APPLICATIONS. LT1300 Micropower High Efficiency 3.3/5V Step-Up DC/DC Converter DESCRIPTION FEATURES TYPICAL APPLICATIONS N

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1 LT3 Micropower High Efficiency 3.3/5V Step-p DC/DC Converter FEATRES p to ma Output Current at 5V from V Supply Supply Voltage as Low as.8v p to 88% Efficiency Small Inductor µh µa Quiescent Current Shutdown to µa Programmable 3.3V or 5V Output I LIM Pin Programs Peak Switch Current Low V CESAT Switch: 7mV at A Typical ses Inexpensive Surface Mount Inductors 8-Lead DIP or SOIC Package APPLICATIONS Palmtop Computers Portable Instruments Bar-Code Scanners DC/DC Converter Module Replacements Battery Backup Supplies Personal Digital Assistants PCMCIA Cards DESCRIPTION The LT3 is a micropower step-up DC/DC converter that utilizes Burst Mode operation. The device can deliver 5V or 3.3V from a two-cell battery input. It features programmable 5V or 3.3V output via a logic-controlled input, noload quiescent current of µa and a shutdown pin which reduces supply current to µa. The on-chip power switch has a low 7mV saturation voltage at a switch current of A, a four-fold reduction over prior designs. A 55kHz internal oscillator allows the use of extremely small surface mount inductors and capacitors. Operation is guaranteed at.8v input. This allows more energy to be extracted from the battery increasing operating life. The I LIM pin can be used to program peak switch current with a single resistor allowing the use of less expensive and smaller inductors and capacitors in lighter load applications. The LT3 is available in an 8-lead SOIC package, minimizing board space requirements. For a 5V/V Selectable Output Converter see the LT3. For increased output current see the LT3. Burst Mode is a trademark of Linear Technology Corporation. TYPICAL APPLICATIONS N AA CELL Two-Cell to 3.3V/5V Step-p Converter C 3 SHTDOWN P L µh D 6 7 LT3 8 5 I LIM N/C C OTPT EFFICIENCY (%) V Output Efficiency =.V = 3.V =.5V =.V L = COILCRAFT DO68-3 OR SMIDA CD5- C = AVX TPSD7MR OR SANYO OS-CON 6SAM LT3 TA LOAD CRRENT (ma) D = MBRS3LT3 OR N587 LT3 TA

2 LT3 ABSOLTE MAXIMM RATINGS W W W Voltage... V Voltage... V Sense Voltage... V SHTDOWN Voltage... V Voltage... V I LIM Voltage....5V Maximum Power Dissipation... 5mW Operating Temperature Range... C to 7 C Storage Temperature Range C to 5 C Lead Temperature (Soldering, sec)... 3 C PACKAGE/ORDER INFORMATION SEL 3 N8 PACKAGE 8-LEAD PLASTIC DIP TOP VIEW 8 P I LIM S8 PACKAGE 8-LEAD PLASTIC SOIC W ORDER PART NMBER LT3CN8 LT3CS8 S8 PART MARKING T JMAX = C, θ JA = 5 C/ W 3 Consult factory for Industrial grade parts. ELECTRICAL CHARACTERISTICS T A = 5 C, = V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX NITS I Q Quiescent Current V =.5V, V SEL = 5V, V = 5.5V µa V =.8V 7 5 µa Input Voltage Range.8 V. V V OT Output Sense Voltage V SEL = 5V V V SEL = V V Output Referred V SEL = 5V (Note ) 5 mv Comparator Hysteresis V SEL = V (Note ) 35 mv Oscillator Frequency Current Limit not Asserted. See Test Circuit khz Oscillator TC. %/ C DC Maximum Duty Cycle % t ON Switch On Time Current Limit not Asserted. 5.6 µs Output Line Regulation.8V < < 6V.6.5 %/V V CESAT Switch Saturation Voltage I = 7mA 3 mv Switch Leakage Current V = 5V, Switch Off. µa Peak Switch Current I LIM Floating (See Typical Application) A (Internal Trip Point) I LIM Grounded. A V H Shutdown Pin High.8 V V L Shutdown Pin Low.5 V V SELH Select Pin High.5 V V SELL Select Pin Low.8 V I Shutdown Pin Bias Current V = 5V 9 µa V = V 3 µa V = V. µa I SEL Select Pin Bias Current V < V SEL < 5V 3 µa The denotes specifications which apply over the C to 7 C temperature range. Note : Hysteresis specified is DC. Output ripple may be higher if output capacitance is insufficient or capacitor ESR is excessive. See applications section.

3 LT3 TYPICAL PERFORMANCE CHARACTERISTICS W EFFICIENCY (%) Efficiency V OT = 3.3V L = µh = 3V =.5V = V LOAD CRRENT (ma) LT3 G INPT CRRENT (µa) No-Load Battery Current V OT = 5V V OT = 3.3V INPT VOLTAGE (V) LT3 G I IVIN I (µa) Total Quiescent Current in Shutdown INPT VOLTAGE (V) 8 LT3 G3 SHTDOWN CRRENT (µa) Shutdown Pin Bias Current T A = 5 C V CESAT (mv) V CESAT vs I OTPT CRRENT (ma) Maximum Output Current vs Input Voltage V OT = 5V, I LIM FLOATING L = µh COILCRAFT DO336-3 L = µh COILCRAFT DO SHTDOWN VOLTAGE (V) ITCH CRRENT (A) INPT VOLTAGE (V) LT3 G LT3 G5 LT3 G6 Maximum Output Current vs Input Voltage Transient Response = V, V OT = 5V Startup Response VOT = 3.3V ILIM FLOATING V OT mv/div AC COPLED V OT V/DIV LOAD CRRENT (ma) L = µh ma I LOAD µs/div LT3 G8 V V/DIV V OT = 5V R LOAD = Ω 5µs/DIV LT3 G INPT VOLTAGE (V) LT3 G7 3

4 LT3 PIN FNCTIONS (Pin ): Signal Ground. Sel (Pin ): Output Select. When tied to or V OT converter regulates at 5V. When grounded converter regulates at 3.3V. (Pin 3): Shutdown. Pull high to effect shutdown. Tie to ground for normal operation. Sense (Pin ): Output Pin. I LIM (Pin 5): Float for A switch current limit. Tie to ground for approximately ma. A resistor between I LIM and ground sets peak current to some intermediate value (see Figure 5). (Pin 6): Supply Pin. Must be bypassed with a large value electrolytic to ground. A.µF ceramic capacitor close to the pin may be needed in some cases. (Pin 7): Switch Pin. Connect inductor and diode here. Keep layout short and direct to minimize electronic radiation. P (Pin 8): Power Ground. Tie to signal ground (pin ) under the package. Bypass capacitor from should be tied directly to the pin. BLOCK DIAGRAM W C L D V OT C 7 5k A CRRENT COMPARATOR 8mV R 73Ω R 3Ω.5V REFERENCE k A COMPARATOR ENABLE OFF OSCILLATOR 55kHZ A3 DRIVER Q x Q 6x 6k BIAS Q3 SHTDOWN 3 8.5k I P LIM 5 8 LT3 F Figure.

5 OPERATION LT3 TEST CIRCITS Oscillator Test Circuit 5V V I L SEL Ω f OT LT3 P Operation of the LT3 is best understood by referring to the Block Diagram in Figure. When A s negative input, related to the Sense pin voltage by the appropriate resistor-divider ratio, is higher that the.5v reference voltage, A s output is low. A, A3 and the oscillator are turned off, drawing no current. Only the reference and A consume current, typically µa. When the voltage at A s negative input decreases below.5v, overcoming A s 6mV hysteresis, A s output goes high, enabling the oscillator, current comparator A, and driver A3. Quiescent current increases to ma as the device prepares for high current switching. Q then turns on in a controlled saturation for (nominally) 5.3µs or until current comparator A trips, whichever comes first. After a fixed off-time of (nominally).µs, Q turns on again. The LT3 s switching causes current to alternately build up in L and dump into capacitor C via D, increasing the output voltage. When the output is high enough to cause A s output to go to low, switching action ceases. C is left to supply current to the load until V OT decreases enough to force A s output high, and the entire cycle repeats. If switch current reaches A, causing A to trip, switch ontime is reduced and off-time increases slightly. This allows continuous mode operation during bursts. Current comparator A monitors the voltage across 3Ω resistor R which is directly related to inductor L s current. Q s collector current is set by the emitter-area ratio to.6% of Q s collector current. When R s voltage drop exceeds 8mV, corresponding to A inductor current, A s output goes high, truncating the on-time portion of the oscillator cycle and increasing off-time to about µs as shown in Figure, trace A. This programmed peak current can be TRACE A 5mA/DIV I LIM PIN OPEN TRACE B 5mA/DIV I LIM PIN GRONDED µs/div reduced by tying the I LIM pin to ground, causing 5µA to flow through R into Q3 s collector. Q3 s current causes a.mv drop in R so that only an additional 7.6mV is required across R to turn off the switch. This corresponds to a ma switch current as shown in Figure, trace B. The reduced peak switch current reduces I R loses in Q, L, C and D. Efficiency can be increased by doing this provided that the accompanying reduction in full load output current is acceptable. Lower peak currents also extend alkaline battery life due to the alkaline cell s high internal impedance. Typical operating waveforms are shown in Figure 3. Figure 3. Burst Mode Operation in Action LT3 F Figure. Switch Pin Current With I LIM Floating or Grounded V OT mv/div AC COPLED V 5V/DIV I IA/DIV µs/div LT3 F 5

6 LT3 APPLICATIONS INFORMATION Output Voltage Selection W The LT3 can be selected to 3.3V or 5V under logic control or fixed at either by tying to ground or respectively. It is permissible to tie to a voltage higher than as long as it does not exceed V. Efficiency in 3.3V mode will be slightly less that in 5V mode due to the fact that the diode drop is a greater percentage of 3.3V than 5V. Since the bipolar switch in the LT3 gets its base drive from, no reduction in switch efficiency occurs when in 3.3V mode. When exceeds the programmed output voltage the output will follow the input. This is characteristic of the simple step-up or boost converter topology. A circuit example that provides a regulated output with an input voltage above or below the output (called a buck-boost or SEPIC) is shown in the Typical Applications section. Figure. Addition of R and C3 Limit Input Current at Startup V OT VDIV C P L µh D LT3 I LIM R M C3.µF OTPT C Shutdown The converter can be turned off by pulling (pin 3) high. Quiescent current drops to µa in this condition. Bias current of 3µA to 5µA flows into the pin (at.5v input). It is recommended that not be left floating. Tie the pin to ground if the feature is not used. I BATTERY 5mA/DIV V V/DIV 5µs/DIV REP RATE = Hz LT3 F5 I LIM Function The LT3 s current limit (I LIM ) pin can be used for soft start. pon start-up, switching regulators require maximum current from the supply. The high currents flowing can create IR drops along supply and ground lines and are especially demanding on alkaline batteries. By installing an R and C3 as shown in Figure, the switch current in the LT3 is limited to ma until the 5µA flowing out of the I LIM pin charges up the.µf capacitor. Input current is held to under 5mA while the output voltage ramps up to 5V as shown in Figure 5. The Meg resistor provides a discharge path for the capacitor without appreciably decreasing peak switch current. When the full capability of the LT3 is not required, peak current can be reduced by changing the value of R3 as shown in Figure 6. With R3 =, switch current is limited to approximately ma. Figure 5. Startup Waveforms using Soft-Start Circuitry I LOAD = ma, V OT = 5V ITCH CRRENT (ma) V VIN 5V 3 k k k M R LIM (Ω) LT3 FB Figure 6. Peak Switch Current vs. R LIM 6

7 LT3 APPLICATIONS INFORMATION Table. Recommended Inductors W EFFICIENCY.5, 5V OT COMPONENT PART NMBER VENDOR L (µh) DCR (Ω) I LIM PIN 5mA LOAD ma LOAD HEIGHT (mm) PHONE NMBER DO68-3 Coilcraft. Float (78) DO336-3 Coilcraft.5 Float DO68-3 Coilcraft.3 Ground CTX- Coiltronics.38 Float (7) 7876 CTX- Coiltronics.75 Ground 86. LQH3CKM Murata-Frie.7 Ground 8. () 36 3 CD5-M Sumida. Float (78) CDRH6-M Sumida.38 Ground 8 3. CDRH6-M Sumida.7 Float GA-K Gowanda.38 Float Through-Hole (76) 53 3 Inductor Selection For full output power, the inductor should have a saturation current rating of.5a for worst-case current limit, although it is acceptable to bias an inductor % or more into saturation. Smaller inductors can be used in conjunction with the I LIM pin. Efficiency is significantly affected by inductor DCR. For best efficiency limit the DCR to.3ω or less. Toroidal types are preferred in some cases due to their closed design and inherent EMI/RFI superiority. Recommended inductors are listed in Table. Capacitor Selection Low ESR capacitors are required for both input and output of the LT3. ESR directly affects ripple voltage and efficiency. For surface mount applications AVX TPS series tantalum capacitors are recommended. These have been specially designed for SMPS and have low ESR along with high surge current ratings. For through-hole application Sanyo OS-CON capacitors offer extremely low ESR in a small size. Again, if peak switch current is reduced using the I LIM pin, capacitor requirements can be relaxed and smaller, higher ESR units can be used. Low frequency output ripple can be reduced by adding multiple output capacitors. If capacitance is reduced, output ripple will increase. Suggested capacitor sources are listed in Table. Table. Recommended Capacitors VENDOR SERIES TYPE PHONE# AVX TPS Surface Mount (83)8 9 Sanyo OS-CON Through-Hole (69) Panasonic HFQ Through-Hole () 38 5 Diode Selection Best performance is obtained with a Schottky rectifier diode such as the N587. Phillips Components makes this in surface mount as the PRLL587. Motorola makes the MBRS3LT3 which is slightly better and also in surface mount. For lower output power a N8 can be used although efficiency will suffer substantially. Layout Considerations The LT3 is a high speed, high current device. The input capacitor must be no more than." from (pin 6) and ground. Connect the P and (pins 8 and ) together under the package. Place the inductor adjacent to (pin 7) and make the switch pin trace as short as possible. This keeps radiated noise to a minimum. 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. 7

8 LT3 TYPICAL APPLICATIONS N Four-Cell to p-down Converter LCD Contrast Supply.5V 8V AA CELL C** SHTDOWN N/C I LIM LT3 *L, L = GOWANDA GA-7K COILCRAFT DO336-73K SMIDA CD73-7K **C, C, C3 = SANYO OS-CON 6SAM P L* 7µH C** L* 7µH N587 ma 8% EFFICIENT C3** LT3 TA3.8V TO 6V N/C N/C LT3 P 8 I LIM T N589 SHTDOWN 5K K CONTRAST VOT V TO 9V ma MAXIMM FROM.8V SPPLY (77% EFFICIENT) ma MAXIMM FROM 3V SPPLY (83% EFFICIENT) µf 35V Step-p Converter with Automatic Output Disconnect T = DALE LPE-57-AO5 (65) K.µF L* µh N587 7Ω N3 5V, ma PWM IN % TO % CMOS DRIVE V TO 5V LT3 TA6 AA CELL SHTDOWN LT3 NC I LIM P.µF *SMIDA CD5-LC COILCRAFT DO336-3 LT3 TA5 PACKAGE DESCRIPTION.3.3 ( ) Dimensions in inches (millimeters) unless otherwise noted (.3.65).3 ±.5 (3.3 ±.7). (.6) MAX N8 Package 8-Lead Plastic DIP S8 Package 8-Lead Plastic SIC 8.8. (.3.5).9.5 (.9.38) ( ).. (.5.58) Linear Technology Corporation 63 McCarthy Blvd., Milpitas, CA (8) 3-9 FAX: (8) 3-57 TELEX: (.65) TYP.5 ±.5 (.3 ±.38). ±. (.5 ±.5) 8 TYP (.36.75)..9 ( ).5 (3.75) MIN.8 ±.3 (.57 ±.76).5 (.7) BSC *THESE DIMENSIONS DO NOT INCLDE MOLD FLASH OR PROTRSIONS. MOLD FLASH OR PROTRSIONS SHALL NOT EXCEED.6 INCH (.5mm).. (.58) MIN.. (..5).8. ( ) * (.8 5.) ±. (6.35 ±.5) N ( SO8 9 LT/GP 39 K PRINTED IN SA LINEAR TECHNOLOGY CORPORATION 99