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Max745dsEVALUATION KIT MANUAL
FOLLOWS DATA SHEET
The MAX745 provides all functions necessary for ♦ Charges 1 to 4 Lithium-Ion Battery Cells
charging lithium-ion battery packs. It provides a regu- ♦ ±0.75% Voltage-Regulation Accuracy
lated charging current of up to 4A without getting hot, Using 1% Resistors
and a regulated voltage with only ±0.75% total error at ♦ Provides up to 4A without Excessive Heating
the battery terminals. It uses low-cost, 1% resistors toset the output voltage, and a low-cost N-channel MOS- ♦ 90% Efficient
♦ Uses Low-Cost Set Resistors and
The MAX745 regulates the voltage set point and charg- N-Channel Switch
ing current using two loops that work together to transi- ♦ Up to 24V Input
tion smoothly between voltage and current regulation.
♦ Up to 18V Maximum Battery Voltage
The per-cell battery voltage regulation limit is set ♦ 300kHz PWM Operation: Low-Noise,
between 4.0V and 4.4V using standard 1% resistors, Small Components
and then the number of cells is set from 1 to 4 by pin- strapping. Total output voltage error is less than ±0.75%.
Stand-Alone Operation; No Microcontroller
For a similar device with an SMBus™ microcontrollerinterface and the ability to charge NiCd and NiMH cells, ________________________Applications
refer to the MAX1647 and MAX1648. For a low-costlithium-ion charger using a linear-regulator control Ordering Information
Pin Configuration appears on last page.
___________________________________________________Typical Operating Circuit
________________________________________________________________ Maxim Integrated Products
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
ABSOLUTE MAXIMUM RATINGS
DCIN to GND .-0.3V to 26V
Continuous Power Dissipation (TA = +70°C) SSOP (derate 8.00mW/°C above +70°C) .640mW Operating Temperature Range .-40°C to +85°C MAX745 CELL0, CELL1, IBAT, STATUS, CCI, CCV,
REF, SETI, VADJ, DLO, THM/SHDN to GND .-0.3V to (VL + 0.3V) Lead Temperature (soldering, 10sec) .+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.
(VDCIN = 18V, VBATT = 8.4V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
SUPPLY AND REFERENCE
6.0V < VDCIN < 24V, logic inputs = VL SWITCHING REGULATOR
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = 18V, VBATT = 8.4V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
(VDCIN = 18V, VBATT = 8.4V, TA = -40°C to +85°C, unless otherwise noted. Limits over temperature are guaranteed by design.)
SUPPLY AND REFERENCE
SWITCHING REGULATOR (Note 1)
Note 1: When VSETI = 0V, the battery charger turns off.
__________________________________________Typical Operating Characteristics
(TA = +25°C, VDCIN = 18V, VBATT = 4.2V, CELL0 = CELL1 = GND, CVL = 4.7µF, CREF = 0.1µF. Circuit of Figure 1, unlessotherwise noted.) BATTERY VOLTAGE
vs. CHARGING CURRENT
vs. SETI VOLTAGE
vs. VADJ VOLTAGE
VL LOAD REGULATION
REFERENCE LOAD REGULATION
Current-Sense Amplifier’s Analog Current-Source Output. See Monitoring Charge Current section for Charger Input Voltage. Bypass DCIN with a 0.1µF capacitor.
Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL with a 4.7µF capacitor.
Voltage-Regulation-Loop Compensation Point Current-Regulation-Loop Compensation Point Thermistor Sense-Voltage Input. THM/SHDN also performs the shutdown function. If pulled low, 4.2V Reference Voltage Output. Bypass REF with a 0.1µF or greater capacitor. 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 VREF. SETI is externally tied to the resistor-divider between REF and GND to set the charging current. Logic Inputs to Select Cell Count. See Table 1 for cell-count programming.
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 1kΩ to 100kΩ pull-up resistor. STATUS may also drivean LED for visual indication of regulation mode (see MAX745 evaluation kit). Leave STATUS floating if not used.
Battery-Voltage-Sense Input and Current-Sense Negative Input Power Connection for the High-Side Power MOSFET Source Power Input for the High-Side Power MOSFET Driver _______________Detailed Description
current-regulation limit before the voltage limit, causingthe system to regulate current. As the battery charges, The MAX745 is a switch-mode, lithium-ion battery the voltage rises to the point where the voltage limit is charger that can achieve 90% efficiency. The charge reached and the charger switches to regulating volt- voltage and current are set independently by external age. The STATUS pin indicates whether the charger is resistor-dividers at SETI and VADJ, and at pin connec- tions at CELL0 and CELL1. VADJ is connected to aresistor-divider to set the charging voltage. The output Voltage Control
voltage-adjustment range is ±5%, eliminating the need To set the voltage limit on the battery, tie a resistor- for 0.1% resistors while still achieving 0.75% set accu- divider to VADJ from REF. A 0V to VREF change at VADJ sets a ±5% change in the battery limit voltage The MAX745 consists of a current-mode, pulse-width- around 4.2V. Since the 0 to 4.2V range on VADJ results modulated (PWM) controller and two transconductance in only a 10% change on the voltage limit, the resistor- error amplifiers: one for regulating current (GMI) and divider’s accuracy does not need to be as high as the the other for regulating voltage (GMV) (Figure 2). The output voltage accuracy. Using 1% resistors for the error amplifiers are controlled via the SETI and VADJ voltage dividers typically results in no more than 0.1% pins. Whether the MAX745 is controlling voltage or cur- degradation in output voltage accuracy. VADJ is inter- rent at any time depends on the battery state. If the bat- nally buffered so that high-value resistors can be used tery is discharged, the MAX745 output reaches the to set the output voltage. When the voltage at VADJ is _______________________________________________________________________________________
VREF / 2, the voltage limit is 4.2V. Table 1 defines the where VREF = 4.2V and cell count is 1, 2, 3, or 4 The battery limit voltage is set by the following: The voltage-regulation loop is compensated at the CCVpin. 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’s equivalent series resistance (ESR). The GMV amplifier’s output is internally clamped to between one- fourth and three-fourths of the voltage at REF.
Set VADJ by choosing a value for R11 (typically 100k), 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 acrossthe current-sense resistor, between CS and BATT. Thecurrent-sense amplifier’s gain is 6. The voltage on SETI Table 1. Cell-Count Programming Table
is buffered and then divided by 4. This voltage is com- CELL COUNT
pared to the current-sense amplifier’s output.
Therefore, full-scale current is accomplished by con-necting SETI to REF. The full-scale charging current _______________________________________________________________________________________
To set currents below full scale without changing RIBAT must be chosen to limit VIBAT to voltages below R1, adjust the voltage at SETI according to the follow- 2V for the maximum charging current. Connect IBAT to PWM Controller
A capacitor at CCI sets the current-feedback loop’s The battery voltage or current is controlled by a dominant pole. While the current is in regulation, CCV current-mode, PWM DC/DC converter controller. This voltage is clamped to within 80mV of the CCI voltage.
controller drives two external N-channel MOSFETs, This prevents the battery voltage from overshooting which control power from the input source. The con- when the voltage setting is changed. The converse is troller sets the switched voltage’s pulse width so that it true when the voltage is in regulation and the current supplies the desired voltage or current to the battery.
setting is changed. Since the linear range of CCI or Total component cost is reduced by using a dual, CCV is about 2V (1.5V to 3.5V), the 80mV clamp results in negligible overshoot when the loop switches from The heart of the PWM controller is a multi-input com- voltage regulation to current regulation, or vice versa.
parator. This comparator sums three input signals to Monitoring Charge Current
determine the switched signal’s pulse width, setting the The battery-charging current can be externally moni- battery voltage or current. The three signals are the current-sense amplifier’s output, the GMV or GMI error IBAT and GND. IBAT is the output of a voltage-con- amplifier’s output, and a slope-compensation signal trolled current source, with output current given by: that ensures that the current-control loop is stable.
The PWM comparator compares the current-sense amplifier’s output to the lower output voltage of either where VSENSE is the voltage across the current-sense 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.
Minimum Input Voltage
The MAX745 drives external N-channel MOSFETs to The input voltage to the charger circuit must be greater switch the input source generating the battery voltage or than the maximum battery voltage by approximately 2V current. Since the high-side N-channel MOSFET’s gate so the charger can regulate the voltage properly. The must be driven to a voltage higher than the input source input voltage can have a large AC-ripple component voltage, a charge pump is used to generate such a volt- when operating from a wall cube. The voltage at the low age. The capacitor (C7) charges through D2 to approxi- point of the ripple waveform must still be approximately MAX745 mately 5V when the synchronous rectifier (M1B) turns on
2V greater than the maximum battery voltage.
(Figure 1). Since one side of C7 is connected to LX (the Using components as indicated in Figure 1, the minimum source of M1A), the high-side driver (DHI) drives the gate input voltage can be determined by the following formula: up to the voltage at BST, which is greater than the inputvoltage 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’s drain and source) to prevent the synchronous rectifier’s body diode from conducting during the dead RL is the the inductor’s series resistance; time. The body diode typically has slower switching-recovery times, so allowing it to conduct degrades R1 is the current-sense resistor R1’s value.
efficiency. D1 can be omitted if efficiency is not a concern, but the resulting increased power dissipation __________________Pin Configuration
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 bereplaced 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 witha 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 forother functions. To estimate the current required to drivethe MOSFETs, multiply the sum of the MOSFET gate ___________________Chip Information
charges by the switching frequency (typically 300kHz).
Bypass VL with a 4.7µF capacitor to ensure stability.
The MAX745 internal 4.2V reference voltage must bebypassed with a 0.1µF or greater capacitor.
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