LT1305 Micropower High Power DC/DC Converter with Low-Battery Detector DESCRIPTIO

Transcription

1 Micropower High Power DC/DC Converter with Low-Battery Detector APPLICATI O FEATRES 5V at ma from V Input Supply Voltage As Low As 1.V 1µA Quiescent Current Low-Battery Detector Low V CESAT Switch: 31mV at A Typ ses Inexpensive Surface Mount Inductors -Lead SO Package S -Cell and 3-Cell to 5V Conversion EL Panel Drivers Portable Instruments DESCRIPTIO The LT 135 is a micropower step-up DC/DC converter that uses Burst Mode TM operation. Similar to the LT133, the features a A internal low-loss switch and can deliver up to four times the output power of the LT133. Quiescent current is only 1µA and the Shutdown pin further reduces current to 1µA. A low-battery detector provides an open-collector output that goes low when the input voltage drops below a preset level. The is available in an -pin SO, easing board space requirements., LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation TYPICAL APPLICATI O -Cell and 3-Cell to 5V/mA DC/DC Converter with Low-Battery Detect L1 1µH D1 9 Efficiency TO 3 CELLS C1 µf 31k 1k SHTDOWN SHDN P k 31k k C µf LOW BATTERY GOES LOW AT V BAT =.V V OT 5V ma EFFICIENCY (%) 7 =.V = 3.V =.V =.5V C1, C: AVX TPSE71R D1: MOTOROLA MBRS13LT3 L1: COILCRAFT D TA3 1 1 LOAD CRRENT (ma) TA 1

2 ABSOLTE AXI RATI GS W W W Voltage... 1V 1 Voltage... 5V Voltage... 1V Shutdown Voltage... 1V Voltage... 1V Voltage... 1V Maximum Power Dissipation... 5mW Operating Temperature Range... C to 7 C Storage Temperature Range... 5 C to 15 C Lead Temperature (Soldering, 1 sec)... 3 C PACKAGE/ORDER I FOR 1 SHDN 3 TOP VIEW 7 5 S PACKAGE -LEAD PLASTIC SO T JMAX = C, θ JA = C/ W P W Consult factory for Industrial and Military grade parts. ATIO ORDER PART NMBER CS S PART MARKING 135 ELECTRICAL CHARA CTERISTICS T A = 5 C, =.V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX NITS I Q Quiescent Current V SHDN =.5V, V = V 1 µa V SHDN = 1.V 7 15 µa Input Voltage Range V. V Feedback Voltage V Comparator Hysteresis 1.5 mv Feedback Pin Bias Current V = 1V 7 na Oscillator Frequency Current Limit Not Asserted khz Oscillator TC. %/ C DC Maximum Duty Cycle 95 % t ON Switch On Time Current Limit Not Asserted 5. µs Output Line Regulation 1.V < < V..15 %/V V CESAT Switch Saturation Voltage I = 1A 1 mv Switch Leakage Current V = 5V, Switch Off.1 1 µa Peak Switch Current = V A 1..5 A = 5V A Trip Voltage (Note ) V Input Bias Current V = 1V 7 na Output Low I LOAD = µa.11. V Leakage Current V = 1.3V, V = 5V.1 5 µa V SHDNH Shutdown Pin High 1. V V SHDNL Shutdown Pin Low.5 V I SHDN Shutdown Pin Bias Current V SHDN = 5V. µa V SHDN = V 3. µa V SHDN = V.1 1 µa The denotes specifications which apply over the C to 7 C operating temperature range. Note 1: Hysteresis specified is DC. Output ripple may be higher if output capacitance is insufficient or capacitor ESR is excessive. Note : Low-battery detector comparator is inoperative when device is in shutdown.

3 TYPICAL PERFORMANCE CHARACTERISTICS W Switch On Time Oscillator Frequency Maximum Duty Cycle ON TIME (µs) 5 FREQENCY (khz) DTY CYCLE (%) G1 G G3 QIESCENT CRRENT (µa) Quiescent Current ITCH OFF = V QIESCENT CRRENT (µa) 5 3 Quiescent Current T A = 5 C ITCH OFF PEAK ITCH CRRENT (A) Current Limit 1 INPT VOLTAGE (V) 1. TEMPERATRE ( C) G G5 G Pin Bias Current Pin Bias Current Voltage BIAS CRRENT (na) BIAS CRRENT (na) FEEDBACK VOLTAGE (V) G7 G G9 3

4 TYPICAL PERFORMANCE CHARACTERISTICS W VOLTAGE (V) Low-Battery Detect Trip Point ITCH SATRATION VOLTAGE (mv) Switch Saturation Voltage T A = 5 C ITCH SATRATION VOLTAGE (mv) Switch Saturation Voltage I = 1A ITCH CRRENT (A) G1 F1 G13 PI F CTIO S (Pin 1): Signal Ground. Tie to P under the package. (Pin ): Open-Collector Output of Comparator C3. Can sink µa. High impedance when device is in shutdown. SHDN (Pin 3): Shutdown. Pull high to shut down the. Ground for normal operation. (Pin ): Feedback Input. Connects to main comparator C1 input. (Pin 5): Low-Battery Comparator Input. When voltage on this pin is below 1.V, is low. (Pin ): Supply Pin. Must be bypassed with a large value capacitor to gound. Keep bypass within." of the device. (Pin 7): Switch Pin. Connect inductor and diode here. Keep layout short and direct to minimize radio frequency interference. P (Pin ): Power Ground. Tie to signal ground (pin 1) under the package. Bypass capacitor from should be tied directly to P within." of the device.

5 BLOCK DIAGRAM W L1 D1 C5 7 C REFERENCE 1.V HYSTERETIC COMPARATOR C1 OFF OSCILLATOR CRRENT COMPARATOR C A3 DRIVER 3mV 3Ω Q 1 Q1 1 C3 1 5 SHTDOWN 3 P F1 Figure 1. Block Diagram OPERATION Operation of the is best understood by referring to the Block Diagram in Figure 1. When C1 s negative input, related to the output voltage by the appropriate resistordivider ratio, is higher than the 1.V reference voltage, C1 s output is low. C, A3 and the oscillator are turned off, drawing no current. Only the reference and C1 consume current, typically 1µA. When C1 s negative input drops below 1.V and overcomes C1 s mv hysteresis, C1 s output goes high, enabling the oscillator, current comparator C and driver A3. Quiescent current increases to ma as the device goes into active switching mode. Q1 then turns on in controlled saturation for nominally µs or until current comparator C trips, whichever comes first. The switch then turns off for approximately 1.5µs, then turns on again. The s switching causes current to alternately build up in L1 and dump into output capacitor C via D1, increasing the output voltage. When the output is high enough to cause C1 s output to go high, switching action ceases. Capacitor C is left to supply current to the load until V OT decreases enough to force C1 s output high, and the entire cycle repeats. Figure details relevant waveforms. C1 s cycling causes low-to-mid-frequency ripple voltage on the output. Ripple can be reduced by making the output capacitor large. The µf unit specified results in ripple of 5mV to mv on the 5V output. Paralleling two capacitors will decrease ripple by approximately 5%. V OT mv/div AC COPLED V 5V/DIV I L 1A/DIV 5µs/DIV Figure. Burst Mode Operation F 5

6 OPERATION If switch current reaches A, causing C to trip, switch on time is reduced and off time increases slightly. This allows continuous operation during bursts. C monitors the voltage across 3Ω resistor which is directly related to the switch current. Q s collector current is set by the emitter-area ratio to.% of Q1 s collector current. When s voltage drop exceeds 3mV, corresponding to A switch current, C s output goes high, truncating the on time portion of the oscillator cycle and increasing off time to about µs. Response time of C, which determines minimum on time, is approximately 3ns. Low-Battery Detector The low-battery detector is enabled when SHDN is low and disabled when SHDN is high. The comparator has no hysteresis built in, but hysteresis can be added by connecting a high-value resistor from to as shown in Figure 3. The internal reference can be accessed via the comparator as shown in Figure. V BAT 9.9k N39 V REF OTPT 1.V = (V TRIP 1.V) (3.5k) HYSTERESIS 3mV Figure 3. R3 Adds Hysteresis to Low-Battery Detector.µF ( ) V REF = 1.V 1 V REF mv 33k k R3 M F 5V R 7k F3 Inductor Selection Inductors used with the must fulfill two requirements. First, the inductor must be able to handle current of A to.5a without runaway saturation. Rod or drum core units usually saturate gradually and it is acceptable to exceed manufacturer s published saturation current by % or so. Second, the unit must have low DCR, under.5ω so that copper loss is kept low and excess heating is avoided. Inductance value is not critical. Generally, for low voltage inputs below 3V a 1µH inductor is recommended (such as Coilcraft DO331-13). For inputs above V to 5V use a µh unit (such as Coilcraft DO331-3). Switching frequency can reach up to 3kHz so the core material should be able to operate at high frequency without excessive core loss. Ferrite or molypermalloy cores are a better choice than powdered iron. If EMI is a concern, a toroidal inductor is suggested, such as Coiltronics CTX-. Capacitor Selection Output and input capacitors should have low ESR for best performance. Inexpensive aluminum electrolytics sometimes have ESR above 1Ω, even for relatively large values such as µf, 1V units. Since the has a A current limit, V of ripple voltage would result with such a capacitor at the output. Keep ESR below.5ω to.1ω for reasonable ripple voltage. Tantalum capacitors such as AVX TPS series or Sprague 593D have low ESR and are surface mount components. For lowest ESR, use Sanyo OS-CON units (OS-CON is also available from Vishay). These capacitors have superior ESR, small size and perform well at cold temperatures. Diode Selection A A Schottky diode such as Motorola MBRS13LT3 is a good choice for the rectifier diode. A 1N51 or MBRS13T3 are suitable as well. Do not use general purpose diodes such as 1N1. They are much too slow for use in switching regulator applications. Figure. Accessing Internal Reference

7 TYPICAL APPLICATIONS N Setting Output Voltage -Cell-to-5V Converter L1 1N V to.5v L1* µh µf** MBRS13LT3 µf V OT = 1.V 1 P ( ) V OT µf TA3 CELLS µf SHTDOWN SHDN *COILCRAFT DO331-3 OR SMIDA CD15- **SANYO OS-CON P 39k k µh* 5V ma 15µF** TA5 5V Step-p Converter with Reference Output V REF OTPT 1.V 1.V TO.5V INPT N39.µF 33k k µf 1µH* P MBRS13LT3 39k k 5V ma µf *COILCRAFT DO TA EL Panel Driver 1.5V TO V 3.3M 1k 7µF 1/W.1µF 3.3M CERAMIC 1N1 1k ZETEX SHDN FZT5 P 51k SHTDOWN 1Ω T1** 1:15, 5 1, 1 5k 1N51 3.3M M.7µF 1V C1* 5pF EL PANEL C PANEL nf Hz TO Hz SQARE WAVE DRIVE *ADD C1 FOR OPEN-PANEL PROTECTION **DALE LPE57-A13 1:15 TRNS RATIO 1µH PRIMARY INDCTANCE (5) -931 ADJSTS V OT 3V RMS TO 115V RMS AVX TPS OR SANYO OS-CON MST HAVE ESR.15Ω 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. TA 7

8 PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. S Package -Lead Plastic SOIC * (.1 5.) ( ) * ( ) (.3.5).1. (.5.5) 5 TYP.53.9 (1.3 1.)..1 (.11.5) (.355.3) *THESE DIMENSIONS DO NOT INCLDE MOLD FLASH OR PROTRSIONS. MOLD FLASH OR PROTRSIONS SHALL NOT EXCEED. INCH (.15mm)..5 (1.7) BSC SO 9 RELATED PARTS PART NMBER DESCRIPTION COMMENTS LT119 Micropower Low Dropout Regulator 7mA Output Current in SO- Package LT11/3/ LCD and CCFL Backlight Controller High Efficiency and Excellent Backlight Control Range LT131 5V to 1V/mA Step-p DC/DC Converter 1µA Quiescent Current LT13 -Cell to 5V/mA Step-p DC/DC Converter µa Quiescent Current LT133 Micropower DC/DC Converter with Low-Battery Detect V to 5V at ma LT137 5kHz Step-p PWM, 1.5A Switch Low Noise, Fixed Frequency Operation LTC 17 PCMCIA Host Switch with Protection Includes Current Limit and Thermal Shutdown LT/GP 595 1K PRINTED IN SA Linear Technology Corporation 13 McCarthy Blvd., Milpitas, CA () 3-19 FAX: () 3-57 TELEX: LINEAR TECHNOLOGY CORPORATION 1995