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| LT1176/LT1176-5 Step-Down Switching Regulator FEATURES s s s s s s s s 1.2A On-Board Switch 100kHz Switching Frequency Excellent Dynamic Behavior DIP and Surface Mount Packages Only 8mA Quiescent Current Preset 5V Output Available Operates Up to 35V Input Micropower Shutdown Mode tion. The power switch, all oscillator and control circuitry, and all current limit components are included on the chip. The topology is a classic positive "buck" configuration but several design innovations allows this device to be used as a positive-to-negative converter, a negative boost converter, and as a flyback converter. The switch output is specified to swing below ground. The LT1176 uses a true analog multiplier in the feedback loop. This makes the device respond nearly instantaneously to input voltage fluctuations and makes loop gain independent of input voltage. As a result, dynamic behavior of the regulator is significantly improved over previous designs. On-chip pulse by pulse current limiting makes the LT1176 nearly bust-proof for output overloads or shorts. The input voltage range as a buck converter is 8V to 35V, but a selfboot feature allows input voltages as low as 5V in the inverting and boost configurations. The LT1176 is available in a low cost 8-lead DIP package with frequency preset at 100kHz and current limit at 1.7A An adjustable output is offered as well as a preset 5V version. For further design details and application help, see the LT1074/LT1076 data sheet and Application Note 44. APPLICATI s s s s Buck Converter with Output Voltage Range of 2.5V to 30V Positive-to-Negative Converter Negative Boost Converter Mulitple Output Buck Converter DESCRIPTIO The LT1176 is a 1A monolithic bipolar switching regulator which requires only a few external parts for normal opera- TYPICAL APPLICATI Basic 5V Positive Buck Converter L1* 100H 10V T0 35V VIN LT1176-5 SENSE VC R3 2.7k C3 100F C2 0.01F VSW 1N5819 EFFICIENCY (%) 5V, 0.8A GND + *COILTRONICS #100-1-52 LT1176/76-5 * TA01 PULSE ENGINEERING, INC. #PE-92102 HURRICANE #HL-AG210LL THESE ARE LOW COST POWDERED IRON CORES. OPTIMUM EFFICIENCY AND SMALLEST SIZE IS OBTAINED BY USING A LOW LOSS CORE SUCH AS MAGNETICS INC. KOOL M. SEE EFFICIENCY GRAPH. UO U UO S 5V Buck Converter Efficiency 90 L = 100H ON MAGNETICS INC. SIZE T-50 KOOL M CORE VIN = 10V 80 VIN = 20V + C1 470F 25V 70 60 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A) LT1176/76-5 * TA02 1 LT1176/LT1176-5 ABSOLUTE AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW VIN 1 ILIM 2 GND 3 FB/SENSE 4 8 7 6 5 VSW VC STATUS SHUTDOWN Input Voltage .......................................................... 38V Switch Voltage With Respect to Input Voltage ........ 50V Switch Voltage With Respect to Ground Pin (VSW Negative) (Note 6) ......................................... 20V Feedback Pin Voltage....................................... - 2V, 10V Shutdown Pin Voltage (Not to Exceed VIN) ............. 35V Status Pin Voltage (Current Must Be Limited to 5mA When Status Pin Switches "ON") ................................................... 30V ILIM Pin Voltage (Forced)...................................... 5.5V Maximum Operating Ambient Temperature Range LT1176C/LT1176C-5 .............................. 0C to 70C Maximum Operating Junction Temperature Range LT1176C/LT1176C-5 ............................ 0C to 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec) 300C *These thermal resistance numbers are for typical mounting technique. Lower thermal resistance can be obtained with large copper lands, thermal glues or heatsinks. ORDER PART NUMBER LT1176CN8 LT1176CN8-5 N8 PACKAGE 8-LEAD PLASTIC DIP TJ MAX = 125C, JA = 90C/ W* TOP VIEW VIN 1 NC 2 ILIM 3 GND 4 GND 5 GND 6 GND 7 NC 8 FB/SENSE 9 NC 10 20 VSW 19 NC 18 VC 17 GND 16 GND 15 GND 14 GND 13 STATUS 12 SHUTDOWN 11 NC S PACKAGE 20-LEAD PLASTIC SOL TJMAX = 125 C, JA = 50C/ W* ORDER PART NUMBER LT1176CS LT1176CS-5 ELECTRICAL CHARACTERISTICS PARAMETER Switch "ON" Voltage (Note 1) Switch "OFF" Leakage Supply Current (Note 2) Minimum Supply Voltage Switch Current Limit (Note 4) CONDITIONS ISW = 0.2A ISW = 1A TJ = 25C, VIN = 25V, unless otherwise noted MIN q q TYP MAX 1.1 1.4 150 250 UNITS V V A A mA A V V A A A % kHz kHz kHz kHz %/ V V/ V mho A mA VIN = 25V, VSW = 0 VIN = VMAX, VSW = 0 (Note 7) VFB = 2.5V, VIN 35V VSHDN = 0.1V (Device Shutdown) (Note 8) Normal Mode Startup Mode (Note 3) ILIM Open RLIM = 10k (Note 5) RLIM = 7k (Note 5) q q q q q 7.5 140.0 7.3 3.5 1.2 1.7 1.2 0.8 90 100 20 20 10 300 8.0 4.8 2.2 Maximum Duty Cycle Switching Frequency q q 85 90 85 110 120 VFB = 0V Through 2k (Note 4) (LT1176) VSENSE = 0V (Note 4) (LT1176-5) Switching Frequency Line Regulation Error Amplifier Voltage Gain (Note 9) Error Amplifier Transconductance (Note 9) Error Amplifier Source and Sink Current Source (VFB = 2V or VSENSE = 4V) Sink (VFB = 2.5V or VSENSE = 5.5V) 8V VIN VMAX (Note 7) 1V VC 4V 3700 100.0 0.7 q 0.03 2000 5000 140.0 1.0 0.1 8000 225.0 1.6 2 U W U U WW W LT1176/LT1176-5 ELECTRICAL CHARACTERISTICS PARAMETER Feedback Pin Bias Current (LT1176) Reference Voltage (LT1176) Reference Voltage Tolerance (LT1176) CONDITIONS VFB = VREF VC = 2V TJ = 25C, VIN = 25V, unless otherwise noted MIN q q q q q TYP 0.5 2.21 0.5 1.0 MAX 2 2.265 1.5 2.5 5.15 2 3 8 0.02 UNITS A V % % V % % k %/ V V mV/C V 2.155 VREF (Nominal) = 2.21V All Conditions of Input Voltage, Output Voltage, Temperature and Load Current VC = 2V VOUT (Nominal) = 5V All Conditions of Input Voltage, Temperature and Load Current 8V VIN VMAX (Note 7) Over Temperature Sense Voltage (LT1176-5) Sense Voltage Tolerance (LT1176-5) 4.85 5 0.5 1.0 Sense Pin Divider Resistance (LT1176-5) Output Voltage Line Regulation VC Voltage at 0% Duty Cycle q q 3 5 0.005 1.5 - 4.0 24 Multiplier Reference Voltage Shutdown Pin Current VSHDN VTHRESHOLD ( 2.5V) Shutdown Thresholds Status Window Status High Level Status Low Level Status Delay Time Status Minimum Width The q denotes specifications which apply over the full operating temperature range. Note 1: To calculate maximum switch "ON" voltage at current between low and high conditions, a linear interpolation may be used. Note 2: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low clamp level and the switch duty cycle to zero. This approximates the zero load condition where duty cycle approaches zero. The LT1176-5 has VSENSE = 5.5V. Note 3: Total voltage from VIN pin to ground pin must be 8V after startup for proper regulation. Note 4: Switch frequency is internally scaled down when the feedback pin Application Hints Although the LT1176 has a peak switch rating of 1.2A and a maximum duty cycle of 85%, it must be used cautiously in applications which require high switch current and high duty cycle simultaneously, to avoid excessive chip temperature. Thermal resistance is 90C/W for the 8-pin DIP package and 50C/W for the 20-pin SO. This limits continuous chip power dissipation to the 0.5W to 1W range. These numbers assume typical mounting techniques. Extra or thick copper connected to the leads can reduce thermal resistance. Bonding the package to the board or using a clip style heatsink can also help. The following formulas will give chip power dissipation and peak switch current for the standard buck converter. Note that surges less than 30 seconds do not need to be considered from a thermal standpoint, but for proper regulation, they must not result in peak switch currents exceeding the 1.2A limit. VSHDN = 5Vq q 5 q q 10 50 2.2 0.1 4 3.5 20 A 2.45 0.30 5 4.5 0.25 9 30 A 2.7 0.5 6 5.0 0.4 V V % V V s s Switch Duty Cycle = 0 Fully Shut Down As a Percent of Output Voltage ISTATUS = 10A Sourcing ISTATUS = 1.6mA Sinking q q voltage is less than 1.3V to avoid extremely short switch-on times. During testing, VFB or VSENSE is adjusted to give a minimum switch-on time of 1s. Note 5: ILM = (RLIM - 1k)/ 7.65k Note 6: Switch to input voltage limitation must also be observed. Note 7: VMAX = 35V Note 8: Does not include switch leakage. Note 9: Error amplifier voltage gain and transconductance are specified relative to the internal feedback node. To calculate gain and transconductance from the sense pin (Output) to the VC pin on the LT1176-5, multiply by 0.44. Power = ILOAD (VOUT/VIN) + VIN [7mA + 3mA (VOUT/VIN) + 0.012 (ILOAD)] IPEAK = ILOAD(PEAK) + [VOUT (VIN - VOUT)]/2E5(VIN)(L) Example: VIN = 15V, VOUT = 5V, ILOAD = 0.5A Continuous, 0.8A Peak, L = 100H Power (ILOAD = 0.5A) = 0.38W IPEAK (ILOAD = 0.8A) = 0.97A Where component size or height is critical, we suggest using solid tantalum capacitors (singly or in parallel), but be sure to use units rated for switching applications. Coiltronics is a good source for low profile surface mount inductors and AVX makes high quality surface mount tantalum capacitors. For further help, use Application Notes 19 and 44, LTC's SwitcherCAD computer design program, and our knowledgeable application department. Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 3 LT1176/LT1176-5 BLOCK DIAGRA INPUT SUPPLY 10A 0.3V POWER SHUTDOWN 2.35V SHUTDOWN OUTPUT VOLTAGE MONITOR STATUS 2.21V 2.8k FB/SENSE *CONNECTED ON 5V VERSION ONLY PACKAGE DESCRIPTIO 0.300 - 0.320 (7.620 - 8.128) 0.009 - 0.015 (0.229 - 0.381) 0.065 (1.651) TYP 0.125 (3.175) MIN 0.020 (0.508) MIN ( +0.025 0.325 -0.015 +0.635 8.255 -0.381 ) 0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254) 0.291 - 0.299 (7.391 - 7.595) 0.005 (0.127) RAD MIN 0.010 - 0.029 x 45 (0.254 - 0.737) 0.093 - 0.104 (2.362 - 2.642) 0.037 - 0.045 (0.940 - 1.143) 20 19 18 0 - 8 TYP 0.050 (1.270) TYP 0.014 - 0.019 (0.356 - 0.482) TYP 0.394 - 0.419 (10.007 - 10.643) 0.009 - 0.013 (0.229 - 0.330) SEE NOTE 0.016 - 0.050 (0.406 - 1.270) NOTE: PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS. 1 2 3 4 5 6 7 8 9 10 4 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977 U W + 320A 6V REGULATOR AND BIAS 6V TO ALL CIRCUITRY - CURRENT LIMIT SHUTDOWN 500 + - CURRENT LIMIT COMP C2 ILIM 4.5V 10k + - 0.16 FREQ SHIFT 100kHz OSCILLATOR SYNC VIN Z ANALOG X MULTIPLIER XYZ Y VC 24V (EQUIVALENT) 3VP-P S R R/S Q LATCH R G1 + C1 400 + - A1 ERROR AMP - PULSE WIDTH COMPARATOR SWITCH OUTPUT (VSW) LT1176/76-5 * BD 2.2k* Dimensions in inches (milimeters) unless otherwise noted. N Package 8-Lead Plastic DIP 0.045 - 0.065 (1.143 - 1.651) 0.130 0.005 (3.302 0.127) 8 0.400 (10.160) MAX 7 6 5 0.250 0.010 (6.350 0.254) 1 2 3 4 0.018 0.003 (0.457 0.076) S Package 20-Lead Plastic SOL 0.496 - 0.512 (12.598 - 13.005) 17 16 15 14 13 12 11 SEE 0.004 - 0.012 NOTE (0.102 - 0.305) LT/GP 0393 10K REV 0 (c) LINEAR TECHNOLOGY CORPORATION 1993 |
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