LT1612 Synchronous, Step-Down 800kHz PWM DC/DC Converter U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ The LT®1612 is an 800kHz, synchronous step-down DC/ DC converter that operates from an input voltage as low as 2V. Internal 0.45Ω switches deliver output currents up to 500mA, and the 800kHz switching frequency allows the use of small, low value ceramic input and output capacitors. Input voltage ranges from 5.5V down to 2V and output voltage can be set as low as the 620mV reference. The device features Burst ModeTM operation, keeping efficiency high at light loads. Burst Mode operation can be defeated by pulling the MODE pin high, enabling constant switching throughout the load range for low noise. Operates from Input Voltage As Low As 2V Internal 0.7A Synchronous Switches Uses Ceramic Input and Output Capacitors 620mV Reference Voltage 800kHz Fixed Frequency Switching Programmable Burst Mode Operation Low Quiescent Current: 160µA 8-Lead MSOP or SO Package U APPLICATIO S ■ ■ ■ ■ Portable Devices Lithium-Ion Step-Down Converters 5V to 3.3V Conversion 2-Cell Alkaline Step-Down Converters No-load quiescent current is 160µA and shutdown current is less than 1µA. The device is available in 8-lead SO and MSOP packages. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. U TYPICAL APPLICATION Efficiency for LT1612 vs Linear Regulator VOUT = 1.2V 0.1µF L1 10µH BOOST VIN SW SHDN LT1612 C1 10µF MODE VC 90 VOUT 1.2V 500mA 100pF FB GND 33.2k 330pF R2 232k 1% R1 215k 1% C2 68µF 3.15V EFFICIENCY (%) VIN 2V 80 VIN = 2V 70 VIN = 3V 60 VIN = 2V (LINEAR) 50 40 C1: TAIYO-YUDEN JMK325BJ106MN C2: PANASONIC EEFCDOF680R L1: SUMIDA CD43-100 Figure 1. 2V to 1.2V Converter VIN = 3V (LINEAR) 1612 F01a 30 10 100 LOAD CURRENT (mA) 500 1612 • F01b 1 LT1612 U W W W ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage (VIN) ............................................... 5.5V SW Pin Voltage ....................................................... 5.5V FB Pin Voltage ............................................... VIN + 0.3V VC Pin Voltage ........................................................... 2V SHDN Pin Voltage ................................................... 5.5V MODE Pin Voltage .................................................. 5.5V BOOST Pin Voltage ....................................... VIN + 5.5V Junction Temperature ........................................... 125°C Operating Temperature Range (Note 2) ... –40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C W U U PACKAGE/ORDER INFORMATION ORDER PART NUMBER TOP VIEW VC FB VIN GND 1 2 3 4 8 7 6 5 SHDN MODE BOOST SW MS8 PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 125°C, θJA = 200°C/ W LT1612EMS8 MS8 PART MARKING ORDER PART NUMBER TOP VIEW VC 1 8 SHDN FB 2 7 MODE VIN 3 6 BOOST GND 4 5 SW LT1612ES8 S8 PART MARKING S8 PACKAGE 8-LEAD PLASTIC SO LTMS 1612 TJMAX = 125°C, θJA = 120°C/ W Consult factory for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V SYMBOL PARAMETER IQ Quiescent Current VFB FB Voltage CONDITIONS MODE = 5V MODE = 0V, Not Switching SHDN = 0V MIN ● ● ● ● gm UNITS 1 160 2 220 1 mA µA µA 0.62 0.62 0.635 0.635 V V ● 0.02 0.15 %/V FB Pin Bias Current (Note 3) ● 7 50 nA Error Amplifier Transconductance 2 5.5 Oscillator Frequency 700 550 fOSC Line Regulation Maximum Duty Cycle ● Shutdown Threshold µmhos 250 ● 2 MAX FB Line Regulation Min Input Voltage Max Input Voltage fOSC 0.605 0.60 TYP Minimum Voltage for Active Maximum Voltage for Shutdown ● ● 85 80 800 900 1100 V V kHz kHz 1 %/ V 90 % % 2 0.2 V V LT1612 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V SYMBOL PARAMETER CONDITIONS MIN SHDN Pin Current SHDN = 2V SHDN = 5V BOOST Pin Current BOOST = VIN + 2V Switch Current Limit (Note 4) Duty Cycle = 0% ● ● TYP MAX 10 30 15 45 UNITS µA µA 4 mA MODE = OV 600 710 900 mA MODE = 5V 550 650 900 mA Burst Mode Operation Current Limit MODE = 0V 180 Switch Voltage Drop ISW = 500mA 200 280 mV Rectifier Voltage Drop IRECT = 500mA 300 400 mV SW Pin Leakage VSW = 5V 1 µA Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1612E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating ● mA temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Bias current flows out of the FB pin. Note 4: Duty cycle affects current limit due to slope compensation. U W TYPICAL PERFOR A CE CHARACTERISTICS FB Voltage vs Temperature Quiescent Current vs Temperature 0.64 FB Pin Bias Current 0 190 MODE = 0V 0.62 0.61 0.60 0.59 – 50 –25 0 25 50 TEMPERATURE (°C) 75 100 1612 • G01 –2 170 –4 FB PIN CURRENT (nA) QUIESCENT CURRENT (µA) FB VOLTAGE (V) 0.63 180 160 150 140 130 120 –6 –8 –10 –12 –14 –16 110 –18 100 – 50 –20 – 50 –25 0 25 50 TEMPERATURE (°C) 75 100 1612 • G02 – 25 25 50 0 TEMPERATURE (°C) 75 100 1612 • G03 3 LT1612 U W TYPICAL PERFOR A CE CHARACTERISTICS 800 80 700 SWITCH CURRENT (mA) 60 50 40 30 20 MODE = 5V 600 500 400 300 10 0 0 1 2 4 3 SHDN PIN VOLTAGE (V) 200 –50 5 –25 0 25 50 TEMPERATURE (°C) 1612 • G04 500 86 82 78 74 0 25 50 TEMPERATURE (°C) 700 600 500 400 –50 100 75 100 1612 • G07 –25 0 25 50 TEMPERATURE (°C) 100 Rectifier Voltage Drop 600 400 300 200 100 0 75 1612 • G06 RECTIFIER VOLTAGE DROP (mV) 90 SWITCH VOLTAGE DROP (mV) 600 –25 800 Switch Voltage Drop 94 70 –50 75 900 1612 • G05 Maximum Duty Cycle vs Temperature MAXIMUM DUTY CYCLE (%) OSCILLATOR FREQUENCY (kHz) SHDN PIN CURRENT (µA) 1000 MODE = 0V 70 4 Oscillator Frequency vs Temperature Switch Current Limit vs Temperature SHDN Pin Bias Current 0 100 500 200 300 400 SWITCH CURRENT (mA) 600 1612 • G08 500 400 300 200 100 0 0 100 500 200 300 400 RECTIFIER CURRENT (mA) 600 1612 • G09 LT1612 U U U PIN FUNCTIONS VC (Pin 1): Compensation Pin. This is the current sink/ source output of the error amplifier. By connecting an RC network from this pin to ground, frequency response can be tuned for a wide range of circuit configurations. The voltage at this pin also sets the current limit, and if grounded, the switch will remain in the OFF state. SW (Pin 5): Switch Pin. Connect inductor and boost capacitor here. Minimize trace area at this pin to keep EMI down. BOOST (Pin 6): This is the supply pin for the switch driver and must be above VIN by 1.5V for proper switch operation. Connect the boost capacitor to this pin. FB (Pin 2): Feedback Pin. This pin is the negative input to the error amplifier. Connect the resistor divider tap to this point which sets VOUT according to: MODE (Pin 7): Burst Mode Operation Disable Pin. For continuous switching operation (low noise), pull this pin above 2V. For Burst Mode operation which gives better light load efficiency, tie to ground. Output ripple voltage in Burst Mode operation is typically 30mVP-P. See applications section for more information about this function. VOUT = 0.62V (1 + R1/R2) VIN (Pin 3): Supply Pin. Bypass capacitor C1 must be right next to this pin. SHDN (Pin 8): Shutdown Pin. Pull this pin low for shutdown mode. Tie to a voltage between 2V and 5.5V for normal operation. GND (Pin 4): Ground Pin. Connect directly to local ground plane. W BLOCK DIAGRA RSENSE 0.08Ω BOOST DIODE 6 BOOST VIN 3 VC 1 FB 2 – + + A1 V/I – SLOPE COMPENSATION A2 0.62V SWITCH MODE + – 7 0.7V SWITCH DRIVER 5 SW A3 FLIP-FLOP R ENABLE SHDN 8 SHUTDOWN Q RECTIFIER DRIVE RECTIFIER S OSCILLATOR 4 GND 1612 BD 5 LT1612 U OPERATIO The LT1612 employs fixed frequency, current mode control. This type of control uses two feedback loops. The main control loop sets output voltage and operates as follows: A load step causes VOUT and the FB voltage to be perturbed slightly. The error amplifier responds to this change in FB by driving the VC pin either higher or lower. Because switch current is proportional to the VC pin voltage, this change causes the switch current to be adjusted until VOUT is once again satisfied. Loop compensation is taken care of by an RC network from the VC pin to ground. Layout Hints Inside this main loop is another that sets current limit on a cycle-by-cycle basis. This loop utilizes current comparator A2 to control peak current. The oscillator runs at 800kHz and issues a set pulse to the flip-flop at the beginning of each cycle, turning the switch on. With the switch now in the ON state the SW pin is effectively connected to VIN. Current ramps up in the inductor linearly at a rate of (VIN – VOUT)/L. Switch current is set by the VC pin voltage and when the voltage across RSENSE trips the current comparator, a reset pulse will be generated and the switch will be turned off. Since the inductor is now loaded up with current, the SW pin will fly low and trigger the rectifier to turn on. Current will flow through the rectifier decreasing at a rate of VOUT/L until the oscillator issues a new set pulse, causing the cycle to repeat. The ground terminal of input capacitor C1 should tie close to Pin 4 of the LT1612. Doing this reduces dI/dt in the ground copper which keeps high frequency spikes to a minimum. The DC/DC converter ground should tie to the PC board ground plane at one place only, to avoid introducing dI/dt in the ground plane. If the load is light and VC decreases below A3’s trip point, the device will enter the Burst Mode operation region (the MODE pin must be at ground or floating). In this state the oscillator and all other circuitry except the reference and comparator A3 are switched on and off at low frequency. This mode of operation increases efficiency at light loads but introduces low frequency voltage ripple at the output. For continuous switching and no low frequency output voltage ripple, pull the MODE pin high. This will disable comparator A3 which forces the oscillator to run continuously. 6 The LT1612 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layout. Figure 2 shows recommended component placement for a buck (step-down) converter. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor C1 must be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation. R1 R2 CC RC 1 2 VIN C1 LT1612 8 SHDN 7 MODE 3 6 4 5 C3 C2 L1 MULTIPLE VIAs 1612 F02 GND VOUT Figure 2. Recommended Component Placement. Traces Carrying High Current are Direct. Trace Area at FB Pin and VC Pin Is Kept Low. Lead Length to Battery Should Be Kept Short LT1612 U OPERATIO Burst Mode Operation Defeat To maintain high efficiency at light loads, the LT1612 will automatically shift into Burst Mode operation (MODE = 0V or floating). In this mode of operation the oscillator and switch drive circuitry is alternately turned on and off, reducing quiescent current to 160µA. This reduces power consumption but also adds low frequency voltage ripple to the output. Figure 3 shows switching waveforms for a 5V to 3.3V converter running in Burst Mode operation. Output voltage ripple is approximately 20mVP-P. If the MODE pin is pulled high, Burst Mode operation will be inhibited and the oscillator runs continuously with no low frequency ripple at the output. See Figures 4 and 5. VOUT 200mV/DIV AC COUPLED VOUT 20mV/DIV AC COUPLED IL 200mA/DIV IL 200mA/DIV ILOAD 10mA TO 310mA 5µs/DIV 0.1ms/DIV 1612 F03 Figure 3. Output Voltage Ripple is 20mVP-P for the Circuit of Figure 1 1612 F04 Figure 4. Transient Response for the Circuit of Figure 1 with the MODE Pin Tied to Ground or Floating VOUT 200mV/DIV AC COUPLED IL 200mA/DIV ILOAD 10mA TO 300mA 0.1ms/DIV 1612 F05 Figure 5. With the MODE Pin Tied High, Low Frequency Output Voltage Ripple Is No Longer Present 7 LT1612 U TYPICAL APPLICATIONS Single Li-Ion to 2V Converter Li-Ion to 2V Converter Efficiency 85 0.1µF L1 10µH BOOST VIN SW SHDN LT1612 MODE VC 10µF CERAMIC 30.1k 680pF 80 VOUT 2V 500mA VIN = 2.8V 75 EFFICIENCY (%) VIN 2.7V TO 4.2V 20pF FB GND 1M 1% 453k 1% 22µF CERAMIC VIN = 4.2V 70 65 VIN = 3.5V 60 55 C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100 50 1612 TA02 1 10 100 LOAD CURRENT (mA) 1000 1612 TA04 Transient Response Burst Mode Operation VOUT 20mV/DIV VOUT 50mV/DIV IL 200mA/DIV IL 100mA/DIV LOAD STEP 125mA TO 300mA VIN = 4V VOUT = 2V MODE = HIGH 100µs/DIV MODE = LOW 1612 TA03 Inrush Current at Start-Up VOUT 2V/DIV INRUSH CURRENT 200mA/DIV VSHDN 5V/DIV 0.2ms/DIV 8 1612 TA06 5µs/DIV 1612 TA05 LT1612 U TYPICAL APPLICATIONS 5V to 2.5V Converter 5V to 2.5V Converter Efficiency 85 0.1µF L1 10µH BOOST VIN SW SHDN LT1612 MODE VC C1 10µF CERAMIC 20pF FB GND 1M 30.1k 332k 680pF 80 VOUT 2.5V 500mA C2 22µF CERAMIC 75 EFFICIENCY (%) VIN 5V 70 65 60 55 50 C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100 1 1612 TA07 10 100 LOAD CURRENT (mA) 1000 1612 TA08 2V to 0.9V Converter Efficiency for LT1612 vs Linear Regulator. VOUT = 0.9V. 0.1µF L1 10µH BOOST VIN SW SHDN LT1612 C1 10µF MODE VC 33.2k 80 VOUT 0.9V 500mA 100pF 60 FB GND R2 232k 330pF C1: TAIYO-YUDEN JMK325BJ106MN C2: PANASONIC EEFCDOF680R L1: SUMIDA CD43-100 R1 105k VIN = 2V 70 C2 68µF 3.15V EFFICIENCY (%) VIN 2V VIN = 3V 50 VIN = 2V (LINEAR) 40 VIN = 3V (LINEAR) 30 20 1612 TA09 10 1 10 100 LOAD CURRENT (mA) 1000 1612 TA10 9 LT1612 U PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted. MS8 Package 8-Lead Plastic MSOP (LTC DWG # 05-08-1660) 0.118 ± 0.004* (3.00 ± 0.102) 8 7 6 5 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 (4.90 ± 0.15) 1 2 3 4 0.043 (1.10) MAX 0.007 (0.18) 0° – 6° TYP 0.021 ± 0.006 (0.53 ± 0.015) SEATING PLANE 0.009 – 0.015 (0.22 – 0.38) 0.0256 (0.65) BSC * DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE 10 0.034 (0.86) REF 0.005 ± 0.002 (0.13 ± 0.05) MSOP (MS8) 1100 LT1612 U PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 8 7 6 5 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) SO8 1298 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0°– 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.014 – 0.019 (0.355 – 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 2 3 4 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) BSC 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. 11 LT1612 U TYPICAL APPLICATIO 5V to 3.3V Converter Efficiency C3 0.1µF L1 10µH BOOST VIN SW SHDN LT1612 MODE C1 10µF VC VOUT 3.3V 500mA 20pF FB GND R3 33.2k C4 680pF R2 232k 1% R1 1M 1% VIN = 5V VOUT = 3.3V 80 75 EFFICIENCY (%) VIN 5V 85 70 65 60 C2 22µF 55 50 C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100 1612 TA01a 1 10 100 LOAD CURRENT (mA) 1000 1612 TA01b RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC 1474 Low IQ Step-Down Switching Regulator 10µA IQ, VIN from 3V to 18V, MSOP Package up to 300mA LT1616 600mA, 1.4MHz Step-Down Regulator in SOT-23 VIN from 3.6V to 25V, SOT-23 Package LTC1701 SOT-23 Step-Down Switching Regulator 500mA in SOT-23 Package, 1MHz Switching Frequency LTC1707 Monolithic Synchronous Step-Down Switching Regulator 500mA, VIN from 2.65V to 8.5V ® LTC1772 Constant Frequency Step-Down Controller in SOT-23 High Current, High Efficiency: Up to 94% LTC1877/LTC1878 High Efficiency, Monolithic Synchronous Step-Down Regulator 10µA IQ, 2.65≤ VIN ≤ 10V, MSOP Package up to 600mA LTC3404 1.4MHz High Efficiency Monolithic Synchronous Step-Down Reg 10µA IQ, High Efficiency: up to 95%, MSOP Package 12 Linear Technology Corporation 1612f LT/TP 1100 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1999