LT1375I

LT1375/LT1376 1.5A, 500kHz Step-Down Switching Regulators FEATURES s s s s s s s s Constant 500kHz Switching Frequency Easily Synchronizable Uses All Surface Mount Components Inductor Size Reduced to 5µH Saturating Switch Design: 0.4Ω Effective Supply Current: 2.5mA Shutdown Current: 20µA Cycle-by-Cycle Current Limiting is current mode for fast transient response and good loop stability. Both fixed output voltage and adjustable parts are available. A special high speed bipolar process and new design techniques achieve high efficiency at high switching frequency. Efficiency is maintained over a wide output current range by using the output to bias the circuitry and by utilizing a supply boost capacitor to saturate the power switch. A shutdown signal will reduce supply current to 20µA on both parts. The LT1375 can be externally synchronized from 550kHz to 1MHz with logic level inputs. The LT1375/LT1376 fit into standard 8-pin PDIP and SO packages, as well as a fused lead 16-pin SO with much lower thermal resistance. Full cycle-by-cycle short-circuit protection and thermal shutdown are provided. Standard surface mount external parts are used, including the inductor and capacitors. For low input voltage applications with 3.3V output, see LT1507. This is a functionally identical part that can operate with input voltages between 4.5V and 12V. , LTC and LT are registered trademarks of Linear Technology Corporation. APPLICATIO S s s s s Portable Computers Battery-Powered Systems Battery Charger Distributed Power DESCRIPTIO The LT ®1375/LT1376 are 500kHz monolithic buck mode switching regulators. A 1.5A switch is included on the die along with all the necessary oscillator, control and logic circuitry. High switching frequency allows a considerable reduction in the size of external components. The topology TYPICAL APPLICATIO 5V Buck Converter D2 1N914 C2 0.1µF VSW LT1376-5 BIAS DEFAULT = ON SHDN GND FB VC CC 3.3nF D2 1N5818 L1** 5µH C3* 10µF TO 50µF + VIN EFFICIENCY (%) INPUT 6V † TO 25V BOOST * RIPPLE CURRENT ≥ IOUT/2 ** INCREASE L1 TO 10µH FOR LOAD CURRENTS ABOVE 0.6A AND TO 20µH ABOVE 1A † FOR INPUT VOLTAGE BELOW 7.5V, SOME RESTRICTIONS MAY APPLY. SEE APPLICATIONS INFORMATION. U U U Efficiency vs Load Current 100 VOUT = 5V VIN = 10V L = 10µH 90 OUTPUT** 5V, 1.25A 80 + C1 100µF, 10V SOLID TANTALUM 70 60 50 0 1375/76 TA01 0.25 0.75 1.00 0.50 LOAD CURRENT (A) 1.25 1375/76 TA02 1 LT1375/LT1376 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage LT1375/LT1376 .................................................. 25V LT1375HV/LT1376HV ........................................ 30V BOOST Pin Voltage LT1375/LT1376 .................................................. 35V LT1375HV/LT1376HV ........................................ 40V SHDN Pin Voltage ..................................................... 7V BIAS Pin Voltage ...................................................... 7V FB Pin Voltage (Adjustable Part) ............................ 3.5V FB Pin Current (Adjustable Part) ............................ 1mA Sense Voltage (Fixed 5V Part) .................................. 7V SYNC Pin Voltage ..................................................... 7V Operating Ambient Temperature Range LT1375C/LT1376C ................................. 0°C to 70°C LT1375I/LT1376I ............................... – 40°C to 85°C Operating Junction Temperature Range LT1375C/LT1376C ............................... 0°C to 125° C LT1375I/LT1376I ............................. – 40°C to 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C PACKAGE/ORDER INFORMATION TOP VIEW BOOST 1 VIN 2 VSW 3 SHDN 4 N8 PACKAGE 8-LEAD PDIP 8 VC 7 FB/SENSE 6 GND 5 SYNC S8 PACKAGE 8-LEAD PLASTIC SO BOOST 1 VIN 2 VSW 3 BIAS 4 N8 PACKAGE 8-LEAD PDIP TOP VIEW 8 VC 7 FB/SENSE 6 GND 5 SHDN S8 PACKAGE 8-LEAD PLASTIC SO θJA = 100°C/ W (N8) θJA = 120°C/ W TO 150°C/W DEPENDING ON PC BOARD LAYOUT (S8) θJA = 100°C/ W (N8) θJA = 120°C/ W TO 150°C/W DEPENDING ON PC BOARD LAYOUT (S8) ORDER PART NUMBER LT1375CN8 LT1375CN8-5 LT1375CS8 LT1375CS8-5 LT1375HVCS8 LT1375IN8 LT1375IN8-5 LT1375IS8 LT1375IS8-5 LT1375HVIS8 ORDER PART NUMBER LT1376CN8 LT1376CN8-5 LT1376CS8 LT1376CS8-5 LT1376HVCS8 LT1376IN8 LT1376IN8-5 LT1376IS8 LT1376IS8-5 LT1376HVIS8 S8 PART MARKING 1375 13755 1375HV 1375I 1375I5 375HVI S8 PART MARKING 1376 13765 1376HV 1376I 1376I5 376HVI Consult factory for Military grade parts. 2 U U W WW U W TOP VIEW GND 1 NC 2 BOOST 3 VIN 4 VSW 5 BIAS 6 NC 7 GND 8 16 GND 15 NC 14 VC 13 FB/SENSE 12 GND 11 SHDN 10 NC 9 GND S PACKAGE 16-LEAD PLASTIC NARROW SO θJA = 50°C/ W WITH FUSED CORNER PINS CONNECTED TO GROUND PLANE OR LARGE LANDS ORDER PART NUMBER LT1376CS LT1376IS LT1376HVCS LT1376HVIS LT1375/LT1376 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open, unless otherwise noted. PARAMETER Reference Voltage (Adjustable) All Conditions Sense Voltage (Fixed 5V) All Conditions Sense Pin Resistance Reference Voltage Line Regulation Feedback Input Bias Current Error Amplifier Voltage Gain Error Amplifier Transconductance VC Pin to Switch Current Transconductance Error Amplifier Source Current Error Amplifier Sink Current VC Pin Switching Threshold VC Pin High Clamp Switch Current Limit VBOOST = VIN + 5V Switch On Resistance (Note 6) Maximum Switch Duty Cycle VSHDN = 1V, VFB = 2.1V or VSENSE = 4.4V VSHDN = 1V, VFB = 2.7V or VSENSE = 5.6V Duty Cycle = 0 VSHDN = 1V VC Open, VFB = 2.1V or VSENSE = 4.4V, DC ≤ 50% DC = 80% q q q q q q ELECTRICAL CHARACTERISTICS CONDITIONS MIN 2.39 2.36 4.94 4.90 7 TYP 2.42 5.0 10 0.01 0.01 MAX 2.45 2.48 5.06 5.10 14 0.03 0.03 1.5 2700 3000 320 UNITS V V V V kΩ %/ V %/V µA µMho µMho A/ V µA mA V V 5V ≤ VIN ≤ 25V 5V ≤ VIN ≤ 30V (LT1375HV/LT1376HV) q 0.5 200 400 2000 2 150 225 2 0.9 2.1 1.50 1.35 2 0.3 90 86 85 460 440 440 93 93 93 500 1500 1100 VSHDN = 1V (Notes 2, 8) VSHDN = 1V, ∆I (VC) = ± 10µA (Note 8) q 3 3 0.4 0.5 A A Ω Ω % % % ISW = 1.5A, VBOOST = VIN + 5V VFB = 2.1V or VSENSE = 4.4V – 40°C ≤ TJ ≤ 125°C TJ = 150°C – 25°C ≤ TJ ≤ 125°C TJ ≤ – 25°C q q q q Switch Frequency VC Set to Give 50% Duty Cycle 540 560 570 0.15 0.15 1.3 5.5 3.5 22 35 1.4 4.0 50 75 75 100 2.46 kHz kHz kHz %/ V %/V V V V mA mA mA mA µA µA µA µA V Switch Frequency Line Regulation Frequency Shifting Threshold on FB Pin Minimum Input Voltage (Note 3) Minimum Boost Voltage (Note 4) Boost Current (Note 5) Input Supply Current (Note 6) Output Supply Current (Note 6) Shutdown Supply Current 5V ≤ VIN ≤ 25V 5V ≤ VIN ≤ 30V (LT1375HV/LT1376HV) ∆f = 10kHz ISW ≤ 1.5A VBOOST = VIN + 5V VBIAS = 5V VBIAS = 5V VSHDN = 0V, VIN ≤ 25V, VSW = 0V, VC Open ISW = 500mA ISW = 1.5A 0.05 0.05 0.8 1.0 5.0 3 12 25 0.9 3.2 15 q q q q q q VSHDN = 0V, VIN ≤ 30V, VSW = 0V, VC Open (LT1375HV/LT1376HV) q 20 q Lockout Threshold VC Open 2.3 2.38 3 LT1375/LT1376 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open, unless otherwise noted. PARAMETER Shutdown Thresholds CONDITIONS VC Open VC Open Device Shutting Down Device Starting Up LT1375HV/LT1376HV Device Shutting Down LT1375HV/LT1376HV Device Starting Up q q q q ELECTRICAL CHARACTERISTICS MIN 0.15 0.25 0.15 0.25 TYP 0.37 0.45 0.37 0.45 1.5 MAX 0.60 0.60 0.70 0.70 2.2 900 UNITS V V V V V kHz kΩ Minimum Synchronizing Amplitude (LT1375 Only) Synchronizing Range (LT1375 Only) SYNC Pin Input Resistance VIN = 5V q 580 40 Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Gain is measured with a VC swing equal to 200mV above the low clamp level to 200mV below the upper clamp level. Note 3: Minimum input voltage is not measured directly, but is guaranteed by other tests. It is defined as the voltage where internal bias lines are still regulated so that the reference voltage and oscillator frequency remain constant. Actual minimum input voltage to maintain a regulated output will depend on output voltage and load current. See Applications Information. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch. Note 5: Boost current is the current flowing into the BOOST pin with the pin held 5V above input voltage. It flows only during switch-on time. Note 6: Input supply current is the bias current drawn by the input pin when the BIAS pin is held at 5V with switching disabled. Output supply current is the current drawn by the BIAS pin when the bias pin is held at 5V. Total input referred supply current is calculated by summing input supply current (ISI) with a fraction of output supply current (ISO): ITOT = ISI + (ISO)(VOUT/VIN)(1.15) With VIN = 15V, VOUT = 5V, ISI = 0.9mA, ISO = 3.6mA, ITOT = 2.28mA. For the LT1375, quiescent current is equal to: ITOT = ISI + ISO(1.15) because the BIAS pin is internally connected to VIN. Note 7: Switch-on resistance is calculated by dividing VIN to VSW voltage by the forced current (1.5A). See Typical Performance Characteristics for the graph of switch voltage at other currents. Note 8: Transconductance and voltage gain refer to the internal amplifier exclusive of the voltage divider. To calculate gain and transconductance refer to sense pin on fixed voltage parts. Divide values shown by the ratio VOUT/2.42. TYPICAL PERFORMANCE CHARACTERISTICS Inductor Core Loss 1.0 VOUT = 5V, VIN = 10V, IOUT = 1A TYPE 52 POWDERED IRON Kool Mµ® PERMALLOY µ = 125 CORE LOSS IS INDEPENDENT OF LOAD CURRENT UNTIL LOAD CURRENT FALLS LOW ENOUGH FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE 0 5 10 15 INDUCTANCE (µH) 20 25 20 12 8 CORE LOSS (W) 0.1 2 1.2 0.8 0.4 FEEDBACK VOLTAGE (V) 4 SWITCH PEAK CURRENT (A) 0.01 0.001 Kool Mµ is a registered trademark of Magnetics, Inc. 4 UW 1375/76 G01 Switch Peak Current Limit 2.5 TYPICAL 2.0 2.43 2.44 Feedback Pin Voltage and Current 2.0 CORE LOSS (% OF 5W LOAD) 1.5 CURRENT (µA) 1.5 GUARANTEED MINIMUM 2.42 VOLTAGE 1.0 1.0 0.2 0.12 0.08 0.04 0.02 CURRENT 2.41 0.5 0.5 0 0 20 60 40 DUTY CYCLE (%) 80 100 1375/76 G08 2.40 –50 –25 100 JUNCTION TEMPERATURE (°C) 0 25 50 75 0 125 1375/76 G09 LT1375/LT1376 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Pin Bias Current 500 400 300 200 8 4 0 –50 –25 CURRENT REQUIRED TO FORCE SHUTDOWN (FLOWS OUT OF PIN). AFTER SHUTDOWN, CURRENT DROPS TO A FEW µA 2.40 STANDBY 2.36 INPUT SUPPLY CURRENT (µA) SHUTDOWN PIN VOLTAGE (V) CURRENT (µA) AT 2.38V STANDBY THRESHOLD (CURRENT FLOWS OUT OF PIN) 50 25 75 0 TEMPERATURE (°C) Shutdown Supply Current 150 125 100 VIN = 25V 75 50 25 0 VIN = 10V INPUT SUPPLY CURRENT (µA) TRANSCONDUCTANCE (µMho) GAIN (µMho) 0 0.1 0.2 0.3 0.4 SHUTDOWN VOLTAGE (V) Frequency Foldback SWITCHING FREQUENCY (kHz) OR CURRENT (µA) 500 400 INPUT VOLTAGE (V) FREQUENCY (kHz) SWITCHING FREQUENCY 300 200 100 FEEDBACK PIN CURRENT 0 0 0.5 1.5 2.0 1.0 FEEDBACK PIN VOLTAGE (V) 2.5 1375/76 G10 UW 100 1375/76 G04 Standby and Shutdown Thresholds 30 Shutdown Supply Current VSHUTDOWN = 0V 25 20 15 10 5 0 2.32 0.8 START-UP 0.4 SHUTDOWN 125 0 50 100 25 75 –50 –25 0 JUNCTION TEMPERATURE (°C) 125 0 5 10 15 INPUT VOLTAGE (V) 20 25 1375/76 G06 1375/76 G05 Error Amplifier Transconductance 2500 3000 Error Amplifier Transconductance 200 PHASE 2500 GAIN 100 VC 2000 150 PHASE (DEG) 1500 2000 1000 1500 VFB 2 • 10–3 ( ) ROUT 200k COUT 12pF 50 500 1000 ERROR AMPLIFIER EQUIVALENT CIRCUIT 0 RLOAD = 50Ω 0.5 1375/76 G07 0 50 0 75 100 25 –50 –25 JUNCTION TEMPERATURE (°C) 125 500 100 1k 10k 100k FREQUENCY (Hz) 1M –50 10M 1375/76 G03 1375/76 G02 Switching Frequency 600 8.5 8.0 550 7.5 7.0 6.5 6.0 5.5 400 –50 5.0 –25 100 JUNCTION TEMPERATURE (°C) 0 25 50 75 125 LT1376 Minimum Input Voltage with 5V Output MINIMUM INPUT VOLTAGE CAN BE REDUCED BY ADDING A SMALL EXTERNAL PNP. SEE APPLICATIONS INFORMATION 500 MINIMUM VOLTAGE TO START WITH STANDARD CIRCUIT MINIMUM VOLTAGE TO RUN WITH STANDARD CIRCUIT 0 10 100 LOAD CURRENT (mA) 1000 1375/76 G12 450 1375/76 G11 5 LT1375/LT1376 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current at VOUT = 10V 1.50 VOUT = 10V 1.25 1.00 0.75 0.50 0.25 0 L = 20µH L = 10µH CURRENT (A) CURRENT (A) CURRENT (A) 0 5 10 15 INPUT VOLTAGE (V) BOOST Pin Current 12 TJ = 25°C 10 BOOST PIN CURRENT (mA) THRESHOLD VOLTAGE (V) 1.2 1.4 8 6 4 2 0 1.0 SWITCH VOLTAGE (V) 0 0.25 0.50 0.75 1.00 SWITCH CURRENT (A) PIN FUNCTIONS BOOST: The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Without this added voltage, the typical switch voltage loss would be about 1.5V. The additional boost voltage allows the switch to saturate and voltage loss approximates that of a 0.3Ω FET structure, but with much smaller die area. Efficiency improves from 75% for conventional bipolar designs to > 87% for these new parts. VSW: The switch pin is the emitter of the on-chip power NPN switch. It is driven up to the input pin voltage during switch on time. Inductor current drives the switch pin negative during switch off time. Negative voltage is clamped with the external catch diode. Maximum negative switch voltage allowed is – 0.8V. SHDN: The shutdown pin is used to turn off the regulator and to reduce input drain current to a few microamperes. Actually, this pin has two separate thresholds, one at 2.38V to disable switching, and a second at 0.4V to force complete micropower shutdown. The 2.38V threshold functions as an accurate undervoltage lockout (UVLO). This is sometimes used to prevent the regulator from operating until the input voltage has reached a predetermined level. 6 UW L = 5µH 20 25 1375/76 G13 Maximum Load Current at VOUT = 3.3V 1.50 L = 20µH 1.25 1.00 0.75 0.50 0.25 VOUT = 3.3V 0 0 0 5 10 15 INPUT VOLTAGE (V) 20 25 1375/76 G14 Maximum Load Current at VOUT = 5V 1.50 L = 20µH 1.25 1.00 0.75 0.50 0.25 VOUT = 5V 0 5 10 15 INPUT VOLTAGE (V) 20 25 1375/76 G15 L = 10µH L = 5µH L = 10µH L = 5µH VC Pin Shutdown Threshold 0.8 SHUTDOWN Switch Voltage Drop TJ = 25°C 0.6 0.4 0.8 0.6 0.2 1.25 0.4 –50 –25 100 JUNCTION TEMPERATURE (°C) 0 25 50 75 125 0 0 0.25 0.50 0.75 1.00 1.25 SWITCH CURRENT (A) 1.50 1375/76 G16 1375/76 G11 1375/76 G18 U U U LT1375/LT1376 PIN FUNCTIONS BIAS (LT1376 Only): The BIAS pin is used to improve efficiency when operating at higher input voltages and light load current. Connecting this pin to the regulated output voltage forces most of the internal circuitry to draw its operating current from the output voltage rather than the input supply. This is a much more efficient way of doing business if the input voltage is much higher than the output. Minimum output voltage setting for this mode of operation is 3.3V. Efficiency improvement at VIN = 20V, VOUT = 5V, and IOUT = 25mA is over 10%. SYNC (LT1375 Only): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 10% and 90% duty cycle. The synchronizing range is equal to initial operating frequency, up to 900kHz. See Synchronizing section in Applications Information for details. FB/SENSE: The feedback pin is used to set output voltage, using an external voltage divider that generates 2.42V at the pin with the desired output voltage. The fixed voltage (-5) parts have the divider included on the chip, and the FB pin is used as a SENSE pin, connected directly to the 5V output. Two additional functions are performed by the FB pin. When the pin voltage drops below 1.7V, switch current limit is reduced. Below 1V, switching frequency is also reduced. See Feedback Pin Function section in Applications Information for details. VC: The VC pin is the output of the error amplifier and the input of the peak switch current comparator. It is normally used for frequency compensation, but can do double duty as a current clamp or control loop override. This pin sits at about 1V for very light loads and 2V at maximum load. It can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4mA. GND: The GND pin connection needs consideration for two reasons. First, it acts as the reference for the regulated output, so load regulation will suffer if the “ground” end of the load is not at the same voltage as the GND pin of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pin and the load ground point. Keep the ground path short between the GND pin and the load, and use a ground plane when possible. The second consideration is EMI caused by GND pin current spikes. Internal capacitance between the VSW pin and the GND pin creates very narrow (