Lockout with Hysteresis

The LM3477/A uses a high switching frequency of

500kHz to reduce the overall solution size. Current-

• 8-lead (VSSOP-8) Package

mode control requires only a single resistor and

capacitor for frequency compensation. The current

APPLICATIONS

mode architecture also yields superior line and load

• Local Voltage Regulation

regulation and cycle-by-cycle current limiting. A 5µA

shutdown state can be used for power savings and

• Distributed Power

for power supply sequencing. Other features include

• Notebook and Palmtop Computers

internal soft-start and output over voltage protection.

• Internet Appliances

The internal soft-start reduces inrush current. Over

voltage protection is a safety feature to ensure that

• Printers and Office Automation

the output voltage stays within regulation.

• Battery operated Devices

The LM3477A is similar to the LM3477. The primary

• Cable Modems

difference between the two is the point at which the

• Battery Chargers

device transitions into hysteretic mode. The hysteretic

threshold of the LM3477A is one-third of the LM3477.

Hysteretic Threshold

(1)

LM3477 ≊ 36% of programmed current limit

LM3477A ≊ 12% of programmed current limit

(1) See Hysteretic Threshold and PROGRAMMING THE

CURRENT LIMIT/HYSTERETIC THRESHOLD for more

information.

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

2All trademarks are the property of their respective owners.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

Summary of content (34 pages)

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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 LM3477 High Efficiency High-Side N-Channel Controller for Switching Regulator Check for Samples: LM3477 FEATURES DESCRIPTION • • • • • The LM3477/A is a high-side N-channel MOSFET switching regulator controller. It can be used in topologies requiring a high side MOSFET such as buck, inverting (buck-boost) and zeta regulators. The LM3477/A's internal push pull driver allows compatibility with a wide range of MOSFETs.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com Typical Application Circuit L 3.3PH Q1 FDC653N RSN 0.02: CSN 0.1PF D 30BG100 6 1 VIN 4.5V - 5.5V CBYP 0.1PF CIN 120PF/20V RFB1 20.5k COUT 47PF ceramic FB DR VOUT 2.5V, 3A RFB2 20.0k 3 7 CB LM3477 2 8 VIN COMP/SD 5 4 SW GND ISEN CBOOT 0.1PF RC 510 CC1 47nF * *RFB1 = RFB2 (Vout - 1.26)/1.26 Figure 1.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 Absolute Maximum Ratings (1) (2) Input Voltage 36V Peak Driver Output Current (<10µs) 1.0A CB Pin Voltage (3) 43V ISEN Pin Voltage 500mV Power Dissipation Internally Limited Storage Temperature Range −65°C to +150°C Junction Temperature +150°C (4) ESD Susceptibilty Human Body Model Machine Model 2kV 200V Lead Temperature for VSSOP Package Vapor Phase (60 sec.) Infared (15 sec.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com Electrical Characteristics(1) (continued) Specifications in Standard type face are for TJ = 25°C, and in bold type face apply over the full Operating Temperature Range. Unless otherwise specified, VIN = 12V.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 Electrical Characteristics(1) (continued) Specifications in Standard type face are for TJ = 25°C, and in bold type face apply over the full Operating Temperature Range. Unless otherwise specified, VIN = 12V. Symbol VSD Parameter Shutdown Threshold (5) Conditions Output = High Output = Low ISD Shutdown Pin Current Typical Limit Units 1.35 V V (max) 0.3 V V (min) 1.15 0.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com Typical Performance Characteristics Unless otherwise specified, VIN = 12V, TJ = 25°C. IQ (Shutdown) vs Temperature & Supply Voltage ISupply vs Temperature & Supply Voltage (Non-Switching) 11 2.75 o -40 C 10 25 C 9 ISUPPLY (mA) 85oC IQ (PA) -40oC 25oC 2.5 o 8 7 2.25 125oC 2 6 1.75 5 1.5 4 0 5 10 15 20 25 30 35 0 40 5 10 20 25 30 35 40 VIN (Volts) VIN (Volts) Figure 3. Figure 4.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 Typical Performance Characteristics (continued) Unless otherwise specified, VIN = 12V, TJ = 25°C. Efficiency vs Load Current (VIN = 24V, VOUT = 12V) Efficiency vs Load Current (VIN = 5V, OUT = 3.3V) 100 100 LM3477 90 90 LM3477A 70 60 50 40 70 60 50 40 30 30 20 20 10 10 0 0.01 0.1 LM3477 80 EFFICIENCY (%) EFFICIENCY (%) 80 LM3477A 1 0 0.01 10 0.1 1 10 LOAD (A) LOAD (A) Figure 9. Figure 10.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified, VIN = 12V, TJ = 25°C. Slope Compensation Ramp vs Slope Compensation Resistor 450 250 LM3477 400 200 350 300 LM3477A 'VSL(mV) SHORT CIRCUIT SENSE VOLTAGE (mV) Short Circuit vs Temperature 250 200 150 150 100 100 50 50 0 -50 0 50 100 0 150 0 1k 2k TEMPERATURE (°C) Figure 15. 4k 5k Figure 16.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 Functional Block Diagram _ ISEN Short-Circuit Detect + _ PWM Comparator + UnderVoltage Lockout One-Shot 500kHz Oscillator VIN Slope Compensation 7.2V Set / Blankout Shutdown Detect Bias Voltages 1.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com FUNCTIONAL DESCRIPTION GENERAL DESCRIPTION The LM3477/A is a switching regulator controller for topologies incorporating a high side switch. The most common of these topologies is the step-down, or buck, converter. Other topologies such as the inverting (buckboost) and inverse SEPIC (zeta) converters can be realized. This datasheet will focus on buck converter applications. The LM3477/A employs current mode control architecture.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3477/A will begin switching again when the feedback voltage reaches VFB + (VOVP - VOVP(HYS)). See Electrical Characteristics for limits on VOVP(HYS). OVP can be triggered by any event that causes the output voltage to rise out of regulation.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com VIN (V) xxxxx 7.2V t VFB (V) OVP (1.315V) 1.265V t The Feedback Voltage Experiences an Oscillation if the Input Voltage Crosses the 7.2V Internal Bias Threshold Figure 21. DEFAULT/ADJUSTABLE SLOPE COMPENSATION The LM3477/A uses a current mode control scheme. There are many advantages in a current mode architecture including inherent cycle-by-cycle current limiting and simple compensation of the control loop.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 The subharmonic oscillation phenomenon is realized when a load excursion is experienced. The way it is analyzed is to calculate how the inductor current settles after such an excursion. Take for example the case when the inductor current experiences a step increase in its average current, shown as the dotted line in Figure 22. In the switching period that the excursion occurs, the inductor current will change by ΔI0.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com COMP/SD LM3477 +5V OFF 0 SHUTDOWN INPUT ON Figure 23. Implementing Shutdown in LM3477 VCC +5V SHUTDOWN INPUT OFF 0 ON COMP/SD LM3477 Figure 24. Implementing Shutdown in LM3477 Design Section GENERAL Power supply design involves making tradeoffs. To achieve performance specifications, limitations will be set on component selection.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 POWER STAGE RSN VIN Q1 L VOUT CIN R RESR LOAD COUT D RFB1 RFB2 RSL 6 1 VSEN + DR FB ISEN CB LM3477 8 4 SW 3 7 5 CBOOT RDR VIN GND 2 COMP/SD CBYP RC CC1 COMPENSATION NETWORK Figure 25. LM3477 Buck Converter Reference Schematic PROGRAMMING THE OUTPUT VOLTAGE The output voltage can be programmed using a resistor divider between the output and the feedback pins, as shown in Figure 25.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com PROGRAMMING THE CURRENT LIMIT/HYSTERETIC THRESHOLD Definitions Current Limit: The current limit is the point at which the LM3477/A begins to limit the peak switch current. The current limit in the LM3477/A varies with duty cycle, which is a function of the VIN − VOUT differential. Hysteric Threshold: Hysteretic threshold is the current at which the LM3477/A enters the hysteretic mode of operation (see OVER VOLTAGE PROTECTION).
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 The hysteretic threshold is derived in a similar manner, the only difference being that VSEN(peak) is compared VC(min) (VHYS). Notice that VHYS does not vary with the duty cycle. The hysteretic threshold is predetermined by the selection of RSN above. The hysteretic threshold is: IHYS = VHYS 0.032 (A), LM3477 = RSN RSN 0.011 = R (A), LM3477A SN (13) Continuing with the example above, 0.032 = 1.6A, LM3477 0.02 0.011 = 0.55A, LM3477A 0.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com POWER INDUCTOR SECTION The LM3477/A operates at a high switching frequency of 500kHz, which allows the use of small inductors. This is made apparent in the following set of equations used to calculate the output voltage ripple. ΔVOUT(Pk-Pk) ≊ ΔiL(Pk-Pk) x RESR (V) 'iL(Pk-Pk) = VOUT (1-D) L x fS (17) (A) (18) As the switching frequency fs increases, the inductance required for a given output voltage ripple decreases.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 It is recommended that: Q(max) = 2, and Q(min) = 0.15 Values for VSL can be found in the Electrical Characteristics. Note: Adding slope compensation with RSL will decrease the current limit. An iterative process may be needed to meet current limit and stability requirements, see PROGRAMMING THE CURRENT LIMIT/HYSTERETIC THRESHOLD. Output Capacitor Selection A capacitance between 47µF - 100µF is typically used.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com Voltage 'Vc1 'Vc2 Upper Voltage Limit VOS (MAX) ESR too large capacitance too small 0 Time Figure 29. Output Voltage Overshoot Violation The ESR and the capacitance of the output capacitor must be carefully chosen so that the output voltage overshoot is within the design's specification VOS(MAX).
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 Voltage 'Vc 'Vr 'Vq 0 tpeak Time Figure 30. Output Voltage Overshoot Peak The intention is to find the capacitance value that will yield, at tpeak, a ΔVC that equals VOS(max).
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com It is sometimes helpful or necessary to slow down the turn on transition of the FET so that less switching noise appears at the ISEN pin. This can be done by inserting a drive resistor RDR in series with the boot-strap capacitor (see Figure 25). This can help reduce sensing noise that may be preventing designs from operating at or near the LM3477/A's minimum duty cycle limit. Gate drive resistors from 2.2Ω to 51Ω are recommended.
PAGE 23
LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 The compensation network is designed around the power components, or the power stage. An isolated schematic of the error amplifier and the various compensation components is shown in Figure 32. The error amplifier in conjunction with the compensation network makes up the compensator block in Figure 31.
PAGE 24
LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 ADC = R 1.8RSN www.ti.com 1 R 1+ fSL mc x D' - 0.5 (37) Se mc = 1+ S n (38) (39) Se = fS(VSL + 50x10−6 RSL) Sn = VIND' x 1.8RSN L (40) 1 1 1 (mc x D' - 0.5) (Hz) + fp1 = 2S COUTR fsLCOUT fESR = 1 2SCOUTRESR (41) (Hz) (42) 1 Q= S mc x D' - 0.5) (43) With the power stage known, a compensator can be designed to achieve improved performance and stability.
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LM3477 www.ti.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 RC = www.ti.com (20 x 103)(50 x 103) (15.5)(0.001)(50 x 103)(0.508)(2.86 x 103) - (20 x 103) = 904: (57) This sets the high frequency gain of the compensator such that a crossover frequency of fC is obtained. The capacitor CC1 sets the compensator zero, fZ2. Set fZ2 between the power pole fP1 and the ½ decade before the target crossover frequency fC: 3.16 1 d CC1 d 2S(2.
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LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 SAMPLING POLE QUALITY FACTOR In a current mode control architecture, there is an inherent resonace at half the switching frequency. The LM3477/A internally compensates for this by adding a negative slope to the PWM control waveform (see DEFAULT/ADJUSTABLE SLOPE COMPENSATION). The factor in the power stage equations above, Q, describes how much resonance will be observed. Q is a function of duty cycle and mc.
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LM3477 SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 www.ti.com REVISION HISTORY Changes from Revision J (March 2013) to Revision K • 28 Page Changed layout of National Data Sheet to TI format ..........................................................................................................
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE OPTION ADDENDUM www.ti.com 1-Nov-2013 (6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish value exceeds the maximum column width. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided.
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PACKAGE MATERIALS INFORMATION www.ti.com 11-Oct-2013 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant LM3477AMM/NOPB VSSOP DGK 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM3477AMMX VSSOP DGK 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM3477AMMX/NOPB VSSOP DGK 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.
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PACKAGE MATERIALS INFORMATION www.ti.com 11-Oct-2013 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM3477AMM/NOPB VSSOP DGK 8 1000 210.0 185.0 35.0 LM3477AMMX VSSOP DGK 8 3500 367.0 367.0 35.0 LM3477AMMX/NOPB VSSOP DGK 8 3500 367.0 367.0 35.0 LM3477MM VSSOP DGK 8 1000 210.0 185.0 35.0 LM3477MM/NOPB VSSOP DGK 8 1000 210.0 185.0 35.0 LM3477MMX VSSOP DGK 8 3500 367.0 367.0 35.
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