DN06044/D Design Note – DN06044/D Switching Charger for Rechargeable NiCd and NiMH Batteries Device Application Input Voltage Current Topology I/O Isolation NCP3066 MC33340(2) Nixx Battery Charger 1-8 Cells (5-25 V) >1.2A BUCK NONE Circuit Description Key Features This circuit is intended for charging NiCd and NiMH batteries with 1-8 cells in series. Power circuit is based on the NCP3066 in the constant current mode oriented buck converter. It operates around 135 kHz switching frequency in continuous conduction mode. Charging is controlled by the MC33340(2) that is searching negative voltage slope to detect a fully charged battery. The MC33340(2) has battery undervoltage and overvoltage protection, charge time or temperature protection. y Switching operation y Wide input and output operation voltage y Regulated output current y High frequency operation y Minimal output current ripple y Reverse battery protection y Output short circuit protection y Time or temperature protection y Small components size y Single layer PCB June 2008, Rev.0 www.onsemi.com 1 DN06044/D Schematic *) These components should be changed for cell count other than 2 cells (see Table 1). Figure 1 – Nixx battery charger schematic Design Notes The charger is separated into a power and a logic part. The power part is controlled by the NCP3066 and the logic part by the MC33340(2). The power part is used to control constant charging current, the logic part is used to control charging circuit and manage protection. There is also a trickle charging circuit (Q1, Q2, R1, R3 and D1) that charges batteries with a small constant current to keep the battery in a charged state after fast charging is completed. Power part Output current is set by resistors R10 and R11. Thanks to the low feedback voltage there is also a small power loss. Output current and power loss can be calculated using equations (1) and (2). I OUT = VFB ⋅ (R10 + R11) 0.235 ⋅ (1 + 1) = = 0.47 A R10 ⋅ R11 1⋅1 PR10 R11 = VFB ⋅ I OUT = 0.235 ⋅ 0.47 = 110mW (1) (2) Inductor is selected according to equation (3). ⎛ VOUT ⎞⎛ ⎞ V 6 6 ⎞ ⎞⎛ ⎟⎟⎜⎜1 − OUT ⎟⎟ = ⎛⎜ Lmin = ⎜⎜ ⎟⎜1 − ⎟ = 95.8μH ⇒ 100 μH 3 ⎝ f ⋅ ΔI L max ⎠⎝ VIN max ⎠ ⎝ 135 ⋅ 10 ⋅ 0.75 ⋅ 0.47 ⎠⎝ 25 ⎠ (3) Operation frequency is set by capacitor C3 and pulse feedback resistor R8 that is used to stabilize convertor operation. Its value is around 135 kHz. Pulse feedback resistor value is experimentally founded for different battery cell count. Peak current is limited by R2, R4, R6 and R7 to 740 mA. June 2008, Rev.0 www.onsemi.com 2 DN06044/D Battery reverse polarity protection is composed by Q3, D3, D4, D6, C5, C6, R9 and R19. A charge pump is used to get enough gate voltage to open the protection P-MOSFET if the NCP3066 is switching. When the battery is connected wrong then the NCP3066 is blocked by the MC33340(2), the charge pump does not allow voltage to open the P-MOSFET and the battery is disconnected. Zener diode D6 protects Q3 gate against high VGS. If this protection is not needed then all protection components should be not used and R5 (0 Ω) will cross MOSFET. But if battery is connected with reverse polarity then short current will flow through R10, R11, D2, L1 and R5. Trickle charging current is set by resistor R1. Charging current value can be calculated by equation (4) ITRICKLE = VBEQ 2 R1 = 0 .6 = 18mA 33 (4) Control part Charging is controlled by the MC33340 or the MC33342. The difference is fast charge hold off time only. For the MC33340 it is 177 s and for the MC33342 it is 708 s. Power supply voltage for the control IC is limited by the shunt regulator (D5 and R12). This IC controls power by utilizing the NCP3066’s ENABLE pin which is used to switch on and off fast charging. The MC33340(2) has a voltage sensor input with under and overvoltage protection. This protection is set by the resistor divider R13 and R15 so that the voltage on the VSEN pin will be between 1 – 2 V. VSEN min = CellCount ⋅ 0.9 ⋅ R15 R13 + R15 (5) VSEN max = CellCount ⋅ 1.5 ⋅ R15 R13 + R15 (6) Suitable resistor divider, pulse feedback resistor value and minimum input voltage are in Table 1. Table 1. Components depended on cells count Cells count 1 2 3 4 5 6 7 8 R13 [kΩ] 3.9 3.9 6.8 12 15 18 22 27 R15 [kΩ] NU 4.7 4.7 4.7 4.7 4.7 4.8 4.9 C3 [nF] 4.7 4.7 4.7 4.7 5.6 8.2 8.2 8.2 R8 [kΩ] 22 22 22 22 27 22 27 33 VINmin [V] 4.5 6.5 8.5 11.5 13 16 18 19.5 Resistors R16, R17 and R18 set the charge time limit or under/over temperature limit. Time setting is made by resistors for R16 – R18 (0 Ω to disable). Temperature sensor (NTC) can be connected to JP1 and by R16 a R17 can be set under and over temperature limit. Time setting is in Table 2 for temperature setting see MC33340(2) datasheet. June 2008, Rev.0 www.onsemi.com 3 DN06044/D Table 2. Temperature and time limit configuration Function monitoring Charger indicates state of charging by one LED diode. There are two states, permanent lighting that indicates end of fast charging and all of errors like under/over voltage, overtime and under/over temperature. Second state is indicated by short LED blink with around 1 s period that indicates fast charging. There is possible to use SMT or thru hole LED on the PCB. Higher charging current Higher charging current is limited by the maximum peak current of the NCP3066 internal switch, but if an external switch is used the current can be higher. Conclusion This circuit is ideal to use for low cost high efficiency charging circuits for example for cameras, portable players and other devices power by NiCd or NiMH batteries. Its feature is small size, low power dissipation (no heatsink is needed) and single layer PCB. All components are the same size and are a standard value (in some cases there are additional footprints for one component to achieve required value). June 2008, Rev.0 www.onsemi.com 4 DN06044/D PC Board Figure 2 – components position on PCB Figure 3 – PCB’s top side (58 x 36 mm) June 2008, Rev.0 www.onsemi.com 5 DN06044/D Measurements Light blue – SWE pin Dark blue – Output voltage Pink – LED cathode Green – Output current Figure 4 – VIN = 12 V, 2x NiMH AA cell Figure 5 – VIN = 12 V, 2x NiMH AA cell, charging switch off for measure June 2008, Rev.0 www.onsemi.com 6 DN06044/D Figure 6 – VIN = 12 V, 2x NiMH AA cell, charging switch on after measure 80 70 Efficiency [%] 60 50 40 30 20 5 7 9 11 13 15 17 19 21 23 25 VIN [V] 1 cell 2 cells 3 cells 4 cells 5 cells 6 cell 7 cells 8 cells Figure 7 – Charger efficiency 1 © 2008 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Tomáš Tichý, e-mail:[email protected] June 2008, Rev.0 www.onsemi.com 7