LT1500/LT1501 Adaptive-Frequency Current Mode Switching Regulators U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Low Noise Adaptive-Frequency Current Mode Operation Avoids Low Frequency Noise at Most Load Currents Can Be Externally Synchronized (LT1500) Micropower Quiescent Current: 200µA Shutdown Current: 8µA Typ Internal Loop Compensation Low-Battery Comparator Active in Shutdown Minimum Input Voltage: 1.8V Typ Additional Negative Voltage Feedback Pin (LT1500) Up to 500kHz Switching Frequency Uses Low Profile, Low Cost Surface Mount Inductors U APPLICATIONS ■ ■ ■ ■ Portable Instrumentation Battery Operated Systems PDA’s Standby Power The LT®1500 is an adaptive-frequency current mode stepup switching regulator with an internal power switch that is rated up to 700mA. In contrast to pulse skipping switching regulators, the LT1500 uses a current mode topology that provides lower noise operation and improved efficiency. Only at very light loads is Burst ModeTM activated to give high efficiency and micropower operation. High switching frequency (up to 500kHz) allows very small inductors to be used, along with ceramic capacitors if desired. The LT1500 operates with input voltages from 1.8V to 15V and has only 200µA operating current dropping to 8µA in shutdown. A low-battery comparator is included which stays alive in shutdown. A second output feedback pin with negative polarity allows negative output voltages to be regulated when the switcher is connected up as a Cuk or a flyback converter. Two package types are available. The LT1500 comes in a 14-pin SO package, with two options available for fixed output (3.3V or 5V) or adjustable operation. A reduced feature part, the LT1501, comes in the smaller 8-pin SO package with internal frequency compensation. It is also available in adjustable and fixed output voltage versions. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. U TYPICAL APPLICATION 2-Cell to 5V Converter 22µH† D1 MBR0520L 5V, 200mA VIN 301k 1% 2 EACH NiCd OR ALKALINE CELLS + ISENSE SW SHDN 33µF* 6V LT1501-5 LBI 301k 1% OUT LBO GND 1nF LOW-BATTERY FLAG (USE EXTERNAL PULL-UP) + 220µF** 10V TANT * AVX, TPSC107M006R0150 ** AVX, TPSD107M010 R0100 † SUMIDA CD73-220, CD54-220 OR CD43-220. SELECT ACCORDING TO MAXIMUM LOAD CURRENT LT1500/01 • TA01 1 LT1500/LT1501 U W W W ABSOLUTE MAXIMUM RATINGS Supply Voltage ........................................................ 20V Switch Voltage (SW)................................................ 30V Shutdown Voltage (SHDN) ...................................... 20V ISENSE Voltage .......................................................... 20V FB Voltage ................................................................. 5V LBI Voltage ................................................................ 5V LBO Voltage ............................................................. 15V Operating Ambient Temperature Range Commercial ............................................. 0°C to 70°C Industrial ............................................ – 40°C to 85°C Operating Junction Temperature Range Commercial ........................................... 0°C to 100°C Industrial .......................................... – 40°C to 100°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C W U U PACKAGE/ORDER INFORMATION TOP VIEW TOP VIEW SHDN 1 14 FB SHDN 1 14 VOUT (3.3V/5V) VC 2 13 NFB VC 2 13 SELECT VIN 3 12 SS VIN 3 12 SS ISENSE 4 11 LBI ISENSE 4 11 LBI NC 5 10 LBO NC 5 GND 6 9 SYNC PGND 7 8 SW TOP VIEW 10 LBO GND 6 9 SYNC PGND 7 8 SW SHDN 1 8 FB/OUT VIN 2 7 LBI ISENSE 3 6 LBO GND 4 5 SW S8 PACKAGE 8-LEAD PLASTIC SO S PACKAGE 14-LEAD PLASTIC SO TJMAX = 100°C, θJA = 100°C/ W S PACKAGE 14-LEAD PLASTIC SO TJMAX = 100°C, θJA = 100°C/ W TJMAX = 100°C, θJA = 120°C/ W ORDER PART NUMBER ORDER PART NUMBER ORDER PART NUMBER LT1500CS LT1500IS LT1500CS-3/5 LT1500IS-3/5 LT1501CS8 LT1501CS8-3.3 LT1501CS8-5 LT1501IS8 LT1501IS8-3.3 LT1501IS8-5 Consult factory for Military grade parts. ELECTRICAL CHARACTERISTICS TJ = 25°C, VIN = 2.3V unless otherwise noted. PARAMETER CONDITIONS Feedback/Output Pin Reference Voltage LT1500/LT1501, TJ = 25°C All Conditions (Note 6) Reference Voltage Line Regulation Feedback Pin Bias Current 2 MIN TYP MAX UNITS 1.240 1.235 1.265 ● 1.290 1.295 V V LT1500-3/5, Select Pin Open All Conditions (Note 6) 3.230 3.200 3.300 ● 3.370 3.400 V V LT1500-3/5, Select Pin Grounded All Conditions (Note 6) 4.900 4.85 5.000 ● 5.100 5.15 V V VIN = 2.3V to 15V ● 0.02 0.06 %/V ● 30 100 nA LT1500/LT1501 ELECTRICAL CHARACTERISTICS TJ = 25°C, VIN = 2.3V unless otherwise noted. PARAMETER CONDITIONS Internal Divider Current LT1500-3.3/LT1501-3.3 LT1500-5/LT1501-5 Operating Quiescent Current Supply Current in Shutdown TYP MAX ● ● 22 33 30 45 µA µA VIN ≤ 5V, VSHDN = 2.3V (Note 1) VIN = 15V ● ● 200 280 320 µA µA VSHDN ≤ 0.2V, Fixed Voltages (Note 7) TJ ≥ 0°C TJ < 0°C ● 8 15 20 µA µA 1.1 V Shutdown Pin Threshold MIN ● 0.4 UNITS Shutdown Pin Input Current VSHDN = 2.3V ● 3 10 µA Input Start-Up Voltage VSHDN = VIN TJ ≥ 0°C TJ < 0°C ● 2.0 2.1 2.2 V V 1.8 2.0 2.1 V V 0.50 0.72 Ω 1.3 A Undervoltage Lockout Light Load Full Load Power Switch Switch On Resistance ISW = 0.7A (Note 2) Peak Switch Current (Note 3) ● ● 0.7 0.85 30 45 Switch Breakdown Voltage ISW = 100µA ● Switch Leakage Current VSW = 5V VSW = 20V ● ● 0.2 0.3 V 5 10 µA µA Switch Turn-On Delay (Note 5) 800 ns Switch Turn-Off Delay (Note 5) 400 ns Current Sense Resistor ● 0.28 0.42 Ω 1.24 1.28 V Low-Battery Comparator Low-Battery Threshold Falling Edge ● 1.20 Threshold Hysteresis 20 LBI Input Bias Current mV ● 20 50 nA 0.1 0.3 0.25 0.5 V V 2 µA 35 µA 1.3 V LBO Output Low State VLBI = 1.2V, ISINK = 100µA ISINK = 2mA ● ● LBO Leakage Current VLBI = 1.3V, VLBO ≤ 15V ● VSYNC = 3.3V ● LT1500 Functions SYNC Pin Bias Current SYNC Pin Threshold ● 15 0.4 Error Amplifier Transconductance 600 µmho VC Pin Source Current 20 µA VC Pin High Clamp Voltage NFB Reference Voltage FB Pin Open NFB Pin Bias Current ● 1.20 1.26 1.32 V 1.230 1.265 1.300 V 12 20 µA ● NFB to FB Transconductance Note 4 Soft Start Bias Current Current Flows Out of Pin µmho 10,000 ● 2 4 7 µA 3 LT1500/LT1501 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range. Note 1: Feedback pin or output is held sightly above the regulated value to force the VC node low and switching to stop. Note 2: See Typical Performance Characteristics for graph of Guaranteed Switch Voltage vs Saturation Voltage. Note 3: Peak switch current is the guaranteed minimum value of switch current available in normal operation. Highest calculated switch current at full load should not exceed the minimum value shown. Note 4: Loading on FB pin will affect NFB reference voltage. ∆VNFB = IFB/gm. Do not exceed 10µA loading on FB when NFB is being used. Note 5: This is the delay between sense pin current reaching its upper or lower threshold and switch transition. Switch delay times cause peak-to- peak inductor current to increase and therefore switching frequency to be low. This effect will be significant for frequencies above 100kHz. See Application Information and Typical Performance Characteristics. Note 6: Reference voltage under all conditions includes VIN = 2.1V to 15V, all loads and full temperature range. Note 7: As with all boost regulators the output voltage of the LT1500 cannot fall to less than input voltage because of the path through the catch diode. This means that the output voltage divider on adjustable parts will still be generating feedback voltage at the FB pin (fixed voltage parts have an internal switch to disconnect the divider in shutdown). If the voltage on FB is greater than 0.6V in shutdown, the internal error amplifier will draw current that adds to shutdown current. See graph of Shutdown Current vs FB voltage in Typical Performance Characteristics. U W TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency (3.3V Output) Switching Frequency (12V Output) Switching Frequency (5V Output) 1000 1000 1000 VIN = 3V VIN = 2.3V VIN = 5V 10µH 10µH 20µH 100 50µH 50µH 100 100µH 100µH 50 100 150 200 LOAD CURRENT (mA) 250 10 0 300 50 100 150 200 LOAD CURRENT (mA) Efficiency (3.3V Output) 300 0 90 L = 100µH 80 60 50 L = 10µH 70 L = 33µH 60 50 TJ = 25°C VIN = 2.3V LOW LOSS INDUCTOR 40 10 100 LOAD CURRENT (mA) 1000 LTC1500/01 • TPC17 L = 10µH 60 40 VIN = 3V LOW LOSS INDUCTOR 30 1 L = 33µH 70 50 40 30 L = 100µH 80 EFFICIENCY (%) L = 10µH EFFICIENCY (%) 80 200 100 90 L = 100µH 70 50 75 100 125 150 175 LOAD CURRENT (mA) Efficiency (12V Output) 100 L = 33µH 25 LTC1500/01 • TPC22 Efficiency (5V Output) 100 4 250 LTC1500/01 • TPC21 LTC1500/01 • TPC20 90 100 BURST REGION 10 10 0 50µH 100µH BURST REGION BURST REGION EFFICIENCY (%) 20µH 20µH FREQUENCY (kHz) FREQUENCY (kHz) FREQUENCY (kHz) 10µH VIN = 5V LOW LOSS INDUCTOR 30 1 10 100 LOAD CURRENT (mA) 1000 LTC1500/01 • TPC18 1 10 100 LOAD CURRENT (mA) 1000 LTC1500/01 • TPC19 LT1500/LT1501 U W TYPICAL PERFORMANCE CHARACTERISTICS Efficiency (5V Output) Efficiency (3.3V Output) 100 EFFICIENCY (%) ILOAD = 10mA 80 70 60 TJ = 25°C L = 33µH LOW LOSS INDUCTOR 40 1.75 2.00 ILOAD = 10mA 90 ILOAD = 100mA 50 100 ILOAD = 100mA 80 70 60 TJ = 25°C L = 33µH LOW LOSS INDUCTOR 50 3.00 2.25 2.50 2.75 INPUT VOLTAGE (V) 60 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 4.5 5.0 TJ = 25°C L = 33µH LOW LOSS INDUCTOR 0 2 10 VIN = 3V VIN = 5V R = 1Ω R = 1Ω EFFICIENCY LOSS (%) EFFICIENCY LOSS (%) R = 0.5Ω R = 0.1Ω R = 0.2Ω R = 0.1Ω 1 12 Inductor Copper Loss (12V Output) 10 R = 0.5Ω R = 0.2Ω 10 4 6 8 INPUT VOLTAGE (V) LT1500/01 • TPC13 Inductor Copper Loss (5V Output) 10 EFFICIENCY LOSS (%) 70 LT1500/01 • TPC12 Inductor Copper Loss (3.3V Output) 1 ILOAD = 10mA 40 2.0 LT1500/01 • TPC11 R = 1Ω 80 50 40 3.25 ILOAD = 50mA 90 EFFICIENCY (%) 90 EFFICIENCY (%) Efficiency (12V Output) 100 R = 0.5Ω 1 R = 0.2Ω VIN = 2.3V 0.1 0.1 0.1 0 50 100 150 200 LOAD CURRENT (mA) 250 0 300 50 100 150 200 LOAD CURRENT (mA) 0 300 Maximum Load Current (3.3V Output) Maximum Load Current (12V Output) 600 500 500 500 L ≥ 33µH L = 10µH 200 400 L ≥ 33µH 300 L = 10µH 200 2.00 2.25 2.50 INPUT VOLTAGE (V) 2.75 3.00 LT1500/01 • TPC08 L = 100µH 300 L = 10µH 200 0 0 1.75 L = 33µH 400 100 100 100 0 1.50 OUTPUT CURRENT (mA) 600 OUTPUT CURRENT (mA) 600 300 200 LTC1500/01 • TPC16 Maximum Load Current (5V Output) 400 50 75 100 125 150 175 LOAD CURRENT (mA) 25 LT1500/01 • TPC15 LT1500/01 • TPC14 OUTPUT CURRENT (mA) 250 2.0 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 4.5 5.0 LT1500/01 • TPC09 0 2 4 6 8 INPUT VOLTAGE (V) 10 12 LT1500/01 • TPC10 5 LT1500/LT1501 U W TYPICAL PERFORMANCE CHARACTERISTICS 1.0 120 TJ = 25°C LOAD CURRENT IS REDUCED UNTIL Burst Mode OPERATION STARTS 220 PEAK-TO-PEAK INDUCTOR CURRENT TJ = 25°C 0.8 SWITCH VOLTAGE (V) 100 LOAD CURRENT (mA) Peak-to-Peak Inductor Ripple Current Switch Saturation Voltage Burst Mode Threshold 80 VOUT = 3.3V 60 VOUT = 5V 40 VOUT = 12V 0.6 0.4 0.2 20 0 2 4 6 8 INPUT VOLTAGE (V) 10 0 12 160 140 120 100 0.2 0.6 0.8 0.4 SWITCH CURRENT (A) 1.0 0 0.1 0.6 0.5 0.2 0.3 0.4 AVERAGE SWITCH CURRENT LT1500/01 • TPC06 LT1500/01 • TPC01 0.6 400 TJ = 25°C VFB OR VOUT HELD 5% HIGH, SO THAT Burst Mode OPERATION IS ACTIVATED. DOES NOT INCLUDE OUTPUT DIVIDER CURRENT 300 250 TJ = 25°C VLBI ≤ 1.2V 0.5 VOLTAGE (V) 350 200 150 0.4 0.3 0.2 100 0.1 50 0 0 0 5 10 15 INPUT VOLTAGE (V) 20 0 25 1 6 LT1500/01 • TPC02 Shutdown Input Current vs Feedback Pin Voltage 140 20 TJ = 25°C VSHDN = 0V TJ = 25°C VIN = 5V ADJUSTABLE PARTS ONLY. FIXED VOLTAGE PARTS DO NOT SHOW SHUTDOWN CURRENT INCREASE WITH FEEDBACK VOLTAGE 120 16 CURRENT (µA) 100 CURRENT (µA) 5 2 3 4 SINK CURRENT (mA) LT1500/01 • TPC04 Input Current in Shutdown 12 8 80 60 40 4 20 0 0 0 5 15 10 INPUT VOLTAGE (V) 20 25 LT1500/01 • TPC03 6 0 0.7 LT1500/01 • TPC07 Low-Battery Output Saturation Voltage Quiescent Input Supply Current SUPPLY CURRENT (µA) 180 80 0 0 VIN = 3.3V VOUT = 5V L = 50µH NOTE THAT RIPPLE CURRENT INCREASES WITH SMALLER INDUCTORS DUE TO PROPAGATION DELAY IN THE CURRENT COMPARATOR 200 0.2 1.2 1.0 0.4 0.6 0.8 FEEDBACK PIN VOLTAGE (V) 1.4 LT1500/01 • TPC05 LT1500/LT1501 U U U PIN FUNCTIONS SHDN: Logic Level Shutdown Pin. This pin must be held high (> 1.1V) for the regulator to run. SHDN can be tied directly to VIN, even with VIN = 18V. The low-battery detector remains active in shutdown, but all other circuitry is turned off. VIN: This pin supplies power to the regulator and is connected to one side of the inductor sense resistor. It should be bypassed close to the chip with a low ESR capacitor. ISENSE: This is one end of the internal inductor-current sense resistor. With most applications, only the external inductor is tied to this pin. GND: This pin carries only low level current in the LT1500, but it carries full switch current in the LT1501. The negative end of the input bypass capacitor should be connected close to this pin and the pin should go directly to the ground plane with the LT1501. PGND (LT1500 Only): This pin is the emitter of the internal NPN power switch. Connect it directly to the ground plane. SW: This is the collector of the internal NPN power switch. To avoid EMI and overvoltage spikes, keep connections to this pin very short. LBI: This is the input to the low-battery detector with a threshold of 1.24V. Maximum pin voltage is 5V. Bypass LBI with a small filter capacitor when used. If unused, tie LBI to ground. The low-battery detector remains active in shutdown. LBO : This is the open collector output of the low-battery detector. It will sink up to 2mA. Leave open if not used. FB/VOUT: FB is the inverting input to the error amplifier with a regulating point of 1.265V and a typical bias current of 30nA. Bias current is reduced with a canceling circuit, so bias current could flow in either direction. FB is replaced with VOUT on fixed voltage parts. VOUT is the top of an internal divider that is connected to the internal FB node. A switch disconnects the divider in shutdown so that the divider current does not load VIN through the inductor and catch diode. NFB/SELECT (LT1500 Only): NFB is a second feedback node used to regulate a negative output voltage. Negative output voltages can be generated by using a transformer flyback circuit, a Cuk converter or a capacitor charge pump added to a boost converter. The regulating point for NFB is 1.265V and the internal resistance to ground is 100kΩ. External divider current should be 300µA or greater to avoid negative output voltage variations due to production variations in the internal resistor value. FB should be left open when using NFB. On fixed voltage parts, NFB is replaced with Select. The Select pin is used to set output voltage at either 3.3V or 5V. VC (LT1500 Only): This is the output of the error amplifier and the input to the current comparator. The VC pin voltage is about 700mV at very light loads and about 1.2V at full load. An internal comparator detects when the VC voltage drops below about 750mV and shuts down the current comparator and the power switch biasing to reduce quiescent current. This forces the regulator to operate in Burst Mode operation. SYNC (LT1500 Only): This is a logic level input used to synchronize switching frequency to an external clock. The sync signal overrides the internal current comparator and turns the switch on. Minimum sync pulse width should be 50ns and maximum width should be 300ns. A continuous high sync signal will force the power switch to stay on indefinitely and current will increase without limit. Don’t do this! SS (LT1500 Only): This is the soft start function using the base of a PNP transistor whose emitter is tied to the VC pin. Grounding SS will turn off switching by pulling VC low. A capacitor tied from SS to ground will force VC to ramp up slowly during start-up at a rate set by the capacitor value and the internal 4µA pull-up current. An external resistor must be used to reset the capacitor voltage completely to 0V at power down. 7 LT1500/LT1501 W BLOCK DIAGRAM LBO SYNC IN ISENSE OUTPUT RSENSE 0.28Ω Rh + 18mV + LBI – 1.24V – + CURRENT COMPARATOR 0.75V – BIAS R1 BURST COMPARATOR 1.265V REFERENCE SW + SHDN FIXED HYSTERESIS I1 + VARIABLE HYSTERESIS ERROR AMP – 100k I2 Q1 – 100k NEGATIVE ERROR AMP 150pF + S1 R2 NFB FB VC GND PGND LTC1500/01 • BD U W U U APPLICATIONS INFORMATION OPERATION (SEE BLOCK DIAGRAM) The LT1500 uses a current mode architecture without the need for an internal oscillator. Switching frequency is determined by the value of the external inductor used. This technique allows the selection of an operating frequency best suited to each application and considerably simplifies the internal circuitry needed. It also eliminates a subharmonic oscillation problem common to all fixed frequency (clocked) current mode switchers. In addition, it allows for high efficiency micropower operation while maintaining higher operating frequencies. Because the power switch (Q1) is grounded, the basic topology used 8 will normally be a boost converter with output voltage always higher than the input voltage. Special topologies such as the SEPIC, flyback and Cuk converter can also be used when the output voltage may not always be higher than the input or when full shutdown of the output voltage is needed. Operation as a boost converter is as follows. Assume that inductor current is continuous, meaning that it never drops to zero. When the switch is on, inductor current will increase with voltage across the inductor equal to VIN. When the switch is off inductor current will decrease with inductor voltage equal to VOUT – VIN. Switching frequency will be determined by the inductor LT1500/LT1501 U W U U APPLICATIONS INFORMATION value, the peak-to-peak inductor current (set internally) and the values for VIN and VOUT. The LT1500 controls output voltage in continuous mode by adjusting the average value of inductor current while maintaining the peakto-peak value of the current relatively constant, hence, the name “current mode architecture.” The LT1500 sets the peak-to-peak value of switch current internally to establish operating frequency. This peak-topeak value is scaled down somewhat at light load currents to avoid as long as possible the characteristic of other micropower converters wherein their switching frequency drops very low (into the audio range) at less than full load currents. At extremely light loads, even the LT1500 can no longer maintain higher frequency operation, and utilizes a Burst Mode operation to control output voltage. Details of Continuous Mode Operation At the start of a switch cycle, inductor current has decreased to the point where the voltage across RSENSE is less than the internally generated voltage across Rh. This causes the current comparator output to go high and turn on the switch. At the same time, extra current is added to Rh via S1 to create hysteresis in the trip point of the comparator. This extra current is composed of a fixed amount (I1), and an amount proportional to average inductor current (I2). The presence of a variable I2 increases switching frequency at lighter loads to extend the load current range where high frequency operation is maintained and no Burst Mode operation exists. With the switch turned on, inductor current will increase until the voltage drop across RSENSE is equal to the higher voltage across Rh. Then the comparator output will go low, the switch will turn off and the current through Rh will be switched back to its lower value. Inductor current will decrease until the original condition is reached, completing one switch cycle. Control of output voltage is maintained by adjusting the continuous current flowing through Rh. This affects both upper and lower inductor current trip levels at the same time. Continuous Rh current is controlled by the error amplifier which is comparing the voltage on the Feedback pin to the internal 1.265V reference. An internal frequency compensation capacitor filters out most the ripple voltage at the amplifier output. Operation at Light Loads At light load currents the lower trip level (switch turn-on) for inductor current drops below zero. At first glance, this would seem to initiate a permanent switch off-state because the inductor current cannot reverse in a boost topology. In fact, what happens is that output voltage drops slightly between switch cycles, causing the error amplifier output to increase and bring the current trip level back up to zero. The switch then turns back on and inductor current increases to a value set by I1 (I2 is near zero at this point). The switch then turns off, and the inductor energy is delivered to the output, causing it to rise back up slightly. One or more switch cycles may be needed to raise the output voltage high enough that the amplifier output drops enough to force a sustained switch off period. The output voltage then slowly drops back low enough to cause the amplifier output to rise high enough to initiate a switch turn-on. Switching operation now consists of a series of bursts where the switch runs at normal frequency for one or more cycles, then turns off for a number of cycles. This Burst Mode operation is what allows the LT1500 to have micropower operation and high efficiency at very light loads. Saving Current in Burst Mode Operation Internal current drain for the LT1500 control circuitry is about 400µA when everything is operating. To achieve higher efficiency at extremely light loads, a special operating mode is initiated when the error amplifier output is toward the low end of its range. The adaptive bias circuit comparator detects that the error amplifier output is below a predetermined level and turns off the current comparator and switch driver biasing. This reduces current drain to about 200µA, and forces a switch off state. Hysteresis in the comparator forces the device to remain in this micropower mode until the error amplifier output rises up beyond the original trip point. The regulated output voltage will fall slightly over a relatively long period of time (remember that load current is very low) until the error amplifier output rises enough to turn off the adaptive bias 9 LT1500/LT1501 U W U U APPLICATIONS INFORMATION mode. Normal operation resumes for one or more switch cycles and the output voltage increases until the error amplifier output falls below threshold, initiating a new adaptive bias shutdown. DESIGN GUIDE Selecting Inductor Value Inductor value is chosen as a compromise between size, switching frequency, efficiency and maximum output current. Larger inductor values become physically larger but provide higher output current and give better efficiency (because of the lower switching frequency). Low inductance minimizes size but may limit output current and the higher switching frequency reduces efficiency. The simplest way to handle these trade-offs is to study the graphs in the Typical Performance Characteristics section. A few minutes with these graphs will clearly show the trade-offs and a value can be quickly chosen that meets the requirements of frequency, efficiency and output current. This leaves only physical size as the final consideration. The concern here is that for a given inductor value, smaller size usually means higher series resistance. The graphs showing efficiency loss vs inductor series resistance will allow a quick estimate of the additional losses associated with very small inductors. One final consideration is inductor construction. Many small inductors are “open frame ferrites” such as rods or barrels. These geometries do not have a closed magnetic path, so they radiate significant B fields in the vicinity of the inductor. This can affect surrounding circuitry that is sensitive to magnetic fields. Closed geometries such as toroids or E-cores have very low stray B fields, but they are larger and more expensive (naturally). Catch Diode The catch diode in a boost converter has an average current equal to output current, but the peak current can be significantly higher. Maximum reverse voltage is equal to output voltage. A 0.5A Schottky diode like MBR0520L works well in nearly all applications. 10 Input Capacitor Input capacitors for boost regulators are less critical than the output capacitor because the input capacitor ripple current is a simple triwave without the higher frequency harmonics found in the output capacitor current. Peak-topeak current is less than 200mA and worst-case RMS ripple current in the input capacitor is less than 70mA. Input capacitor series resistance (ESR) should be low enough to keep input ripple voltage to less than 100mVP-P. This assumes that the capacitor is an aluminum or tantalum type where the capacitor reactance at the switching frequency is small compared to the ESR. C≥ 2 π ( f)(ESR) A typical input capacitor is a 33µF, 6V surface mount solid tantalum type TPS from AVX. It is a “C” case size, with 0.15Ω maximum ESR. Some caution must be used with solid tantalum input capacitors because they can be damaged with turn-on surge currents that occur when a low impedance power source is hot-switched to the input of the regulator. This problem is mitigated by using a capacitor with a voltage rating at least twice the highest expected input voltage. Consult with the manufacturer for additional guidelines. If a ceramic input capacitor is used, different design criteria are used because these capacitors have extremely low ESR and are chosen for a minimum number of microfarads. C (Ceramic) = 1 4f f = switching frequency A typical unit is an AVX or Tokin 3.3µF or 4.7µF. Output Capacitor Output ripple voltage is determined by the impedance of the output capacitor at the switching frequency. Solid tantalum capacitors rated for switching applications are recommended. These capacitors are essentially resistive at frequencies above 50kHz, so ESR is the important factor in determining ripple voltage. A typical unit is a 220µF, 10V LT1500/LT1501 U W U U APPLICATIONS INFORMATION Loop frequency stability is affected by the characteristics of the output capacitor. The ESR of the capacitor should be very low, and the capacitance must be large (> 200µF) to ensure good loop stability under worst-case conditions of low input voltage, higher output voltages, and high load currents. The 14-pin LT1500 can use external frequency compensation on the VC pin to give good loop stability with smaller output capacitors. See Loop Stability section for details. Precautions regarding solid tantalum capacitors for input bypassing do not apply to the output capacitor because turn-on surges are limited by the inductor and discharge surges do not harm the capacitors. Setting Output Voltage Preset 3.3V and 5V parts are available. For other voltage applications the adjustable part uses an external resistor divider to set output voltage. Bias current for the feedback (FB) pin is typically ±30nA (it is internally compensated). Thevenin divider resistance should be 100kΩ or less to keep bias current errors to a minimum. This leads to a value for R1 and R2 (see Figure 1) of: Note that there is an internal switch that disconnects the internal divider for fixed 3.3V and 5V parts in shutdown. This prevents the divider from adding to shutdown current. Without this switch, shutdown current increases because of the divider current directly, but even more so if the FB pin is held above 0.6V by the divider. See graphs in Typical Performance Characteristics. VOUT = 12V ERROR AMPLIFIER 100kΩ(12) R1 = = 949k (use 1M) 1.265 LTC1500/01 • F01 Figure 1. External Voltage Divider Selectable Output (Fixed Voltage Parts) The Select pin (available only on LT1500-3/5) allows the user to select either a 3.3V or 5V output. Floating the pin sets output voltage at 3.3V and grounding the pin sets output voltage at 5V. The equivalent circuit of the Select pin function is shown in Figure 2. VOUT 204k ERROR AMPLIFIER 69k + Example: VOUT = xxV R2 118K 1% 1.265V 100kΩ(VOUT ) 1.265V R1(1.265) R2 = VOUT – 1.265 R1 1M 1% FB – R1 = 1M(1.265) = 118k 12 – 1.265 + 1.2(IOUT )(VOUT ) VRIPPLE = ESR 0.1 + VIN R2 = – type TPS from AVX, or type 595D from Sprague. These have an ESR of 0.06Ω in a “E” case size. At lower output current levels, a 100µF unit in a “D” case size may be sufficient. Output ripple voltage can be calculated from: SELECT 1.265V GND 58k LTC1500/01 • F02 Figure 2. Schematic of Select Pin Function Note that there is a switch in series with the VOUT pin. This switch is turned off in shutdown to eliminate shutdown current drawn by the voltage divider. For adjustable parts 11 LT1500/LT1501 U U W U APPLICATIONS INFORMATION with an external divider no switch exists and the divider current remains. There may be additional current drawn by the adjustable LT1500 in shutdown if the divider voltage at the feedback node exceeds 0.6V. See Typical Performance Characteristics. Loop Stability The LT1501 is internally compensated since the device has no spare pin for a compensation point. The LT1500 brings out the VC pin to which an external series RC network is connected. This provides roll-off for the error amplifier, ensuring overall loop stability. Typical values when using tantalum output capacitors are 1000pF and 100kΩ. Transient response of Figure 3’s circuit with a 30mA to 100mA load step is detailed in Figure 4. The maximum output disturbance is approximately 20mV. The “splitting” of the VOUT trace when load current increases to 100mA is due to ESR of COUT. COUT can be replaced by a ceramic unit, which has lower ESR, size and cost. Figure 5 shows transient response to the same 30mA to 100mA load step, with COUT = 15µF ceramic, CC = 2200pF and RC = 10k. The maximum output disturbance in this case is 100mV. VIN ISENSE SHDN VOUT 5V 1M 100pF SW VCOMP 500mV/DIV VOUT 20mV/DIV AC COUPLED I LOAD 100mA 30mA IL 500mA/DIV 500µs/DIV Figure 4. Transient Response of LT1500 with RC = 100k, CC = 1000pF and COUT = 220µF. VOUT Disturbance is 20mV VCOMP 500mV/DIV I LOAD 100mA 30mA IL 500mA/DIV + COUT* FB LT1500 200µs/DIV 220µF VC GND The low-battery detector is a combined reference and comparator. It has a threshold of 1.24V with a typical input bias current of 20nA. In a typical application a resistor divider is connected across the battery input voltage with the center tap tied to Low Battery Input (LBI), see Figure 6. The suggested parallel resistance of the divider is 150k VOUT 50mV/DIV AC COUPLED 33µH MBR0520L CTX33-1 VIN 2V Low-Battery Detector PGND Figure 5. Transient Response of LT1500 with RC = 10k, CC = 2200pF and COUT =15µF Ceramic. VOUT Disturbance is 100mV 332k RC 100k CC 1000pF *TANTALUM = AVX TPS SERIES CERAMIC = TOKIN 1E156ZY5U LT1500/01 • F03 VCC R3 301k 1% VIN 470k LBI Figure 3. LT1500 2V to 5V Converter R4 274k 1% LT1500 LBO LT1501 GND PULL-UP RESISTOR SHOULD BE AT LEAST FIVE TIMES SMALLER THAN R5 TO ENSURE LBO HIGH STATE R5 10M LT1500/01 • F06 Figure 6. Low Battery Detection 12 LT1500/LT1501 U W U U APPLICATIONS INFORMATION and it should be no more than 300k to keep bias current errors under 1%, giving: R (V ) R3 = DIV BAT 1.24V R4 = R3(1.24) VBAT – 1.24 VBAT = low battery voltage RDIV = Thevenin divider resistance = R3 in parallel with R4 There is about 20mV of hysteresis at the LBI pin. Hysteresis can be increased by adding a resistor (R5) from the output (LBO) back to LBI. This resistor can be calculated from the following equation, but note that the equation for R4 will have to be changed when R5 is added. R5 = R3(VCC ) (VHYST ) – 17mV(VBAT ) VCC = supply voltage for LBO pull-up resistor VHYST = desired hysteresis at the battery R4 (When R5 is Used) = R3(R5)(1.24) R5(VBAT − 1.24) + R3(VCC − 1.24) The LBO pin is open collector. The external pull-up resistor value is determined by user needs. Generally the resistor is 100k to 1M to keep current drain low, but the LBO pin can sink several milliamperes if needed. Example: low battery voltage = 2.5V, desired hysteresis = 200mV, VCC = 5V. Use RDIV = 150k R3 = 150 k(2.5) = 302k (use 301k, 1%) 1.24 R5 = 301k(5V ) = 9.56M (Use 10M) (0.2) – 0.017(2.5) R4 = (301k)(10M)(1.24) 10M(2.5 − 1.24) + 301k(5 − 1.24) = 272k (Use 274k 1%) The total divider resistance will be 274k + 301k = 575k, and this will draw about 7µA from a fully charged battery. Synchronizing The SYNC pin on the LT1500 can be used to synchronize switching frequency to an external clock. The pin should be driven with a 50ns to 300ns pulse which will trigger the switch to an on state. There is a fairly restricted range over which synchronizing will work, because the period between sync pulses must be greater than the natural on-time of the regulator when it is running unsynchronized, and the sync frequency must be greater than the unsynchronized switching frequency. This puts the following restrictions on synchronized operation: fSYNC > fNATURAL f (V ) fSYNC < NATURAL OUT (Use Minimum VIN ) VOUT – VIN fNATURAL is the natural unsynchronized switching frequency of the regulator. It is a function of load current, so a careful check must be done to ensure that the above conditions are met under all load and input voltage conditions. Soft Start (SS) The LT1500 can be soft started by connecting a capacitor to the SS pin. This pin is the base of a PNP transistor whose emitter is tied to the VC pin. Soft start action will occur over the range of 0V to 0.8V on the SS pin and the pin is clamped at 1.2V with an internal clamp. An internal 4µA pull-up current and the external capacitor value determine soft start time. In a typical application a 0.22µF capacitor is sufficient to limit input surges and prevent output overshoot, even with overcompensation on the VC pin. Output voltages greater than 6V with very large output 13 LT1500/LT1501 U U W U APPLICATIONS INFORMATION capacitors may require the capacitor to be larger. To ensure proper reset of the soft start capacitor, an external resistor must be connected in parallel with the capacitor. The resistor value should be 470k or more. Calculating Temperature Rise For most applications, temperature rise in the IC will be fairly low and will not be a problem. However, if load currents are near the maximum allowed and ambient temperatures are also high, a calculation should be done to ensure that the maximum junction temperature of 100°C is not exceeded. The calculations must account for power dissipation in the switch, the drive circuitry and the sense resistor. 2 IOUT ) (RSW )(VOUT )(VOUT − VIN ) ( PTOTAL = (VIN )2 2 IOUT (VOUT − VIN ) RSENSE (IOUT • VOUT ) + + (VIN ) 2 30 U PACKAGE DESCRIPTION PTOTAL = total device power dissipation RSW = switch resistance (0.72Ω max) RSENSE = sense resistance (0.42Ω max) With VIN = –2.2V, VOUT = 5V, IOUT = 150mA, an 8-pin SO package and maximum ambient temperature of 85°C (industrial range), 2 0.15) (0.72)(5)(5 − 2.2) 0.15(5 − 2.2) ( PTOTAL = + 30 (2.2)2 2 0.42(0.15 • 5) + (2.2)2 = 0.47 + 0.014 + 0.049 = 0.11W The SO package has a thermal resistance of 120°C/W, so maximum device temperature will be: TJMAX = 85°C + 0.11W(120°C/W) = 98°C Dimensions 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) 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 *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 14 0.014 – 0.019 (0.355 – 0.483) 2 3 4 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) BSC SO8 0695 LT1500/LT1501 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. S Package 14-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.337 – 0.344* (8.560 – 8.738) 14 13 12 11 10 9 8 0.228 – 0.244 (5.791 – 6.197) 0.150 – 0.157** (3.810 – 3.988) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 4 5 6 0.053 – 0.069 (1.346 – 1.752) 0.004 – 0.010 (0.101 – 0.254) 0° – 8° TYP 0.016 – 0.050 0.406 – 1.270 0.014 – 0.019 (0.355 – 0.483) 7 0.050 (1.270) 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 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. S14 0695 15 LT1500/LT1501 U TYPICAL APPLICATION Typical LT1500 (14-Pin) Application, 2-Cell to 5V Converter 33µH MBR0520L 5V 249k ON(HI) OFF (LO) 2 EACH NiCd OR ALKALINE CELLS + + IN ISENSE SHDN LBI SW OUT 220µF 10V 5V LT1500-3.3/LT1501-5 470k LBO SYNC TO SYSTEM 402k 1nF SELECT SS GND PGND COMP 100k 1M 0.22µF 1000pF LT1500/01 • TA02 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC®1163 Triple High Side Driver for 2-Cell Inputs 1.8V Minimum Input, Drives N-Channel MOSFETs LTC1174 Micropower Step-Down DC/DC Converter 94% Efficiency, 130µA IQ, 9V to 5V at 300mA LT1302 High Output Current Micropower DC/DC Converter 5V/600mA from 2V, 2A Internal Switch, 200µA IQ LT1304 2-Cell Micropower DC/DC Converter Low-Battery Detector Active in Shutdown LTC1440/1/2 Ultralow Power Single/Dual Comparator with Reference 2.8µA IQ, Adjustable Hysteresis LTC1516 2-Cell to 5V Regulated Charge Pump 12µA IQ, No Inductors, 5V at 50mA from 3V Input LT1521 Micropower Low Dropout Linear Regulator 500mV Dropout, 300mA Current, 12µA IQ 16 Linear Technology Corporation LT/GP 0896 7K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1996