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| Order this document by AN1593/D AN1593 Low Cost 1.0 A Current Source For Battery Chargers Prepared by: Ondrej Pauk Industrial System Application Laboratory Roznov, CZ Figure 1. Low Cost Current Source for Battery Chargers Demonstration Board This paper describes two designs of low cost current sources for battery charger applications based on the LM2575-ADJ switching step-down converter and the MC33341 regulator control circuit. General Description Today's most popular rechargeable battery type is NiCd. When overcharged, this type of battery experiences increasing pressure inside the cell. This can cause opening of the cell's vent and release of oxygen. This has a detrimental affect on the battery, although it may still retain some useful capacity. When NiMH batteries are overcharged, they also increase their internal pressure and release some hydrogen, an extremely explosive gas. The schematic diagram is shown in Figure 2. It is a 1.0 A (maximum) "dumb" battery charger that uses the LM2575-ADJ switching converter to step down the input dc voltage, together with the MC33341, which regulates the charging current flowing into the battery. The switching regulator has high efficiency over a wide input voltage range, which allows this design to be universal. Both 12 and 24 V car batteries as well as cheap, poorly regulated, wall adaptors can be used. The term "dumb" battery charger means that it offers only some basic protective features and the main protective and control functions are maintained by a P based main control unit inside the PC or the control function of a cellular phone. This concept allows a very compact and cost effective design. Various charge techniques have to be used to accommodate both NiCd and NiMH type batteries. Both NiCd and NiMH batteries can be charged at a high current rate INTRODUCTION This design is a highly cost effective 1.0 A current source for battery chargers with a rectangular constant-current, constant-voltage charging characteristic. This feature assures a basic protection against overcharge whose results can range from minor damage to catastrophic failure of the whole system. This circuit was designed to implement additional charge control based either on the microcontroller or on any other charging control unit in the system that operates from NiCd or NiMH batteries. The MC33341 and this board may be used in a wide variety of applications. All functions needed are performed by just two integrated circuits and a small number of external components. This allows a very compact printed circuit board design and a very cost effective solution. The LM2575-ADJ Easy SwitchTM step down converter allows the system to operate from 8.0 to 40 Vdc, thus allowing direct operation from both 12 and 24 V board voltages used in the automotive industry. In comparison with linear topologies of battery chargers, this circuit provides much better efficiency, especially over a wide input voltage range. (c) Motorola, Inc. 1997 AN1593 (`c' rate) up until the charge limit is reached. After that, the battery has to be charged by a much lower current at the so called `trickle charge' rate. Trickle charging is a continuous low current charging rate that keeps the battery fully charged. While NiCd batteries have a recommended trickle charge rate of about c/10, for NiMH type it is not recommended to exceed a charging rate of c/40. Some battery manufacturers recommend, for their chemistry, pulse charging instead of continuous current charging. This feature can be accomplished by use of the ON/OFF pin of the LM2575. In the circuit shown in Figure 2, the MC33341 control circuit is configured for high-side current sensing. The voltage drop across the sense resistor RS provides a voltage that is proportional to the charging current. The current regulation threshold Vsen can be adjusted externally (switch S1 in position "2") in the range of 0 V to 200 mV with respect to Pin 4 of U2. When the switch S1 is in position "1", the current regulation threshold level is set internally to 200 mV. Then the regulated current can be calculated as follows: I reg sen 0.2 + VRS + RS Circuit Operation Circuit operation is as follows. When a discharged battery is connected to the charger, the circuit operates as a constant current source. The LM2575-ADJ buck regulator is used to step down an unregulated dc input voltage. This regulator is capable of providing up to 1.0 A of charging current.The amount of charging current flowing into the battery is controlled by the MC33341 regulation control circuit. This IC is used to control the feedback loop in either constant-current or constant-voltage mode with automatic crossover. The MC33341 features the unique ability to perform both high-side and low-side current sensing, each with either internally fixed or externally adjustable threshold level. This feature makes this circuit very universal and ideally suited for use in connection with a microcontroller based intelligent control systems. Resistor R3 is required in those applications where a high peak level of reverse current is possible, if the source outputs are shorted and the diode D2 is not used. The resistor value should be chosen to limit the input current of the internal VCC clamp diode to less than 20 mA. Excessively large values for R3 will degrade the current sensing accuracy. Resistor R3 value can be calculated from the following expression: R3 + pkRS 0.02 I - 0.6 where IpkRS is a peak current flowing through the sense resistor RS. Once the battery voltage reaches a predetermined level, the MC33341 begins to regulate in the constant-voltage mode and the charger starts to regulate the voltage across the battery. This voltage is monitored by Pin 5 of U2, the Figure 2. Low Cost Switching Regulator Performs Constant-Current/Constant-Voltage 3 Cell Charging Function ON/OFF + +Vin 1 Unregulated DC - Input Vin = 10 to 40 V U1 LM2575-ADJ 5 Output 2 C1 + 100 mF/50 V 4 Feedback R4 1.0 kW 3 Gnd D1 1N5819 L1 400 mH + C2 330 mF/16 V ON/OFF Control (from Controller) Gnd RS 0.22 W A D2 +VO 1N4001 C3 33 nF 8 7 R3 27 W 6 R2 39 W + - 3 Battery Cells Under Charge 1 U2 MC33341 5 R1 10 kW -VO 2 3 4 1 2 S1 Charge Current Control (from Controller) 2 MOTOROLA ANALOG IC APPLICATIONS INFORMATION AN1593 non-inverting input of the transconductance amplifier inside the MC33341. This voltage is divided by resistor divider R1, R2 to the 1.2 V internally fixed level Vth. By this arrangement the battery charger output voltage threshold can be set. Moreover, in the low-side current sensing configuration (refer to Figure 3) this threshold level can be externally adjusted over a range of 0 to 1.2 V with respect to the U2 ground at Pin 4. The maximum battery charger output voltage Vreg (the voltage at the point A with respect to Pin 4 of U2) can be calculated as follows: V reg of a single 1.2 V cell may not be possible (depending on the voltage drop on D2, if used). Also the current limit in the case of fully discharged cells or shorted output is given only by the internal current limiting of the LM2575, as mentioned above. This drawback can be solved by using the circuit shown in Figure 3. Improved Battery Charger This circuit is very similar to the previous one. It uses the same source IC, the LM2575 and also the same charger control IC, the MC33341 but now in the low-side current sensing configuration. The second difference is a different connection of the VCC pin of U2 which is the supply voltage pin of the MC33341. Now this VCC pin is connected directly to the unregulated dc input voltage through the supply current limiting resistor R5 and resistor R6. The use of the coupling capacitor C4 is essential to assure a stable operation of the whole system. C4 transfers the ac part of the LM2575 output voltage (or the LM2575 output ripple voltage) through the MC33341 VCC Pin 7 and output Pin 8 into the feedback Pin 4 of the LM2575. The way the LM2575 operates makes this connection necessary. Since the maximum allowable supply voltage of the MC33341 is 18 V, the Zener diode D3 has to be used to clamp the supply voltage of the MC33341to its operating limit when the input voltage exceeds that value. Use of such an arrangement assures that the charging control circuit U2 will always have a supply voltage high enough, even under short circuit conditions at the output of the battery charger. Switch S1 can be used the same way as in the previous design. Switch S2 is used to alter the output voltage threshold. When S2 is in position "1", the voltage threshold on Pin 5 is set internally to 1.2 V and consequently the output voltage threshold can be set only by the resistor divider R1, R2. Switching S2 to position "2" allows an external control of the Pin 5 voltage threshold Vth in the range of 0 V to 1.2 V. This feature contributes to the universality of this battery charger. + Vth R2 R1 ) 1 + 1.2 R2 R1 )1 The current control loop is closed by connecting Pin 8 of U2 directly to the feedback input of the LM2575 (Pin 4 of U1). Under normal working conditions this pin is held at 1.23 V, resistor R4 is added to convert the MC33341 output current to this voltage. The diode D2 protects the batteries against discharge through U2 when the power source U1 is switched off. For the 1N4001 diode, used in this design, the typical forward voltage drop is 0.9 V. This value must be added to the voltage of three fully charged battery cells in series when the output voltage threshold level is chosen. Capacitor C3 is used for frequency compensation of an internal transconductance amplifier. The circuit shown in Figure 2 provides high efficiency battery charging with protection against short circuit accomplished by the LM2575 internal current limiting. Because it is possible to set the output voltage threshold by a simple resistor divider, various types of battery cells, as well as various number of cells, can be charged. Nonetheless, this circuit has some limitations. In the high-side current sensing configuration, shown in Figure 2, the VCC Pin 7 of the MC33341 is connected to the output of the power supply circuit. Such a configuration offers the advantage of a common return path for both ICs, the LM2575-ADJ and the MC33341, but it has also a drawback. The low limit of the MC33341 supply voltage is 1.9 V. That implies that charging MOTOROLA ANALOG IC APPLICATIONS INFORMATION 3 AN1593 Figure 3. Low Cost Switching Regulator Performs Constant-Current/Constant-Voltage 3 Cell Charging Function. Version with Full Short-Circuit Regulation Control Capability. ON/OFF + +Vin 1 Unregulated DC - Input Vin = 10 to 40 V C1 + 100 mF/50 V U1 LM2575-ADJ 5 Output 2 4 Feedback R4 1.0 kW C4 680 nF Threshold Voltage Control (from Controller) +VO 1N4001 R2 39 W 3 Gnd D1 1N5819 L1 400 mH + C2 330 mF/16 V ON/OFF Control (from Controller) Gnd D3 1N4745A R5 560 W R6 100 W D2 A 1 8 7 2 S2 R3 27 W C3 33 nF 6 + - 3 Battery Cells Under Charge 1 U2 MC33341 5 R1 10 kW -VO 2 3 4 1 2 S1 RS 0.22 W Charge Current Control (from Controller) Figure 4. Constant-Voltage/Constant-Current Charging Characteristic of the Improved Current Source for Battery Chargers 6 VO, OUTPUT VOLTAGE (V) 5 4 3 2 1 0 0 0.4 0.2 0.6 0.8 1.0 IO, OUTPUT CURRENT (A) Figure 4 shows the V/I charging characteristic of the improved circuit (see schematic in Figure 3). The small voltage drop in the beginning of the V/I characteristic is caused by dynamic resistance of the diode D2. This phenomenon can be eliminated either by reconnection of the resistor R2 from the point "A" to the cathode of the diode D2, or this diode might be replaced by shorting wire. Both actions would cause also another effect on the V/I characteristic of this circuit. When the output voltage drops below approximately 1.2 V, the output current will fall down accordingly, thus creating a typical "foldback" V/I characteristic. 4 MOTOROLA ANALOG IC APPLICATIONS INFORMATION A AAAAAAAAAAAAAAAAAAAAAAAAAA A A A AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A A AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA A A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AA A A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA A A AA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA A A A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA A A AA A A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAA A A A AA A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAA NOTES: 1. Inductor L1: Inductance Pins 4, 6. Manufacturer: TECH 39 Power Electronic, Tel. +33/1-4115-1681, Fax +33/1-4409-5051. 2. All tolerances 10%, unless otherwise specified. MOTOROLA ANALOG IC APPLICATIONS INFORMATION Figure 5. PCB Layout Component Side Performance of the Sample Design (Refer to the Circuit Shown in Figure 2) Gnd +Vin + C1 Input voltage range . . . . . . . . . . . Battery charging current . . . . . . . Open output voltage . . . . . . . . . . Power converter efficiency . . . . . C3 Component L1 RS U2 U1 D2 D1 C3 C2 C1 R4 R3 R2 R1 S1 L1 U1 R4 Table 1. Parts List (Refer to the Circuit Shown in Figure 2) ON/OFF SW1 U2 + D1 C2 Current Control Quantity 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -VO +VO 8.0 to 40 Vdc 0.88 A 5.45 V 77%, Vin = 24 V 400 H, 1.6 A 0.22 , 1/2 W 1.0 k, 1/4 W 39 k, 1/4 W 10 k, 1/4 W 330 F, 16 V 100 F, 50 V 1.0 A, 100 V Value/Rating 27 , 1/4 W 1.0 A, 40 V 33 nF - - - Figure 6. PCB Layout Copper Side Schottky Diode, 1N5819 Capacitor Electrolytic Capacitor Electrolytic Inductor, 77 458 BV Capacitor Ceramic IC, LM2575-ADJ Diode, 1N4001 IC, MC33341 Description Resistor Resistor Resistor Resistor Resistor Switch AN1593 5 R2 R3 D2 RS R1 AAAAAAAAAAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAA A A A A A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA A A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA A A AA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA A A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAA A A AA A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAA A A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AA AAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAA AAAAAAAAA A A AA A A AAAAAAAAAAAAAAAAAAAAAAAAAA AA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAA A A AA A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAA AA AAAAAAAAA A AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AA AAAAAAAAAAAAAAAAAAAAAAAAAA A AA AN1593 NOTES: 1. Inductor L1: Inductance Pins 4, 6. Manufacturer: TECH 39 Power Electronic, Tel. +33/1-4115-1681, Fax +33/1-4409-5051 2. All tolerances 10%, unless otherwise specified. 6 +Vin Performance of the Sample Design (Refer to the Circuit Shown in Figure 3) MOTOROLA ISAL Roznov Gnd Figure 7. PCB Layout Component Side Input voltage range . . . . . . . . . . . Battery charging current . . . . . . . Open output voltage . . . . . . . . . . Power converter efficiency . . . . . C1 + Component RS U2 U1 D3 D2 D1 C4 C3 C2 C1 R6 R5 R4 R3 R2 R1 S2 S1 L1 Table 2. Parts List (Refer to the Circuit Shown in Figure 3) R4 U1 ON/OFF C4 C2 + Quantity U2 C3 R1 L1 S1 Current Control Voltage Control 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 D2 R2 8.0 to 40 Vdc 0.88 A 5.45 V 77%, Vin = 24 V S2 +VO -VO 400 H, 1.6 A 0.22 , 1/2 W 1.0 k, 1/4 W 100 , 1/4 W 560 , 1.0 W 39 k, 1/4 W 10 k, 1/4 W 330 F, 16 V 100 F, 50 V 1.0 A, 100 V Value/Rating 27 , 1/4 W 16 V, 1.0 W 1.0 A, 40 V 680 nF 33 nF MOTOROLA ANALOG IC APPLICATIONS INFORMATION R6 R5 RS R3 D1 D3 - - - - Figure 8. PCB Layout Copper Side Schottky Diode, 1N5819 Zener Diode, 1N4745A Capacitor Electrolytic Capacitor Electrolytic Inductor, 77 458 BV Capacitor Ceramic Capacitor Ceramic IC, LM2575-ADJ Diode, 1N4001 Description IC, MC33341 Resistor Resistor Resistor Resistor Resistor Resistor Resistor Switch Switch AN1593 Post-Design Recommendation In some cases, there is a need to compensate for a small voltage drop on the constant-voltage part of the V/I characteristic of this current source or to compensate a voltage drop across wires leading from the output of the current source to the charged battery. In that case, it is possible to add a resistor between Pin 5 of the MC33341 and the ground of the circuit shown in Figure 3. Conclusion The LM2575-ADJ step-down converter and the MC33341 regulator control circuit used in the battery charger applications offer a solution which is simple, highly efficient and cost effective. The two current sources for battery chargers described in this paper can operate directly from both 12 and 24 V board voltages with no change of the electrical connection. This fact makes these applications especially suitable for the automotive industry. REFERENCES 1. LM2575 Data Sheet and Application Note; order from Motorola by LM2575/D. 2. MC33341 Data Sheet and Application Note; order from Motorola by MC33341/D. MOTOROLA ANALOG IC APPLICATIONS INFORMATION 7 AN1593 Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. "Typical" parameters which may be provided in Motorola data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be validated for each customer application by customer's technical experts. Motorola does not convey any license under its patent rights nor the rights of others. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury or death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorola and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that Motorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal Opportunity/Affirmative Action Employer. Mfax is a trademark of Motorola, Inc. How to reach us: USA / EUROPE / Locations Not Listed: Motorola Literature Distribution; P.O. Box 5405, Denver, Colorado 80217. 1-303-675-2140 or 1-800-441-2447 Customer Focus Center: 1-800-521-6274 MfaxTM: RMFAX0@email.sps.mot.com - TOUCHTONE 1-602-244-6609 ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park, Motorola Fax Back System - US & Canada ONLY 1-800-774-1848 51 Ting Kok Road, Tai Po, N.T., Hong Kong. 852-26629298 - http://sps.motorola.com/mfax/ HOME PAGE: http://motorola.com/sps/ JAPAN: Nippon Motorola Ltd.: SPD, Strategic Planning Office, 4-32-1, Nishi-Gotanda, Shinagawa-ku, Tokyo 141, Japan. 81-3-5487-8488 8 MOTOROLA ANALOG IC APPLICATIONS INFORMATION AN1593/D |
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