LT1308A/LT1308B High Current, Micropower Single Cell, 600kHz DC/DC Converters U FEATURES DESCRIPTIO ■ The LT ®1308A/LT1308B are micropower, fixed frequency step-up DC/DC converters that operate over a 1V to 10V input voltage range. They are improved versions of the LT1308 and are recommended for use in new designs. The LT1308A features automatic shifting to power saving Burst Mode operation at light loads and consumes just 140μA at no load. The LT1308B features continuous switching at light loads and operates at a quiescent current of 2.5mA. Both devices consume less than 1μA in shutdown. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 5V at 1A from a Single Li-Ion Cell 5V at 800mA in SEPIC Mode from Four NiCd Cells Fixed Frequency Operation: 600kHz Boost Converter Outputs up to 34V Starts into Heavy Loads Automatic Burst ModeTM Operation at Light Load (LT1308A) Continuous Switching at Light Loads (LT1308B) Low VCESAT Switch: 300mV at 2A Pin-for-Pin Upgrade Compatible with LT1308 Lower Quiescent Current in Shutdown: 1μA (Max) Improved Accuracy Low-Battery Detector Reference: 200mV ±2% Available in 8-Lead SO and 14-Lead TSSOP Packages U APPLICATIO S ■ ■ ■ ■ ■ ■ ■ GSM/CDMA Phones Digital Cameras LCD Bias Supplies Answer-Back Pagers GPS Receivers Battery Backup Supplies Handheld Computers Low-battery detector accuracy is significantly tighter than the LT1308. The 200mV reference is specified at ±2% at room and ±3% over temperature. The shutdown pin enables the device when it is tied to a 1V or higher source and does not need to be tied to VIN as on the LT1308. An internal VC clamp results in improved transient response and the switch voltage rating has been increased to 36V, enabling higher output voltage applications. The LT1308A/LT1308B are available in the 8-lead SO and the 14-lead TSSOP packages. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. U TYPICAL APPLICATIO L1 4.7μH Li-Ion CELL VIN C1 47μF SHUTDOWN 95 5V 1A SW LBO LBI LT1308B SHDN VC V IN = 4.2V 85 R1* 309k FB GND 47k V IN = 3.6V 90 + C2 220μF R2 100k 100pF EFFICIENCY (%) + Converter Efficiency D1 80 V IN = 2.5V V IN = 1.5V 75 70 65 60 55 C1: AVX TAJC476M010 C2: AVX TPSD227M006 D1: IR 10BQ015 L1: MURATA LQH6C4R7 *R1: 887k FOR VOUT = 12V 50 1308A/B F01a Figure 1. LT1308B Single Li-Ion Cell to 5V/1A DC/DC Converter 1 10 100 LOAD CURRENT (mA) 1000 1308A/B F01b 1308abfa 1 LT1308A/LT1308B W W W AXI U U ABSOLUTE RATI GS (Note 1) VIN, SHDN, LBO Voltage ......................................... 10V SW Voltage ............................................... – 0.4V to 36V FB Voltage ....................................................... VIN + 1V VC Voltage ................................................................ 2V LBI Voltage ................................................. – 0.1V to 1V Current into FB Pin .............................................. ±1mA Operating Temperature Range Commercial ............................................ 0°C to 70°C Extended Commerial (Note 2) ........... – 40°C to 85°C Industrial ........................................... – 40°C to 85°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C U U U PI CO FIGURATIO TOP VIEW TOP VIEW VC 1 14 LBO FB 2 13 LBI VC 1 8 LBO SHDN 3 12 VIN FB 2 7 LBI GND 4 11 VIN SHDN 3 6 VIN GND 5 10 SW GND 4 5 SW GND 6 9 SW GND 7 8 SW S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 125°C, θJA = 190°C/W F PACKAGE 14-LEAD PLASTIC TSSOP (Note 6) TJMAX = 125°C, θJA = 80°C/W NOT RECOMMENDED FOR NEW DESIGNS Contact Linear Technology for Potential Replacement U W U ORDER I FOR ATIO LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT1308ACS8#PBF LT1308ACS8#TRPBF 1308A 8-Lead Plastic SO 0°C to 70°C LT1308AIS8#PBF LT1308AIS8#TRPBF 1308AI 8-Lead Plastic SO –40°C to 85°C LT1308BCS8#PBF LT1308BCS8#TRPBF 1308B 8-Lead Plastic SO 0°C to 70°C LT1308BIS8#PBF LT1308BIS8#TRPBF 1308BI 8-Lead Plastic SO –40°C to 85°C LT1308ACF#PBF LT1308ACF#TRPBF LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C LT1308BCF#PBF LT1308BCF#TRPBF LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT1308ACS8 LT1308ACS8#TR 1308A 8-Lead Plastic SO 0°C to 70°C LT1308AIS8 LT1308AIS8#TR 1308AI 8-Lead Plastic SO –40°C to 85°C LT1308BCS8 LT1308BCS8#TR 1308B 8-Lead Plastic SO 0°C to 70°C LT1308BIS8 LT1308BIS8#TR 1308BI 8-Lead Plastic SO –40°C to 85°C LT1308ACF LT1308ACF#TR LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C LT1308BCF LT1308BCF#TR LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 1308abfa 2 LT1308A/LT1308B ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. Commercial Grade 0°C to 70°C. VIN = 1.1V, VSHDN = VIN, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Not Switching, LT1308A Switching, LT1308B VSHDN = 0V (LT1308A/LT1308B) VFB Feedback Voltage IB FB Pin Bias Current (Note 3) Reference Line Regulation 1.1V ≤ VIN ≤ 2V 2V ≤ VIN ≤ 10V MIN TYP MAX UNITS 140 2.5 0.01 240 4 1 μA mA μA 1.22 1.24 V ● 27 80 nA ● 0.03 0.01 0.4 0.2 %/V %/V 0.92 1 ● 1.20 Minimum Input Voltage gm Error Amp Transconductance AV Error Amp Voltage Gain fOSC Switching Frequency ΔI = 5μA VIN = 1.2V Maximum Duty Cycle V 60 μmhos 100 V/V ● 500 600 ● 82 90 2 3 4.5 A 350 400 mV mV Switch Current Limit Duty Cyle = 30% (Note 4) Switch VCESAT ISW = 2A (25°C, 0°C), VIN = 1.5V ISW = 2A (70°C), VIN = 1.5V 290 330 Burst Mode Operation Switch Current Limit (LT1308A) VIN = 2.5V, Circuit of Figure 1 400 Shutdown Pin Current VSHDN = 1.1V VSHDN = 6V VSHDN = 0V ● ● ● LBI Threshold Voltage ● 196 194 700 kHz % mA 2 20 0.01 5 35 0.1 μA μA μA 200 200 204 206 mV mV LBO Output Low ISINK = 50μA ● 0.1 0.25 V LBO Leakage Current VLBI = 250mV, VLBO = 5V ● 0.01 0.1 μA LBI Input Bias Current (Note 5) VLBI = 150mV 33 100 Low-Battery Detector Gain Switch Leakage Current 3000 VSW = 5V ● 0.01 nA V/V 10 μA The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. Industrial Grade – 40°C to 85°C. VIN = 1.2V, VSHDN = VIN, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Not Switching, LT1308A Switching, LT1308B VSHDN = 0V (LT1308A/LT1308B) VFB Feedback Voltage IB FB Pin Bias Current (Note 3) ● 27 80 nA Reference Line Regulation 1.1V ≤ VIN ≤ 2V 2V ≤ VIN ≤ 10V ● ● 0.05 0.01 0.4 0.2 %/V %/V 0.92 1 Error Amp Transconductance AV Error Amp Voltage Gain ● ● ● ● Minimum Input Voltage gm MIN ΔI = 5μA 1.19 TYP MAX UNITS 140 2.5 0.01 240 4 1 μA mA μA 1.22 1.25 V V 60 μmhos 100 V/V 1308abfa 3 LT1308A/LT1308B ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. Industrial Grade – 40°C to 85°C. VIN = 1.2V, VSHDN = VIN, unless otherwise noted. SYMBOL PARAMETER fOSC CONDITIONS Switching Frequency Maximum Duty Cycle MIN TYP MAX UNITS ● 500 600 750 kHz ● 82 90 2 3 4.5 A 350 400 mV mV Switch Current Limit Duty Cyle = 30% (Note 4) Switch VCESAT ISW = 2A (25°C, – 40°C), VIN = 1.5V ISW = 2A (85°C), VIN = 1.5V 290 330 Burst Mode Operation Switch Current Limit (LT1308A) VIN = 2.5V, Circuit of Figure 1 400 Shutdown Pin Current VSHDN = 1.1V VSHDN = 6V VSHDN = 0V ● ● LBI Threshold Voltage 196 193 ● % mA 2 20 0.01 5 35 0.1 μA μA μA 200 200 204 207 mV mV LBO Output Low ISINK = 50μA ● 0.1 0.25 V LBO Leakage Current VLBI = 250mV, VLBO = 5V ● 0.01 0.1 μA LBI Input Bias Current (Note 5) VLBI = 150mV 33 100 nA Low-Battery Detector Gain 3000 Switch Leakage Current ● VSW = 5V Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT1308ACS8, LT1308ACF, LT1308BCS8 and LT1308BCF are designed, characterized and expected to meet the industrial temperature limits, but are not tested at – 40°C and 85°C. I grade devices are guaranteed over the –40°C to 85°C operating temperature range. Note 3: Bias current flows into FB pin. V/V 0.01 10 μA Note 4: Switch current limit guaranteed by design and/or correlation to static tests. Duty cycle affects current limit due to ramp generator (see Block Diagram). Note 5: Bias current flows out of LBI pin. Note 6: Connect the four GND pins (Pins 4–7) together at the device. Similarly, connect the three SW pins (Pins 8–10) together and the two VIN pins (Pins 11, 12) together at the device. U W TYPICAL PERFORMANCE CHARACTERISTICS LT1308B 3.3V Output Efficiency V IN = 1.8V 80 V IN = 1.2V 75 95 90 90 V IN = 1.8V 85 EFFICIENCY (%) EFFICIENCY (%) 85 V IN = 2.5V 95 70 65 V IN = 2.5V 80 75 V IN = 1.2V 70 65 65 60 55 55 50 1000 1308A/B G01 1 100 10 LOAD CURRENT (mA) 1000 V IN = 2.5V 70 60 100 10 LOAD CURRENT (mA) V IN = 1.5V 75 55 1 V IN = 3.6V 80 60 50 V IN = 4.2V 85 EFFICIENCY (%) 95 90 LT1308A 5V Output Efficiency LT1308A 3.3V Output Efficiency 50 1 10 100 LOAD CURRENT (mA) 1000 1308A/B G02 1308A/B G03 1308abfa 4 LT1308A/LT1308B U W TYPICAL PERFORMANCE CHARACTERISTICS LT1308B 12V Output Efficiency 4.0 90 500 V IN = 5V 400 3.5 CURRENT LIMIT (A) V IN = 3.3V 80 75 70 65 SWITCH VCESAT (mV) 85 EFFICIENCY (%) Switch Saturation Voltage vs Current Switch Current Limit vs Duty Cycle 3.0 2.5 60 85°C 300 25°C 200 –40°C 100 55 50 2.0 1 100 10 LOAD CURRENT (mA) 1000 0 20 60 40 DUTY CYCLE (%) 80 1308A/B G04 Low Battery Detector Reference vs Temperature 80 203 70 202 60 201 LBI 50 VREF (mV) 85°C 20 BIAS CURRENT (nA) SHDN PIN CURRENT (μA) – 40°C 25°C 40 30 2 6 8 4 SHDN PIN VOLTAGE (V) 10 198 197 10 196 –25 0 25 50 TEMPERATURE (°C) 75 195 –50 100 Oscillator Frequency vs Temperature 1.25 750 170 1.24 550 500 –2.5 0 25 50 TEMPERATURE (°C) 75 100 1308 • G10 100 1.23 150 140 1.22 1.21 130 1.20 120 1.19 110 450 400 –50 160 VFB (V) QUIESCENT CURRENT (μA) 180 600 75 Feedback Pin Voltage vs Temperature 800 650 0 25 50 TEMPERATURE (°C) 1308 • G09 LT1308A Quiescent Current vs Temperature 700 –25 1308 • G08 1308 G07 FREQUENCY (kHz) 199 20 0 –50 0 200 FB 10 0 2.0 1308 G06 FB, LBI Bias Current vs Temperature 50 30 1.0 0.5 1.5 SWITCH CURRENT (A) 0 1308 • G05 SHDN Pin Bias Current vs Voltage 40 0 100 100 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 1308 • G11 1.18 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 1308 • G12 1308abfa 5 LT1308A/LT1308B U U U PIN FUNCTIONS (SO/TSSOP) VC (Pin 1/Pin 1): Compensation Pin for Error Amplifier. Connect a series RC from this pin to ground. Typical values are 47kΩ and 100pF. Minimize trace area at VC. FB (Pin 2/Pin 2): Feedback Pin. Reference voltage is 1.22V. Connect resistive divider tap here. Minimize trace area at FB. Set VOUT according to: VOUT = 1.22V(1 + R1/R2). SHDN (Pin 3/Pin 3): Shutdown. Ground this pin to turn off switcher. To enable, tie to 1V or more. SHDN does not need to be at VIN to enable the device. GND (Pin 4/Pins 4, 5, 6, 7): Ground. Connect directly to local ground plane. Ground plane should enclose all components associated with the LT1308. PCB copper connected to these pins also functions as a heat sink. For the TSSOP package, connect all pins to ground copper to get the best heat transfer. This keeps chip heating to a minimum. SW (Pin 5/Pins 8, 9, 10): Switch Pins. Connect inductor/ diode here. Minimize trace area at these pins to keep EMI down. For the TSSOP package, connect all SW pins together at the package. VIN (Pin 6/Pins 11, 12): Supply Pins. Must have local bypass capacitor right at the pins, connected directly to ground. For the TSSOP package, connect both VIN pins together at the package. LBI (Pin 7/Pin 13): Low-Battery Detector Input. 200mV reference. Voltage on LBI must stay between –100mV and 1V. Low-battery detector does not function with SHDN pin grounded. Float LBI pin if not used. LBO (Pin 8/Pin 14): Low-Battery Detector Output. Open collector, can sink 50μA. A 220kΩ pull-up is recommended. LBO is high impedance when SHDN is grounded. 1308abfa 6 LT1308A/LT1308B W BLOCK DIAGRA S VIN VIN 6 R6 40k 2VBE Q4 VIN R5 40k SHDN + VC gm VOUT 1 LBI – R1 (EXTERNAL) FB 2 FB Q1 R2 (EXTERNAL) + ERROR AMPLIFIER Q2 ×10 BIAS + 7 * – LBO 8 ENABLE A1 R3 30k R4 140k 3 SHUTDOWN – 200mV A4 SW COMPARATOR – RAMP GENERATOR + Σ + Q3 Q R + 5 DRIVER FF S A2 + 0.03Ω A=3 600kHz OSCILLATOR – 4 *HYSTERESIS IN LT1308A ONLY GND Figure 2a. LT1308A/LT1308B Block Diagram (SO-8 Package) 1308 BD2a VIN VIN 11 VIN 12 R5 40k R6 40k 2VBE Q4 VIN SHDN + VC gm VOUT R1 (EXTERNAL) FB R2 (EXTERNAL) Q1 Q2 ×10 LBI + ERROR AMPLIFIER BIAS – + 13 * R3 30k R4 140k 3 1 – FB 2 SHUTDOWN A1 LBO 14 ENABLE – 200mV A4 SW SW SW 8 COMPARATOR – RAMP GENERATOR + Σ + A2 Q3 Q R 10 DRIVER FF + 9 S + A=3 600kHz OSCILLATOR 0.03Ω – *HYSTERESIS IN LT1308A ONLY 4 5 6 7 GND GND GND GND 1308 BD2b Figure 2b. LT1308A/LT1308B Block Diagram (TSSOP Package) 1308abfa 7 LT1308A/LT1308B U W U U APPLICATIONS INFORMATION OPERATION The LT1308A combines a current mode, fixed frequency PWM architecture with Burst Mode micropower operation to maintain high efficiency at light loads. Operation can be best understood by referring to the block diagram in Figure 2. Q1 and Q2 form a bandgap reference core whose loop is closed around the output of the converter. When VIN is 1V, the feedback voltage of 1.22V, along with an 80mV drop across R5 and R6, forward biases Q1 and Q2’s base collector junctions to 300mV. Because this is not enough to saturate either transistor, FB can be at a higher voltage than VIN. When there is no load, FB rises slightly above 1.22V, causing VC (the error amplifier’s output) to decrease. When VC reaches the bias voltage on hysteretic comparator A1, A1’s output goes low, turning off all circuitry except the input stage, error amplifier and lowbattery detector. Total current consumption in this state is 140μA. As output loading causes the FB voltage to decrease, A1’s output goes high, enabling the rest of the IC. Switch current is limited to approximately 400mA initially after A1’s output goes high. If the load is light, the output voltage (and FB voltage) will increase until A1’s output goes low, turning off the rest of the LT1308A. Low frequency ripple voltage appears at the output. The ripple frequency is dependent on load current and output capacitance. This Burst Mode operation keeps the output regulated and reduces average current into the IC, resulting in high efficiency even at load currents of 1mA or less. If the output load increases sufficiently, A1’s output remains high, resulting in continuous operation. When the LT1308A is running continuously, peak switch current is controlled by VC to regulate the output voltage. The switch is turned on at the beginning of each switch cycle. When the summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscillations at duty factors greater than 50%) exceeds the VC signal, comparator A2 changes state, resetting the flip-flop and turning off the switch. Output voltage increases as switch current is increased. The output, attenuated by a resistor divider, appears at the FB pin, closing the overall loop. Frequency compensation is provided by an external series RC network connected between the VC pin and ground. Low-battery detector A4’s open-collector output (LBO) pulls low when the LBI pin voltage drops below 200mV. There is no hysteresis in A4, allowing it to be used as an amplifier in some applications. The entire device is disabled when the SHDN pin is brought low. To enable the converter, SHDN must be at 1V or greater. It need not be tied to VIN as on the LT1308. The LT1308B differs from the LT1308A in that there is no hysteresis in comparator A1. Also, the bias point on A1 is set lower than on the LT1308B so that switching can occur at inductor current less than 100mA. Because A1 has no hysteresis, there is no Burst Mode operation at light loads and the device continues switching at constant frequency. This results in the absence of low frequency output voltage ripple at the expense of efficiency. The difference between the two devices is clearly illustrated in Figure 3. The top two traces in Figure 3 shows an LT1308A/LT1308B circuit, using the components indicated in Figure 1, set to a 5V output. Input voltage is 3V. Load current is stepped from 50mA to 800mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on Trace B. At light loads, the LT1308B will begin to skip alternate cycles. The load point at which this occurs can be decreased by increasing the inductor value. However, output ripple will continue to be significantly less than the LT1308A output ripple. Further, the LT1308B can be forced into micropower mode, where IQ falls from 3mA to 200μA by sinking 40μA or more out of the VC pin. This stops switching by causing A1’s output to go low. TRACE A: LT1308A VOUT, 100mV/DIV AC COUPLED TRACE B: LT1308B VOUT, 100mV/DIV AC COUPLED 800mA ILOAD 50mA VIN = 3V 200μs/DIV (CIRCUIT OF FIGURE 1) 1308 F03 Figure 3. LT1308A Exhibits Burst Mode Operation Output Voltage Ripple at 50mA Load, LT1308B Does Not 1308abfa 8 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION Waveforms for a LT1308B 5V to 12V boost converter using a 10μF ceramic output capacitor are pictured in Figures 4 and 5. In Figure 4, the converter is operating in continuous mode, delivering a load current of approximately 500mA. The top trace is the output. The voltage increases as inductor current is dumped into the output capacitor during the switch off time, and the voltage decreases when the switch is on. Ripple voltage is in this case due to capacitance, as the ceramic capacitor has little ESR. The middle trace is the switch voltage. This voltage alternates between a VCESAT and VOUT plus the diode drop. The lower trace is the switch current. At the beginning of the switch cycle, the current is 1.2A. At the end of the switch on time, the current has increased to 2A, at which point the switch turns off and the inductor current flows into the output capacitor through the diode. Figure 5 depicts converter waveforms at a light load. Here the converter operates in discontinuous mode. The inductor current reaches zero during the switch off time, resulting in some ringing at the switch node. The ring frequency is set by switch capacitance, diode capacitance and inductance. This ringing has little energy, and its sinusoidal shape suggests it is free from harmonics. Minimizing the copper area at the switch node will prevent this from causing interference problems. LAYOUT HINTS The LT1308A/LT1308B switch current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layout. Figure 6 shows recommended component placement for an SO-8 package boost (step-up) 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. The negative terminal of output capacitor C2 should tie close to the ground pin(s) of the LT1308A/LT1308B. 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. C1 R1 1 R2 VOUT 100mV/DIV LBI LBO GROUND PLANE SHUTDOWN 2 3 LT1308A LT1308B GND 7 L1 6 5 + MULTIPLE VIAs ISW 1A/DIV VIN 8 4 VSW 10V/DIV + D1 C2 VOUT 500ns/DIV Figure 4. 5V to 12V Boost Converter Waveforms in Continuous Mode. 10μF Ceramic Capacitor Used at Output 1308 F04 Figure 6. Recommended Component Placement for SO-8 Package Boost Converter. Note Direct High Current Paths Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground Plane. Use Vias at One Location Only to Avoid Introducing Switching Currents into the Ground Plane VOUT 20mV/DIV VSW 10V/DIV ISW 500mA/DIV 500ns/DIV Figure 5. Converter Waveforms in Discontinuous Mode Figure 7 shows recommended component placement for a boost converter using the TSSOP package. Placement is similar to the SO-8 package layout. 1308abfa 9 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION C1 R1 R2 SHUTDOWN 1 14 2 13 + VIN L1 12 3 4 LT1308A LT1308B 10 6 9 7 8 + VIN SW R1 309k LT1308B SHDN D1 GND R2 100k 47k C2 680pF VOUT VOUT 5V 500mA FB VC GND D1 L1B C1 47μF SHUTDOWN + C2 4.7μF CERAMIC L1A CTX10-2 VIN 3V TO 10V 11 5 MULTIPLE VIAs A SEPIC (Single-Ended Primary Inductance Converter) schematic is shown in Figure 8. This converter topology produces a regulated output over an input voltage range that spans (i.e., can be higher or lower than) the output. Recommended component placement for an SO-8 package SEPIC is shown in Figure 9. LBI LBO GROUND PLANE C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006 D1: IR 10BQ015 L1: COILTRONICS CTX10-2 + C3 220μF 6.3V 1308A/B F08 1308 F07 Figure 7. Recommended Component Placement for TSSOP Boost Converter. Placement is Similar to Figure 4. Figure 8. SEPIC (Single-Ended Primary Inductance Converter) Converts 3V to 10V Input to a 5V/500mA Regulated Output LBI LBO GROUND PLANE C1 R1 + 1 R2 2 3 SHUTDOWN 8 LT1308A LT1308B 7 6 L1A L1B 5 4 MULTIPLE VIAs VIN C3 C2 + GND D1 VOUT 1308 F09 Figure 9. Recommended Component Placement for SEPIC 1308abfa 10 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION SHDN PIN The LT1308A/LT1308B SHDN pin is improved over the LT1308. The pin does not require tying to VIN to enable the device, but needs only a logic level signal. The voltage on the SHDN pin can vary from 1V to 10V independent of VIN. Further, floating this pin has the same effect as grounding, which is to shut the device down, reducing current drain to 1μA or less. A cross plot of the low-battery detector is shown in Figure 12. The LBI pin is swept with an input which varies from 195mV to 205mV, and LBO (with a 100k pull-up resistor) is displayed. VLBO 1V/DIV LOW-BATTERY DETECTOR The low-battery detector on the LT1308A/LT1308B features improved accuracy and drive capability compared to the LT1308. The 200mV reference has an accuracy of ±2% and the open-collector output can sink 50μA. The LT1308A/ LT1308B low-battery detector is a simple PNP input gain stage with an open-collector NPN output. The negative input of the gain stage is tied internally to a 200mV reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 10 details hookup. R1 and R2 need only be low enough in value so that the bias current of the LBI pin doesn’t cause large errors. For R2, 100k is adequate. The 200mV reference can also be accessed as shown in Figure 11. 5V R1 VIN LBI + LT1308A LT1308B 100k LBO R2 100k TO PROCESSOR – 200mV INTERNAL REFERENCE GND VBAT R1 = VLB – 200mV 2μA 1308 F10 195 200 VLBI (mV) 205 1308 F12 Figure 12. Low-Battery Detector Input/Output Characteristic START-UP The LT1308A/LT1308B can start up into heavy loads, unlike many CMOS DC/DC converters that derive operating voltage from the output (a technique known as “bootstrapping”). Figure 13 details start-up waveforms of Figure 1’s circuit with a 20Ω load and VIN of 1.5V. Inductor current rises to 3.5A as the output capacitor is charged. After the output reaches 5V, inductor current is about 1A. In Figure 14, the load is 5Ω and input voltage is 3V. Output voltage reaches 5V in 500μs after the device is enabled. Figure 15 shows start-up behavior of Figure 5’s SEPIC circuit, driven from a 9V input with a 10Ω load. The output reaches 5V in about 1ms after the device is enabled. VOUT 2V/DIV IL1 1A/DIV Figure 10. Setting Low-Battery Detector Trip Point VSHDN 5V/DIV 1ms/DIV 1308 F13 200k 2N3906 VIN LBO VBAT VREF 200mV LBI + 10k 10μF LT1308A LT1308B Figure 13. 5V Boost Converter of Figure 1. Start-Up from 1.5V Input into 20Ω Load GND 1308 F11 Figure 11. Accessing 200mV Reference 1308abfa 11 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION when operating from a battery composed of alkaline cells. The inrush current may cause sufficiency internal voltage drop to trigger a low-battery indicator. A programmable soft-start can be implemented with 4 discrete components. A 5V to 12V boost converter using the LT1308B is detailed in Figure 16. C4 differentiates VOUT, causing a current to flow into R3 as VOUT increases. When this current exceeds 0.7V/33k, or 21μA, current flows into the base of Q1. Q1’s collector then pulls current out the VC pin, creating a feedback loop where the slope of VOUT is limited as follows: VOUT 1V/DIV IL1 2A/DIV VSHDN 5V/DIV 500μs/DIV 1308 F14 Figure 14. 5V Boost Converter of Figure 1. Start-Up from 3V Input into 5Ω Load ΔVOUT 0.7V = Δt 33k • C 4 VOUT 2V/DIV ISW 2A/DIV VSHDN 5V/DIV 500μs/DIV 1308 F15 Figure 15. 5V SEPIC Start-Up from 9V Input into 10Ω Load Soft-Start In some cases it may be undesirable for the LT1308A/ LT1308B to operate at current limit during start-up, e.g., With C4 = 33nF, VOUT/t is limited to 640mV/ms. Start-up waveforms for Figure 16’s circuit are pictured in Figure 17. Without the soft-start circuit implemented, the inrush current reaches 3A. The circuit reaches final output voltage in approximately 250μs. Adding the soft-start components reduces inductor current to less than 1A, as detailed in Figure 18, while the time required to reach final output voltage increases to about 15ms. C4 can be adjusted to achieve any output slew rate desired. L1 4.7μH VIN 5V VIN + SHUTDOWN C1 47μF D1 VOUT 12V 500mA SW SHDN LT1308B 100k 330pF 10k C2 10μF FB GND VC C4 33nF R4 33k Q1 R3 33k 11.3k RC 47k CC 100pF SOFT-START COMPONENTS C1: AVX TAJ476M010 C2: TAIYO YUDEN TMK432BJ106MM D1: IR 10BQ015 L1: MURATA LQH6C4R7 Q1: 2N3904 1308 F16 Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4. 1308abfa 12 LT1308A/LT1308B U W U U APPLICATIONS INFORMATION so that copper loss is minimized. Acceptable inductance values range between 2μH and 20μH, with 4.7μH best for most applications. Lower value inductors are physically smaller than higher value inductors for the same current capability. 12V VOUT 5V/DIV 5V IL1 1A/DIV VSHDN 10V/DIV 50μs/DIV 1308 F17 Figure 17. Start-Up Waveforms of Figure 16’s Circuit without Soft-Start Components 12V VOUT 5V Table 1 lists some inductors we have found to perform well in LT1308A/LT1308B application circuits. This is not an exclusive list. Table 1 VENDOR PART NO. VALUE PHONE NO. Murata LQH6C4R7 4.7μH 770-436-1300 Sumida CDRH734R7 4.7μH 847-956-0666 CTX5-1 5μH 561-241-7876 LPO2506IB-472 4.7μH 847-639-6400 Coiltronics Coilcraft IL1 1A/DIV VSHDN 10V/DIV Capacitors 5ms/DIV 1308 F18 Figure 18. Start-Up Waveforms of Figure 16’s Circuit with Soft-Start Components Added COMPONENT SELECTION Diodes We have found ON Semiconductor MBRS130 and International Rectifier 10BQ015 to perform well. For applications where VOUT exceeds 30V, use 40V diodes such as MBRS140 or 10BQ040. Height limited applications may benefit from the use of the MBRM120. This component is only 1mm tall and offers performance similar to the MBRS130. Inductors Suitable inductors for use with the LT1308A/LT1308B must fulfill two requirements. First, the inductor must be able to handle current of 2A steady-state, as well as support transient and start-up current over 3A without inductance decreasing by more than 50% to 60%. Second, the DCR of the inductor should have low DCR, under 0.05Ω Equivalent Series Resistance (ESR) is the main issue regarding selection of capacitors, especially the output capacitors. The output capacitors specified for use with the LT1308A/ LT1308B circuits have low ESR and are specifically designed for power supply applications. Output voltage ripple of a boost converter is equal to ESR multiplied by switch current. The performance of the AVX TPSD227M006 220μF tantalum can be evaluated by referring to Figure 3. When the load is 800mA, the peak switch current is approximately 2A. Output voltage ripple is about 60mVPP, so the ESR of the output capacitor is 60mV/2A or 0.03Ω. Ripple can be further reduced by paralleling ceramic units. Table 2 lists some capacitors we have found to perform well in the LT1308A/LT1308B application circuits. This is not an exclusive list. Table 2 VENDOR SERIES PART NO. VALUE PHONE NO. AVX TPS TPSD227M006 220μF, 6V 803-448-9411 AVX TPS TPSD107M010 100μF, 10V 803-448-9411 Taiyo Yuden X5R LMK432BJ226 22μF, 10V 408-573-4150 Taiyo Yuden X5R TMK432BJ106 10μF, 25V 408-573-4150 1308abfa 13 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION Ceramic Capacitors Multilayer ceramic capacitors have become popular, due to their small size, low cost, and near-zero ESR. Ceramic capacitors can be used successfully in LT1308A/LT1308B designs provided loop stability is considered. A tantalum capacitor has some ESR and this causes an "ESR zero" in the regulator loop. This zero is beneficial to loop stability. Ceramics do not have appreciable ESR, so the zero is lost when they are used. However, the LT1308A/LT1308B have external compensation pin (VC) so component values can be adjusted to achieve stability. A phase lead capacitor can also be used to tune up load step response to optimum levels, as detailed in the following paragraphs. Figure 19 details a 5V to 12V boost converter using either a tantalum or ceramic capacitor for C2. The input capacitor has little effect on loop stability, as long as minimum capacitance requirements are met. The phase lead capacitor CPL parallels feedback resistor R1. Figure 20 shows load step response of a 50mA to 500mA load step using a 47μF tantalum capacitor at the output. Without the phase lead capacitor, there is some ringing, suggesting the phase margin is low. CPL is then added, and response to the same load step is pictured in Figure 21. Some phase margin is restored, improving the response. Next, C2 is replaced by a 10μF, X5R dielectric, ceramic capacitor. L1 4.7μH VIN 5V D1 Without CPL, load step response is pictured in Figure 22. Although the output settles faster than the tantalum case, there is appreciable ringing, again suggesting phase margin is low. Figure 23 depicts load step response using the 10μF ceramic output capacitor and CPL. Response is clean and no ringing is evident. Ceramic capacitors have the added benefit of lowering ripple at the switching frequency due to their very low ESR. By applying CPL in tandem with the series RC at the VC pin, loop response can be tailored to optimize response using ceramic output capacitors. VOUT 500mV/DIV IL1 1A/DIV LOAD CURRENT 50mA 200μs/DIV 1308 F20 Figure 20. Load Step Response of LT1308B 5V to 12V Boost Converter with 47μF Tantalum Output Capacitor VOUT 500mV/DIV IL1 1A/DIV VOUT 12V 500mA LOAD CURRENT VIN 500mA SW 500mA 50mA 200μs/DIV 1308 F21 SHDN LT1308B R3 10k R1 100k FB C2 GND VC + Figure 21. Load Step Response with 47μF Tantalum Output Capacitor and Phase Lead Capacitor CPL CPL 330pF VOUT 1V/DIV C1 47μF 47k R2 11.3k IL1 1A/DIV 100pF C1: AVX TAJC476M010 C2: AVX TPSD476M016 (47μF) OR TAIYO YUDEN TMK432BJ106MM (10μF) D1: IR 10BQ015 L1: MURATA LQH6C4R7 Figure 19. 5V to 12V Boost Converter LOAD CURRENT 1308 F19 500mA 50mA 200μs/DIV 1308 F22 Figure 22. Load Step Response with 10μF X5R Ceramic Output Capacitor 1308abfa 14 LT1308A/LT1308B U U W U APPLICATIONS INFORMATION VOUT 500mV/DIV VOUT VIN = 4.2V VOUT VIN = 3.6V IL1 1A/DIV LOAD CURRENT VOUT VIN = 3V ILOAD 1A 10mA 500mA 50mA 200μs/DIV VOUT TRACES = 200mV/DIV 1308 F23 Figure 23. Load Step Response with 10μF X5R Ceramic Output Capacitor and CPL 200μs/DIV 1308 F25 Figure 25. LT1308A Li-Ion to 5V Boost Converter Transient Response to 1A Load Step GSM AND CDMA PHONES The LT1308A/LT1308B are suitable for converting a single Li-Ion cell to 5V for powering RF power stages in GSM or CDMA phones. Improvements in the LT1308A/LT1308B error amplifiers allow external compensation values to be reduced, resulting in faster transient response compared to the LT1308. The circuit of Figure 24 (same as Figure 1, printed again for convenience) provides a 5V, 1A output from a Li-Ion cell. Figure 25 details transient response at the LT1308A operating at a VIN of 4.2V, 3.6V and 3V. Ripple voltage in Burst Mode operation can be seen at 10mA load. Figure 26 shows transient response of the LT1308B under the same conditions. Note the lack of Burst Mode ripple at 10mA load. L1 4.7μH + Li-Ion CELL VIN C1 47μF SHUTDOWN VOUT VIN = 4.2V VOUT VIN = 3.6V VOUT VIN = 3V ILOAD 1A 10mA VOUT TRACES = 200mV/DIV 100μs/DIV 1308 F26 Figure 26. LT1308B Li-Ion to 5V Boost Converter Transient Response to 1A Load Step D1 5V 1A SW R1 309k LT1308B SHDN VC FB GND 47k + C2 220μF R2 100k 100pF C1: AVX TAJC476M010 C2: AVX TPSD227M006 D1: IR 10BQ015 L1: MURATA LQH6N4R7 1308A/B F24 Figure 24. Li-Ion to 5V Boost Converter Delivers 1A 1308abfa 15 LT1308A/LT1308B U TYPICAL APPLICATIO S Triple Output TFTLCD Bias Supply D2 VOFF –9V 10mA C4 1μF D3 C5 1μF 0.22μF 0.22μF VON 27V 15mA D4 C6 1μF 0.22μF L1 4.7μH VIN 5V 3 C1 4.7μF D1 6 5 VIN SW AVDD 10V 500mA SHDN 76.8k C2, C3 10μF ×2 LT1308B 1 220k VC FB 2 GND 10.7k 4 100pF C1:TAIYO-YUDEN JMK212BJ475MG C2, C3:TAIYO-YUDEN LMK325BJ106MN C4, C5, C6:TAIYO-YUDEN EMK212BJ105MG D1: MBRM120 D2,D3,D4: BAT54S L1: TOKO 817FY-4R7M 1308 TA02 TFTLCD Bias Supply Transient Response AVDD 500mV/DIV VON 500mV/DIV VOFF 500mV/DIV ILOAD 800mA 200mA 100μs/DIV 1308abfa 16 LT1308A/LT1308B U TYPICAL APPLICATIO S 40nF EL Panel Driver T1 1:12 VBAT 3V TO 6V + D2 D3 4 3 C1 47μF 1 6 D1 3.3V REGULATED 1μF 100k Q1 VIN 47k 17k C2 1μF 200V 324k LBI VC 3.3k 4.3M FB LT1308A 2M 150k SW LBO 22nF 49.9k SHUTDOWN SHDN GND Q2 400V EL PANEL ≤40nF 100pF 10k 47pF 1308 TA03 C1: AVX TAJC476M010 C2: VITRAMON VJ225Y105KXCAT D1: BAT54 D2, D3: BAV21 Q1: MMBT3906 Q2: ZETEX FCX458 T1: MIDCOM 31105 SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output High Voltage Supply 350V at 1.2mA 10nF 250V VIN 2.7V TO 6V T1 1:12 + 3 C1 47μF D1 D3 10nF 250V D2 4 1 10nF 250V 6 SHUTDOWN VIN 3V TO 10V VIN SW LT1308A SHDN GND 47k FB VC VOUT 5V 500mA FB 10M 47k R1 309k LT1308B VC SHDN D1 L1B C1 47μF SHUTDOWN SW C2 4.7μF CERAMIC L1A CTX10-2 + D4 VIN VOUT 350V 1.2mA R2 100k 680pF + C3 220μF 6.3V GND 100pF 34.8k C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006 10nF D1, D2, D3: BAV21 200mA, 250V D4: MBR0540 T1: MIDCOM 31105R LP = 1.5μH D1: IR 10BQ015 L1: COILTRONICS CTX10-2 1308A/B TA05 1308 TA04 1308abfa 17 LT1308A/LT1308B U PACKAGE DESCRIPTION S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .189 – .197 (4.801 – 5.004) NOTE 3 .045 ±.005 .050 BSC 8 .245 MIN 7 6 5 .160 ±.005 .150 – .157 (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) .030 ±.005 TYP 1 RECOMMENDED SOLDER PAD LAYOUT .010 – .020 × 45° (0.254 – 0.508) .008 – .010 (0.203 – 0.254) 0°– 8° TYP .016 – .050 (0.406 – 1.270) NOTE: 1. DIMENSIONS IN .053 – .069 (1.346 – 1.752) .014 – .019 (0.355 – 0.483) TYP INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) 2 3 4 .004 – .010 (0.101 – 0.254) .050 (1.270) BSC SO8 0303 1308abfa 18 LT1308A/LT1308B U PACKAGE DESCRIPTION F Package 14-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1650) 4.90 – 5.10* (.193 – .201) 14 13 12 11 10 9 8 1.05 ±0.10 6.60 ±0.10 6.40 (.252) BSC 4.50 ±0.10 0.45 ± 0.05 0.65 BSC 1 2 3 4 5 6 7 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50** (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 1.10 (.0433) MAX 0° – 8° 0.65 (.0256) BSC 0.19 – 0.30 (.0075 – .0118) TYP 0.05 – 0.15 (.002 – .006) F14 TSSOP 0204 3. DRAWING NOT TO SCALE *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .152mm (.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE 1308abfa 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. 19 LT1308A/LT1308B U TYPICAL APPLICATIO Li-Ion to 12V/300mA Step-Up DC/DC Converter L1 4.7μH 2.7V TO 4.2V VIN + Li-Ion CELL C1 47μF D1 12V 300mA SW R1 887k LT1308B SHUTDOWN SHDN VC FB GND 47k + C2 100μF R2 100k 330pF C1: AVX TAJC476M010 C2: AVX TPSD107M016 D1: IR 10BQ015 L1: MURATA LQH6C4R7 1308A/B TA01 RELATED PARTS PART NUMBER DESCRIPTION LT1302 High Output Current Micropower DC/DC Converter 5V/600mA from 2V, 2A Internal Switch, 200μA IQ LT1304 2-Cell Micropower DC/DC Converter 5V/200mA, Low-Battery Detector Active in Shutdown LT1307/LT1307B Single Cell, Micropower, 600kHz PWM DC/DC Converters LT1316 Burst Mode Operation DC/DC with Programmable Current Limit COMMENTS 3.3V at 75mA from One Cell, MSOP Package 1.5V Minimum, Precise Control of Peak Current Limit LT1317/LT1317B Micropower, 600kHz PWM DC/DC Converters 100μA IQ, Operate with VIN as Low as 1.5V LTC®1474 Micropower Step-Down DC/DC Converter 94% Efficiency, 10μA IQ, 9V to 5V at 250mA LTC1516 2-Cell to 5V Regulated Charge Pump 12μA IQ, No Inudctors, 5V at 50mA from 3V Input LTC1522 Micropower, 5V Charge Pump DC/DC Converter Regulated 5V ±4% Output, 20mA from 3V Input LT1610 Single-Cell Micropower DC/DC Converter 3V at 30mA from 1V, 1.7MHz Fixed Frequency LT1611 Inverting 1.4MHz Switching Regulator in 5-Lead SOT-23 – 5V at 150mA from 5V Input, Tiny SOT-23 package LT1613 1.4MHz Switching Regulator in 5-Lead SOT-23 5V at 200mA from 4.4V Input, Tiny SOT-23 package LT1615 Micropower Step-Up DC/DC in 5-Lead SOT-23 20μA IQ, 36V, 350mA Switch LT1617 Micropower Inverting DC/DC Converter in SOT-23 VIN = 1V to 15V; VOUT to –34V LTC1682 Doubler Charge Pump with Low Noise LDO Adjustable or Fixed 3.3V, 5V Outputs, 60μVRMS Output Noise LT1949 600kHz, 1A Switch PWM DC/DC Converter 1.1A, 0.5Ω, 30V Internal Switch, VIN as Low as 1.5V LT1949-1 1.1MHz, 1A Switch DC/DC Converter 1.1MHz Version of LT1949 1308abfa 20 Linear Technology Corporation LT 0807 REV A • 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