MAY 2002 VOLUME XII NUMBER 2

LINEAR TECHNOLOGYLINEAR TECHNOLOGYLINEAR TECHNOLOGY

, LTC, LT, Burst Mode, OPTI-LOOP and Over-The-Top are registered trademarks of Linear Technology Corporation.Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD, MultimodeDimming, No Latency ∆Σ, No RSENSE, Operational Filter, PolyPhase, PowerSOT, SoftSpan, SwitcherCAD, ThinSOT andUltraFast are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companiesthat manufacture the products.

IN THIS ISSUE…COVER ARTICLE

LTC1733: Thermal RegulationMaximizes Lithium-Ion BatteryCharging Rate Without Risk ofOverheating .................................. 1Trevor Barcelo

Issue Highlights ............................ 2

LTC® in the News ........................... 2

DESIGN FEATURES

LT®3420 Charges PhotoflashCapacitors Quickly and EfficientlyWhile Using Minimal Board Space..................................................... 5

Albert Wu

Small 1.25A Step-Down RegulatorSwitches at 4MHz for Space-SensitiveApplications .................................. 9Damon Lee

Dual DC/DC Controller Brings 2-PhaseBenefits to Low Input VoltageApplications ................................ 12Jason Leonard

ThinSOT™ RF Power Controllers SaveCritical Board Space and Power inPortable RF Products ................... 15Ted Henderson and Shuley Nakamura

DESIGN IDEAS.............................................. 21–35

(complete list on page 21)

New Device Cameos ..................... 36

Design Tools ................................ 39

Sales Offices ............................... 40

continued on page 3

IntroductionLinear battery chargers are typicallysmaller, simpler and less expensivethan switcher-based solutions, butthey have one major disadvantage:excessive power dissipation. Whenthe input voltage is high and thebattery voltage is low (dischargedbattery) a linear charger could gener-ate enough heat to damage itself orother components. Typically, suchconditions are temporary—as thebattery voltage rises with its charge—but it is worst case situations thatone must account for when determin-ing the maximum allowable valuesfor charge current and IC tempera-ture. To solve this problem, theLTC1733 employs internal thermal

LTC1733: ThermalRegulation MaximizesLithium-Ion BatteryCharging Rate WithoutRisk of Overheating

by Trevor Barcelofeedback to regulate the charge cur-rent and limit the die temperature.This feature translates to faster chargetimes, because a designer can pro-gram a high charge current (tominimize charging time) without therisk of damaging the LTC1733 or anyother components. The need for ther-mal over-design is also eliminated. Tofurther improve heat transfer, theLTC1733 is housed in a thermallyenhanced 10-pin MSOP package. Forsimplicity, the LTC1733 provides acomplete lithium-ion charger solu-tion requiring only three externalcomponents, as shown in Figure 1.

An internal power MOSFET allowscharge current to be programmed up

BAT

PROGTIMER

GND NTC

VCC

VIN = 5V

1.5k1%

4.7µF

0.1µF

IBAT = 1A

1733TA01

LTC1733 1-CELLLi-IonBATTERY*

*AN OUTPUT CAPACITOR MAY BE REQUIRED DEPENDING ON BATTERY LEAD LENGTH

Figure 1. Standalone Li-Ion battery charger

DESIGN FEATURES

Linear Technology Magazine • May 2002 3

LTC1733, continued from page 1to 1.5A, with 7% accuracy, to ensurea fast and complete charge. The inter-nal MOSFET also eliminates the needfor an external current sense resistoror blocking diode. The final float volt-age is pin selectable to either 4.1V or4.2V with 1% accuracy to preventdangerous overcharging or reducedbattery capacity due to undercharg-ing.

Following battery manufacturers’guidelines, the LTC1733 includes aprogrammable charge terminationtimer and thermistor input for tem-perature qualified charging. Statusoutputs include C/10 charge detec-tion to indicate a near end-of-chargecondition, wall adapter present de-tection to determine whether chargingmay proceed or not, charge currentmonitoring for gas gauging, and faultdetection for identifying bad cells.Low battery charge conditioning(trickle charging) safely charges anover-discharged cell, and automaticrecharge ensures that the battery isalways fully charged. To conservebattery power, the LTC1733 batterydrain current drops to less than 5µAwhen a wall adapter is not present orwhen the part is shutdown.

Charging a BatteryTo charge a single cell Li-ion battery,the user must apply an input voltage(typically, a wall adapter) of at least4.5V to the VCC pin. The ACPR pin willsubsequently pull low to indicate thatthe input voltage condition has beenmet. Furthermore, a 1% resistor mustbe connected from PROG to GND toprogram the nominal charge currentto 1500V/RPROG. The CHRG pin willthen pull low to indicate that a chargecycle has commenced. A capacitorconnected between the TIMER pinand GND will program the chargetermination time to 3 hours per 100nF.

If the BAT pin voltage is below2.48V at the beginning of a chargecycle then the charge current will beone-tenth of the programmed valuein order to safely bring the cell voltagehigh enough to allow full charge cur-rent. If the cell is damaged, and thevoltage does not rise above 2.48V

within one-quarter of the programmedtermination time, the charge cyclewill terminate, and the FAULT statusoutput will latch low indicating a badcell. All three of these status outputpins, ACPR, CHRG and FAULT, haveenough current sinking capability tolight an LED.

Once the battery voltage rises above2.48V (which typically occurs soonafter the start of a charge cycle), theLTC1733 will provide a constant cur-rent to the battery as programmed byRPROG. The LTC1733 will remain inconstant-current mode until the BATpin voltage approaches the selectedfinal float voltage (4.1V for SEL = 0Vand 4.2V for SEL = VCC). At this pointthe part enters constant-voltage mode.

In constant-voltage mode, theLTC1733 begins to decrease thecharge current to maintain a con-stant voltage at the BAT pin rather

than a constant current out of theBAT pin. When the current drops to10% of the full-scale programmedcharge current, an internal compara-tor latches off the strong pull-down atthe CHRG pin and connects a weakcurrent source (about 25µA) to groundto indicate a near end-of-charge (C/10) condition.

Unlike battery chargers that ter-minate when the current reaches C/10, the LTC1733 continues to chargethe battery after the C/10 point, aslong as the termination time has notelapsed, to ensure that the battery isfully charged. Terminating chargingat C/10 can leave a battery charged toonly 90% to 95% capacity, while charg-ing past C/10 and terminating basedon time can charge a battery to 100%capacity. Upon termination, theCHRG pin assumes a high impedancestate.

Recharging a BatteryThe LTC1733 has the ability to re-charge a battery assuming that thebattery voltage has been chargedabove 3.95V (SEL = 0V) or 4.05V (SEL= VCC) during the initial charge cycle.Once above these thresholds, a newcharge cycle begins if the battery volt-age drops below 3.9V (SEL = 0V) or4.0V (SEL = VCC) due to either a loadon the battery, or the self-dischargecurrent of the battery. The rechargecircuit integrates the BAT pin voltagefor a few milliseconds to prevent tran-sients from restarting the charge cycle.This feature ensures that the batteryremains charged even if left connectedto the powered charger for very longperiods of time.

Thermal RegulationAn additional key feature of theLTC1733 is the internal thermal regu-lation loop. If high power operationand/or high ambient temperatureconditions cause the junction tem-perature of the LTC1733 to approach105°C, the charge current is auto-matically reduced to maintain thejunction temperature at roughly105°C (board temperatures typicallyremain below about 85°C). This iscalled constant-temperature mode.

–

+

–

+

–

+

1733 F02

RNTC10k

RHOT1%

7/8 VCC

1/2 VCC

3/160 VCC

TOOCOLD

TOOHOT

DISABLENTC

LTC1733

VCC

NTC

Figure 3. Temperature qualification circuitry

Figure 2. Full featured single cellLi-Ion charger

Linear Technology Magazine • May 20024

DESIGN FEATURES

This feature allows the user to pro-gram a charge current based on typicaloperating conditions and eliminatesthe need for the complicated thermalover-design necessary in many linearcharger applications. Worst-case con-ditions are automatically taken careof by the LTC1733. In addition toprotecting the LTC1733, this featureeliminates “hot spots” on the board,thereby protecting surrounding com-ponents. The thermal shutdownfeatures of other battery chargers sim-ply turn off the charger at very hightemperatures (typically, in excess of130°C). This junction-temperature-based type of shutdown allows boththe battery charger and the surround-ing board to get extremely hot, soeven though the shutdown “protec-tion” exists, the application must bepainstakingly designed to avoidreaching the thermal shutdown tem-perature under all scenarios. TheLTC1733 simplifies thermal designby automatically balancing chargecurrent, power dissipation and oper-ating temperature.

To further improve the thermalperformance of the LTC1733, it ispackaged in a 10-pin thermallyenhanced MSOP package. The appli-cation board pictured in Figure 2occupies just 76mm2 of board spaceand can dissipate over 2W of power atroom temperature. That equates to amaximum charge current of about1.5A, with a 5V input supply. Thisassumes that a Li-ion battery spendsmost of its time at 3.7V during charge.

In fact, this is a conservative assump-tion, since a typical Li-ion battery willrise above 3.8V within the first fewminutes of charging. The powerfulthermal features of the LTC1733 andthe 7% accuracy of the programmedcharge current allow very fast andaccurate charging of single cell Li-ionbatteries.

PROG Current MonitorFor gas gauging applications, thePROG pin provides very accurate in-formation regarding the currentflowing out of the BAT pin. The rela-tionship is given by:

IBAT = (VPROG/RPROG) • 1000During constant-current mode, the

PROG pin voltage is always 1.5V,indicating that the programmedcharge current is flowing out of theBAT pin. In constant-temperature orconstant-voltage mode, the BAT pincurrent is reduced and can be deter-mined by measuring the PROG pinvoltage and applying the above for-mula. The PROG pin, along with thethree open-drain status outputs(ACPR, CHRG, and FAULT), informthe user of exactly what the LTC1733is doing at all times.

NTC ThermistorIn addition to the programmable timerand low battery charge qualification,the LTC1733 adds temperature quali-fied charging to the list of batterymanufacturer recommended safetyfeatures. The battery temperature is

measured by placing a negative tem-perature coefficient (NTC) thermistorclose to the battery pack. Using thecircuitry shown in Figure 3, theLTC1733 can temporarily suspendthe internal timer and stop chargingwhen the battery temperature fallsbelow 0°C or rises above 50°C. Toperform this function, RHOT should bechosen to be the value of the selectedNTC thermistor at 50°C. This willensure that the internal comparator’strip point of 1/2VCC corresponds toan NTC temperature of 50°C. Fur-thermore, the selected NTC thermistorshould have a value at 0°C that is asclose to seven times the value at 50°Cas possible. A 7:1 cold to hot NTCratio ensures that the internalcomparator’s trip point of 7/8VCCcorresponds to an NTC temperatureof 0°C. The hot and cold comparatorseach have approximately 2°C of hys-teresis to prevent oscillation aboutthe trip point. In addition, the NTCfunction can be disabled without anyexternal components by simplygrounding the NTC pin.

ConclusionThe LTC1733 is a full-featured,standalone Li-ion battery charger. Inits simplest form, the LTC1733 onlyrequires three external componentsand can safely and accurately chargehigh-capacity batteries very quicklywith up to 1.5A of charge current. AnNTC thermistor and a few LEDs canbe added to take advantage of thesafety and status features.