MAX745 switch-mode lithium-ion battery-charger ________________________________________________________________ maxim integrated products 1 general description the MAX745 provides all functions necessary for charging lithium-ion (li+) battery packs. it provides a regulated charging current of up to 4a without getting hot, and a regulated voltage with only ?.75% total error at the battery terminals. it uses low-cost, 1% resis- tors to set the output voltage, and a low-cost n-channel mosfet as the power switch. the MAX745 regulates the voltage set point and charg- ing current using two loops that work together to transi- tion smoothly between voltage and current regulation. the per-cell battery voltage regulation limit is set between 4v and 4.4v using standard 1% resistors, and then the number of cells is set from 1 to 4 by pin-strap- ping. total output voltage error is less than ?.75%. for a similar device with an smbus microcontroller interface and the ability to charge nicd and nimh cells, refer to the max1647 and max1648. for a low-cost li+ charger using a linear-regulator control scheme, refer to the max846a. ________________________applications li+ battery packs desktop cradle chargers cellular phones notebook computers hand-held instruments ____________________________features charges 1 to 4 li+ battery cells 0.75% voltage-regulation accuracy using 1% resistors provides up to 4a without excessive heating 90% efficient uses low-cost set resistors and n-channel switch up to 24v input up to 18v maximum battery voltage 300khz pulse-width modulated (pwm) operation low-noise, small components stand-alone operation?o microcontroller needed typical operating circuit 19-1182; rev 3; 10/01 part MAX745eap 40? to +85? temp range pin-package 20 ssop evaluation kit manual follows data sheet ordering information pin configuration appears at end of data sheet. MAX745c/d 0? to +70? dice* * dice are tested at t a = +25?. smbus is a trademark of intel corp. (up to 24v) ref dcin v in bst vl dhi dlo lx cs batt cell count select set per cell voltage with 1% resistors on off vadj status seti cell0 cell1 cci pgnd gnd ibat ccv n n i charge r sense vout 1? li+ cells (up to 18v) MAX745 thm/shdn for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com.
MAX745 switch-mode lithium-ion battery charger 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v dcin = 18v, v batt = 8.4v, t a = 0? to +85? . typical values are at t a = +25?, unless otherwise noted.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. dcin to gnd ............................................................-0.3v to 26v bst, dhi to gnd ......................................................-0.3v to 30v bst to lx ....................................................................-0.3v to 6v dhi to lx............................................(lx - 0.3v) to (bst + 0.3v) lx to gnd ................................................-0.3v to (dcin + 0.3v) vl to gnd...................................................................-0.3v to 6v cell0, cell1, ibat, status, cci, ccv, ref, seti, vadj, dlo, thm/ shdn to gnd.................-0.3v to (vl + 0.3v) batt, cs to gnd .....................................................-0.3v to 20v pgnd to gnd..........................................................-0.3v to 0.3v vl current ...........................................................................50ma continuous power dissipation (t a = +70?) ssop (derate 8.00mw/? above +70?) ....................640mw operating temperature range ...........................-40? to +85? storage temperature.........................................-60? to +150? lead temperature (soldering, 10s) .................................+300? 6.0v < v dcin < 24v, logic inputs = vl vl < 3.2v, v cs = 12v vl < 3.2v, v batt = 12v output high or low 0 < i ref < 1ma 6.0v < v dcin < 24v, no load t a = +25? output high or low conditions v 019 batt, cs input voltage range ? 5 cs input current ? 5 batt input current ? 614 dlo on-resistance ? 47 dhi on-resistance ma 46 dcin quiescent supply current v 624 dcin input voltage range % 89 93 dhi maximum duty cycle khz 270 300 330 oscillator frequency mv/ma 10 20 ref output load regulation v 5.15 5.40 5.65 vl output voltage v 4.17 4.2 4.23 ref output voltage units min typ max parameter 4v < v batt < 16v 6.0v < v dcin < 24v (note 1) mv ?.5 cs to batt offset voltage seti = v ref (full scale) mv 170 185 205 cs to batt current-sense voltage not including vadj resistor tolerance % -0.65 +0.65 absolute voltage accuracy with 1% tolerance vadj resistors 4.16 4.2 4.24 -0.75 +0.75 switching regulator supply and reference vl > 5.15v, v batt = 12v vl > 5.15v, v cs = 12v 400 500 seti = 400mv 14 18 22
MAX745 _______________________________________________________________________________________ 3 switch-mode lithium-ion battery charger note 1: when v seti = 0v, the battery charger turns off. electrical characteristics (continued) (v dcin = 18v, v batt = 8.4v, t a = 0? to +85? . typical values are at t a = +25?, unless otherwise noted.) electrical characteristics (v dcin = 18v, v batt = 8.4v, t a = -40? to +85? , unless otherwise noted. limits over temperature are guaranteed by design.) 6.0v < v dcin < 24v 6.0v < v dcin < 24v, no load output high or low output high or low conditions mv 165 205 cs to batt full-scale current-sense voltage not including vadj resistors % -1.0 +1.0 absolute voltage accuracy v 4.14 4.26 ref output voltage v 5.10 5.70 vl output voltage ? 14 dlo on-resistance ? 7 dhi on-resistance khz 260 340 oscillator frequency units min typ max parameter ibat compliance voltage range 02 v v ibat = 2v ibat output current vs. current-sense voltage 0.9 ?/mv charger in voltage-regulation mode, v status = 5v status output leakage current 1 ? charger in current-regulation mode, status sinking 1ma status output low voltage 0.2 v thm/ shdn falling threshold 2.01 2.1 2.19 v thm/ shdn rising threshold 2.20 2.3 2.34 v 1.1v < v cci < 3.5v ccv clamp voltage with respect to cci parameter min typ max units gmv amplifier output current ?30 ? gmi amplifier transconductance 200 ?/v gmi amplifier output current ?20 ? cci clamp voltage with respect to ccv 25 80 200 mv 25 80 200 mv cell0, cell1 input bias current -1 +1 ? seti input voltage range 0v ref v seti, vadj input bias current -10 +10 na vadj adjustment range 10 % conditions vadj input voltage range 0 v ref 1.1v < v ccv < 3.5v v (note 1) switching regulator (note 1) supply and reference gmv amplifier transconductance 800 ?/v control inputs/outputs error amplifiers
MAX745 switch-mode lithium-ion battery charger 4 _______________________________________________________________________________________ 4.5 0 0 0.1 0.2 0.4 1.0 battery voltage vs. charging current 1.0 4.0 MAX745/toc-01 charging current (a) battery voltage (v) 0.3 0.5 0.6 0.7 0.8 0.9 3.0 2.0 0.5 3.5 2.5 1.5 r1 = 0.2 ? r16 = short r12 = open circuit 200 0 0 0.5 1.5 4.0 current-sense voltage vs. seti voltage 40 160 MAX745/toc-02 seti voltage (v) current-sense voltage (mv) 1.0 2.0 2.5 3.0 3.5 120 80 180 20 140 100 60 r1 = 0.2 ? 4.45 3.95 0 0.5 1.0 2.0 4.5 voltage limit vs. vadj voltage 4.05 4.35 MAX745/toc-03 vadj voltage (v) per-cell voltage limit (v) 1.5 2.5 3.0 3.5 4.0 4.25 4.15 4.40 4.00 4.30 4.20 4.10 4.205 4.195 02550 reference voltage vs. temperature 4.197 4.203 MAX745/toc-06 temperature ( c) reference voltage (v) 75 100 4.201 4.199 4.204 4.196 4.202 4.200 4.198 __________________________________________typical operating characteristics (t a = +25?, v dcin = 18v, v batt = 4.2v, cell0 = cell1 = gnd, c vl = 4.7? c ref = 0.1?. circuit of figure 1, unless otherwise noted.) 5.50 0 0 5 10 25 vl load regulation 5.10 5.40 MAX745/toc-04 vl output current (ma) vl output voltage (v) 15 20 5.30 5.20 5.45 5.05 5.35 5.25 5.15 4.25 4.15 0 500 1000 3000 reference load regulation 4.17 4.23 MAX745/toc-05 reference current ( a) reference voltage (v) 1500 2000 2500 4.21 4.19 4.24 4.16 4.22 4.20 4.18
_______________detailed description the MAX745 is a switch-mode, li+ battery charger that can achieve 90% efficiency. the charge voltage and current are set independently by external resistor- dividers at seti and vadj, and at pin connections at cell0 and cell1. vadj is connected to a resistor- divider to set the charging voltage. the output voltage- adjustment range is ?%, eliminating the need for 0.1% resistors while still achieving 0.75% set accuracy using 1% resistors. the MAX745 consists of a current-mode, pulse-width- modulated (pwm) controller and two transconductance error amplifiers: one for regulating current (gmi) and the other for regulating voltage (gmv) (figure 2). the error amplifiers are controlled through the seti and vadj pins. whether the MAX745 is controlling voltage or current at any time depends on the battery state. if the battery is discharged, the MAX745 output reaches the current-regulation limit before the voltage limit, causing the system to regulate current. as the battery charges, the voltage rises to the point where the volt- age limit is reached and the charger switches to regu- lating voltage. the status pin indicates whether the charger is regulating current or voltage. voltage control to set the voltage limit on the battery, connect a resis- tor- divider to vadj from ref. a 0v to v ref change at vadj sets a ?% change in the battery limit voltage around 4.2v. since the 0 to 4.2v range on vadj results in only a 10% change on the voltage limit, the resistor- divider? accuracy does not need to be as high as the output voltage accuracy. using 1% resistors for the voltage dividers typically results in no more than 0.1% degradation in output voltage accuracy. vadj is inter- nally buffered so that high-value resistors can be used to set the output voltage. when the voltage at vadj is MAX745 switch-mode lithium-ion battery charger _______________________________________________________________________________________ 5 ______________________________________________________________pin description ibat current-sense amplifier? analog current-source output. see the monitoring charge current section for a detailed description. 2 dcin charger input voltage. bypass dcin with a 0.1? capacitor. 3 vl chip power supply. output of the 5.4v linear regulator from dcin. bypass vl with a 4.7? capacitor. 1 4 ccv voltage-regulation-loop compensation point 5 cci current-regulation-loop compensation point 8 vadj voltage-adjustment pin. vadj is tied to a 1% tolerance external resistor-divider to adjust the voltage set point by 10%, eliminating the need for precision 0.1% resistors. the input voltage range is 0v to v ref . 7 ref 4.2v reference voltage output. bypass ref with a 0.1? or greater capacitor. 6 thm/ shdn thermistor sense-voltage input. thm/ shdn also performs the shutdown function. if pulled low, the charger turns off. 13 status an open-drain mosfet sinks current when in current-regulation mode, and is high impedance when in volt- age-regulation mode. connect status to vl through a 1k ? to 100k ? pullup resistor. status can also drive an led for visual indication of regulation mode (see MAX745 ev kit). leave status floating if not used. 11, 12 cell1, cell0 logic inputs to select cell count. see table 1 for cell-count programming. 10 gnd analog ground 9 seti seti is externally tied to the resistor-divider between ref and gnd to set the charging current. 14 batt battery-voltage-sense input and current-sense negative input 15 cs current-sense positive input 16 pgnd power ground 17 dlo low-side power mosfet driver output 18 dhi high-side power mosfet driver output 19 lx power connection for the high-side power mosfet source 20 bst power input for the high-side power mosfet driver name function pin
MAX745 switch-mode lithium-ion battery charger 6 _______________________________________________________________________________________ v ref / 2, the voltage limit is 4.2v. table 1 defines the battery cell count. the battery limit voltage is set by the following: solving for v adj , we get: set v adj by choosing a value for r11 (typically 100k ? ), and determine r3 by: r3 = [1 - (v adj / v ref )] x r11 (figure 1) where v ref = 4.2v and cell count is 1, 2, 3, 4 (table 1). the voltage-regulation loop is compensated at the ccv pin. typically, a series-resistor-capacitor combination can be used to form a pole-zero doublet. the pole introduced rolls off the gain starting at low frequencies. the zero of the doublet provides sufficient ac gain at mid-frequencies. the output capacitor (c1) rolls off the mid-frequency gain to below unity. this guarantees sta- bility before encountering the zero introduced by the c1? equivalent series resistance (esr). the gmv amplifier? output is internally clamped to between one- fourth and three-fourths of the voltage at ref. current control the charging current is set by a combination of the cur- rent-sense resistor value and the seti pin voltage. the current-sense amplifier measures the voltage across the current-sense resistor, between cs and batt. the current-sense amplifier? gain is 6. the voltage on seti is buffered and then divided by 4. this voltage is com- pared to the current-sense amplifier? output. therefore, full-scale current is accomplished by con- necting seti to ref. the full-scale charging current (i fs) is set by the following: i fs = 185mv / r1 (figure 1) v = 9.523 v cell count 9.023v adj batt ref () ? v = cell count x v v 1 2 v 9.523 batt ref adj ref () + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? cell0 cell1 gnd gnd 1 vl gnd 2 gnd vl 3 vl vl 4 cell count ref (up to 24v) vl dcin v in bst dhi dlo lx pgnd cs batt thm 1 vadj seti cci c3 47nf r11 100k ? 1% r3 100k ? 1% r16 d2 c6 0.1 f c7 0.1 f m1a 1/2 irf7303 m1b d6 mbrs 340t3 d1 mbrs 340t3 r1 0.2 ? c1 68 f l1 22 h 1/2 irf7303 in4148 r12 r15 10k ? c4 0.1 f c5 4.7 f r2 c2, 0.1 f 10k ? gnd ibat ccv MAX745 battery thm/shdn status figure 1. standard application circuit table 1. cell-count programming table
MAX745 switch-mode lithium-ion battery charger _______________________________________________________________________________________ 7 to set currents below full scale without changing r1, adjust the voltage at seti according to the follow- ing formula: i chg = i fs (v seti / v ref ) a capacitor at cci sets the current-feedback loop? dominant pole. while the current is in regulation, ccv voltage is clamped to within 80mv of the cci voltage. this prevents the battery voltage from overshooting when the voltage setting is changed. the converse is true when the voltage is in regulation and the current setting is changed. since the linear range of cci or ccv is about 2v (1.5v to 3.5v), the 80mv clamp results in negligible overshoot when the loop switches from voltage regulation to current regulation, or vice versa. monitoring charge current the battery-charging current can be externally moni- tored by placing a scaling resistor (r ibat ) between ibat and gnd. ibat is the output of a voltage-con- trolled current source, with output current given by: where v sense is the voltage across the current-sense resistor (in millivolts) given by: v sense = v cs - v batt = i chg x r1 the voltage across r ibat is then given by: r ibat must be chosen to limit v ibat to voltages below 2v for the maximum charging current. connect ibat to gnd if unused. pwm controller the battery voltage or current is controlled by a current-mode, pwm dc/dc converter controller. this controller drives two external n-channel mosfets, which control power from the input source. the con- troller sets the switched voltages pulse width so that it supplies the desired voltage or current to the battery. total component cost is reduced by using a dual, n-channel mosfet. the heart of the pwm controller is a multi-input com- parator. this comparator sums three input signals to determine the switched signal? pulse width, setting the battery voltage or current. the three signals are the current-sense amplifier? output, the gmv or gmi error amplifier? output, and a slope-compensation signal that ensures that the current-control loop is stable. the pwm comparator compares the current-sense amplifier? output to the lower output voltage of either the gmv or gmi amplifiers (the error voltage). this cur- rent-mode feedback reduces the effect of the inductor on the output filter lc formed by the output inductor (l1) and c1 (figure 1). this makes stabilizing the cir- cuit much easier, since the output filter changes to a first-order rc from a complex, second-order rlc. v= a v ibat chg ibat irr 09 1 0 1 3 . ? i = 0.9 a mv v ibat sense batt 1 / 4 ibat dcin current sense a v = 6 on cs seti cci vadj ccv cell0 vl bst vl status ref dhi lx dlo pgnd gnd gmv gmi cell1 pwm logic 5.4v reg 4.2 ref cell logic clamp ref 2 thm/shdn figure 2. functional diagram
MAX745 switch-mode lithium-ion battery charger 8 _______________________________________________________________________________________ mosfet drivers the MAX745 drives external n-channel mosfets to switch the input source generating the battery voltage or current. since the high-side n-channel mosfet? gate must be driven to a voltage higher than the input source voltage, a charge pump is used to generate such a volt- age. the capacitor (c7) charges through d2 to approxi- mately 5v when the synchronous rectifier (m1b) turns on (figure 1). since one side of c7 is connected to lx (the source of m1a), the high-side driver (dhi) drives the gate up to the voltage at bst, which is greater than the input voltage while the high-side mosfet is on. the synchronous rectifier (m1b) behaves like a diode but has a smaller voltage drop, improving efficiency. a small dead time is added between the time when the high-side mosfet is turned off and when the synchro- nous rectifier is turned on, and vice versa. this prevents crowbar currents during switching transitions. place a schottky rectifier from lx to ground (d1, across m1b? drain and source) to prevent the synchronous rectifier? body diode from conducting during the dead time. the body diode typically has slower switching- recovery times, so allowing it to conduct degrades efficiency. d1 can be omitted if efficiency is not a concern, but the resulting increased power dissipation in the synchronous rectifier must be considered. since the bst capacitor is charged while the synchro- nous rectifier is on, the synchronous rectifier may not be replaced by a rectifier. the bst capacitor will not fully charge without the synchronous rectifier, leaving the high- side mosfet with insufficient gate drive to turn on. however, the synchronous rectifier can be replaced with a small mosfet (such as a 2n7002) to guarantee that the bst capacitor is allowed to charge. in this case, the majority of the high charging currents are carried by d1, and not by the synchronous rectifier. internal regulator and reference the MAX745 uses an internal low-dropout linear regula- tor to create a 5.4v power supply (vl), which powers its internal circuitry. the vl regulator can supply up to 25ma. since 4ma of this current powers the internal cir- cuitry, the remaining 21ma can be used for external cir- cuitry. mosfet gate-drive current comes from vl, which must be considered when drawing current for other functions. to estimate the current required to drive the mosfets, multiply the sum of the mosfet gate charges by the switching frequency (typically 300khz). bypass vl with a 4.7 f capacitor to ensure stability. the MAX745 internal 4.2v reference voltage must be bypassed with a 0.1 f or greater capacitor. minimum input voltage the input voltage to the charger circuit must be greater than the maximum battery voltage by approximately 2v so the charger can regulate the voltage properly. the input voltage can have a large ac-ripple component when operating from a wall cube. the voltage at the low point of the ripple waveform must still be approximately 2v greater than the maximum battery voltage. using components as indicated in figure 1, the minimum input voltage can be determined by the following formula: v in x [v batt + v d6 + i chg ( r ds(on) + r l + r1)] 0.89 where: v in is the input voltage; v d6 is the voltage drop across d6 (typically 0.4v to 0.5v); i chg is the charging current; r ds(on) is the high-side mosfet m1a? on-resistance; r l is the the inductor? series resistance; r1 is the current-sense resistor r1 s value. 18 17 16 15 14 13 19 20 1 2 3 4 5 6 7 8 top view 12 11 9 10 bst lx dhi dlo pgnd cs batt status cell0 cell1 ccv vl dcin ibat vadj ref thm/shdn cci gnd seti ssop MAX745 __________________pin configuration ___________________chip information transistor count: 1695 substrate connected to gnd
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