DESIGN FEATURES L Integrated High Voltage Switching Charger/PowerPath Controller Minimizes Power Dissipation and Fits into 2cm2 by Tage Bjorklund Introduction GPS navigators, PDAs, MP3 players and other handheld devices draw on an increasing array of power sources for recharging their batteries. These sources include USB (4.5V), low voltage AC adaptors (4.5V–5V), high voltage AC adaptors (12V–24V), Firewire and automotive batteries. USB has the advantage of convenience while the high voltage sources offer faster charging at home and in the car. One issue with high voltage sources is that the voltage difference between the high voltage source and the battery is large enough that a linear charger cannot handle the power dissipation, thus dictating the need for a switching regulator. The LTC4089 and LTC4089-5 integrate a high voltage, wide input range (6V–36V) monolithic 1.2A buck switching regulator, a USB input, a PowerPath controller and a linear charger into a compact thermally enhanced 3mm × 6mm DFN package. The LTC4089’s buck regulator output voltage follows the battery voltage, thus minimizing the overall power dissipa- 0.1µF HIGH (6V-36V) VOLTAGE INPUT HVIN 1µF Linear Technology Magazine • September 2006 10µF HVEN HVOUT 5V (NOM) FROM USB CABLE VBUS IN HVPR LTC4089 4.7µF 1k 4.7µF OUT TO LDOs REGS, ETC. BAT TIMER CLPROG GND PROG 0.1µF 2k 100k + Li-Ion BATTERY VOUT (TYP) VBAT +0.3V 5V 5V VBAT AVAILABLE INPUT HV INPUT (LTC4089) HV INPUT (LTC4089-5) USB ONLY BAT ONLY Figure 1. Typical application of the LTC4089 tion, while the LTC4089-5 has a fixed 5V output. The USB is current limited power, so the LTC4089’s PowerPath controller distributes the available power, with the load taking precedence and any remaining current used to charge the Li-ion battery. If the load current exceeds available current from USB, the additional current needed is drawn from the battery. If a high voltage source is connected to the input of the buck regulator (HVIN), the current is drawn from this source instead of the USB. Figure 1 shows a complete solution that fits into less than 2cm2 with all components on one side of the PCB (Figure 2). Operation Figure 2. A complete LTC4089/LTC4089-5 USB/high voltage/Li-ion charger application fits into 2cm2 10µH SW BOOST As shown in the simplified block diagram (Figure 3), both the LTC4089 and LTC4089-5 consist of an integrated high voltage monolithic buck regulator, a PowerPath controller and a Li-ion battery charger. They are designed to manage power from a high voltage source (e.g., FireWire/IEEE1394, 12V–24V automotive batteries, 12V– 20V wall adaptors, etc.), a low voltage source (e.g., USB or 5V wall adaptor) and a single-cell Li-ion battery. When an external power source is connected to the supply pins, it delivers power to the OUT pin and charges a battery connected to the BAT pin. When high voltage is present at the HVIN pin, the monolithic high voltage switching regulator regulates the HVOUT voltage. An external PFET between HVOUT (connected to the drain) and OUT (connected to the source) is controlled by the HVPR pin, allowing OUT to supply the power to the load and charge the battery. The LTC4089 maintains about 300mV between the OUT pin and the BAT pin, while the LTC4089-5 provides a fixed 5V OUT voltage. The HVIN input takes priority over the IN input (i.e., if both HVIN and IN are connected to power sources, load current and charge current are provided by the HVIN input). Power supplies with limited current capability (such as USB) are connected 19 L DESIGN FEATURES HVIN SW Q1 ies, Firewire, and other high voltage sources—no extra conversion to a lower voltage is needed. L1 D1 HIGH VOLTAGE BUCK REGULATOR HVOUT C1 + 4.25V (RISING) 3.15V (FALLING) – HVPR 19 + – IN + – LOAD 75mV (RISING) 25mV (FALLING) OUT 21 USB CURRENT LIMIT + – 25mV CC/CV REGULATOR CHARGER + – ENABLE OUT 25mV IDEAL DIODE + EDA 21 GATE – BAT 21 4089 F01 BAT + LI-ION Figure 3. A block diagram of the LTC4089 and LTC4089-5 shows the PowerPath controller, wide-input-range buck regulator and battery charging features. to the IN pin, which has a programmable current limit via a resistor connected at CLPROG pin. Battery charge current is adjusted to ensure the sum of the load current (which takes priority) and the charge current does not exceed the programmed input current. The high voltage buck regulator operates at 750kHz in constant frequency current mode, allowing the use of a small 10µH–33µH inductor while providing 1.2A nominal output current and minimizing the number of the external compensation components. Features High Voltage Switching Converter Saves an Adaptor The LTC4089 and LTC4089-5’s input voltage range is 6V to 36V, well within the range of automotive batter- Adaptive Buck Output Voltage Minimizes Total Power Loss The LTC4089’s buck converter output voltage, VOUT, regulates to 0.3V above the battery voltage so that the battery can be charged efficiently with the linear charger. Figure 4 shows the overall efficiency at various input voltages. The minimum VOUT is 3.6V to ensure the system can operate even if the battery is excessively discharged. USB PowerPath Controller/Charger Maximizes Power Available to the System and Solves Other Problems In a traditional battery powered device, the input charges the battery and the system’s power is directly taken from the battery. This simple topology presents some significant problems: qCase 1. The load current is restricted to the trickle charge current. If the battery is excessively drained, the charger enters trickle charge mode, thus reducing the available system current to the 50mA to 100mA trickle charge. This may not be enough to start up the system, forcing the user to wait until the charger is in constant-current mode qCase 2. The system will not work without a battery. If a battery is not present, some systems will not turn on because this is considered a fault, or the charger output oscillates. Table 1. Comparison of traditional dual input charger and LT power manager/charger Case 20 Scenario Traditional Dual Input Charger LT Power Manager/Charger 1 Battery voltage below Available current to system is only trickle charge Full adaptor/USB power available to system trickle charging voltage current (50mA–100mA), which may not enough to start the system 2 Battery is not present Most chargers consider this as a fault. System can’t start Full adaptor/USB power available to system 3 VBATT = 3.2V or USB input Available power to system is limited to 1.6W. Worst case 2.2W available to system. 4 System consuming close to input power Can’t distinguish the available charging current. Charger timer increases charging time with Charger timer runs out before battery is fully decreasing available charging current. Battery charged always fully charged. Linear Technology Magazine • September 2006 DESIGN FEATURES L 90 CC CURRENT = 970mA 85 NO OUTPUT LOAD FIGURE 5 SCHEMATIC 80 WITH R PROG = 52k EFFICIENCY (%) qCase 3. Available power reduces with battery voltage. Because the available system load power depends on the battery voltage, when USB input is used, the available system power is restricted to 1.6W (3.2V battery voltage). qCase 4. The battery cannot be fully charged. In this scenario, the battery slowly charges because the system draws the bulk of the available power, leaving little current for the charger. The problem arises because the safety timer runs out before the battery can be fully charged. The LT4089/LTC4089-5’s PowerPath controller/charger solves the above problems (see Table 1) and provides other benefits (see “Additional Features” below). from a wall adaptor and 98% from the USB (0.1V drop on the 0.2Ω FET). This means that the available power to the system is at least 2.2W (assuming a 4.5V USB) versus 1.6W when battery is at low 3.2V (Case 3). The LTC4085 has a smart, adaptive safety timer, whose time extends inversely to the charging current in constant-current charging mode. This solves the problem in Case 4. LTC4089 75 70 65 LTC4089-5 60 55 HVIN = 8V HVIN = 12V HVIN = 24V HVIN = 36V 50 45 40 3.5 3 4 BATTERY VOLTAGE (V) 2.5 4.5 Additional Features Figure 4. The efficiency of LTC4089/ LTC4089-5 when charging from HVIN For instance, in Case 1, the system gets the current it needs—anything left over is available to trickle-charge the battery. The removal of the battery (Case 2) doesn’t affect the system’s available power, which is over 99% The LT4089/LTC4089-5 offers other advantages over a basic charger in line with a battery: qSeamless transition between the three power sources: AC adaptor, USB input, and Li-ion battery. q200mΩ monolithic ideal diode from battery to system load. D2 SD101AWS VIN 6V TO 36V VIN E1 + E2 GND 21 C9 22µF 50V R1 1M 1% C1 1µF 50V 20 HVIN BOOST SW 19 L1 C2 10µH 0.1µF 6.3V SLF6028T-100M1R3 E16 HVOUT C3 22µF 6.3V D1 DLFS160 JP1 VIN 1 ON 22 2 C7 1000pF 50V 3 OFF USB E3 4.35V TO 5.5V JP2 CURRENT USB 500mA 100mA 1 2 3 E8 HPWR JP3 USB ON/OFF 1 OFF 15 16 C4 0.1µF 10% R3 2.1k 1% RPROG 52k 1% ON 17 HVPR IN OUT HPWR SUSP GATE BAT TIMER CHRG 14 9 CLPROG NTC 4 3 VNTC PROG VC 2 HVOUT HVOUT 12 C5 4.7µF 6.3V R2 1Ω LTC4089 HVEN 10pF GND GND 2 1 3 R7 680 18 7 13 Q1 Si2333DS R6 1k 1% 10 D3 HVPR RED E4 OUT C6 4.7µF 6.3V Q2 Si2333DS GND 11 8 R8 680 6 5 D4 CHGR GRN E6 LI-ION+ C8 4.7µF 6.3V R9 1Ω E7 GND R5 10k 1% E9 CHGR E11 NTC JP4 NTC 1 E13 SUSP EXT 2 3 E10 CLPROG R10 10k 1% E12 PROG INT Figure 5. The typical application circuit schematic diagram Linear Technology Magazine • September 2006 21 L DESIGN FEATURES An external FET gate signal is provided if user wants to use an external switch between the battery and the load to reduce RDS(ON) losses. qCharging current is system load dependent, guaranteeing the compliance to USB current limits qConstant-current/constant-voltage battery charge operation qThermal foldback to maximize charging rate without risk of overheating qAccurate monitoring of USB current: 5% for 500mA and 10% for 100mA qPreset 4.2V charge voltage with 0.8% accuracy qNTC thermistor input for temperature qualified charging qC/10 charge current detection output (CHRG) qHigh voltage present indication (HVPR) Applications Figure 5 shows a typical LTC4089/ LTC4089-5 circuit schematic. Designing a complete USB, high voltage, battery charger circuit is relatively easy—only a few external components are needed to set the operating parameters: qIN pin (USB) current limit is set by resistor connected to CLPROG pin (2.1kΩ for 475mA USB current limit with maximum of 500mA considering component tolerances) qCharge current is set by a resistor connected to PROG pin (71.5kΩ for 700mA charge current) qCharge safety timer is a function of RPROG (R4) and capacitor C4 connected to TIMER pin. A typical value is 0.15µF for a 3-hour charging time for the constant charging current of 700mA. The time for the constant current charge portion increases with decreasing available charging current to ensure the battery is always fully charged. The increase in charge time is reflected on the frequency of the triangular waveform on C4. 22 Application Caveats High Voltage Buck Input Surge Protection The small size and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor at LTC4089/LTC4089-5’s buck HVIN pin. However, these capacitors can cause problems if the circuit is plugged into a live supply (see Linear Technology Application Note AN88 for a complete discussion). The low loss ceramic capacitor combined with parasitic inductance in series with the source forms an under-damped LC tank circuit and the voltage at the HVIN pin can ring as much as twice the nominal input voltage, possibly exceeding the maximum voltage rating and damaging the part. If the input supply is poorly regulated or the user can hot plug the LTC4089/LTC40895 into an energized supply, an input network should be designed to prevent the overshoot. Figure 6a shows the waveforms that result when an LTC4089 circuit is connected to a 24V supply through six feet of 24-gauge twisted wire. The first plot is the response with a 2.2µF ceramic capacitor at the input. The input voltage HVIN rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 6b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance dampens the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 6c. A 1Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1µF capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V. continued on page 32 CLOSING SWITCH SIMULATES HOT PLUG IIN VIN VIN 20V/DIV + LOW IMPEDANCE ENERGIZED 24V SUPPLY + 2.2µF RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING IIN 5A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 10µF 35V AI.EI. DANGER! 20µs/DIV (6a) VIN 20V/DIV + 2.2µF IIN 5A/DIV (6b) 1Ω + 0.1µF 20µs/DIV VIN 20V/DIV 2.2µF IIN 5A/DIV (6c) 20µs/DIV Figure 6. A well chosen input network prevents input voltage overshoot and ensures reliable operation when the LTC4089/LTC4089-5 is connected to a live supply. Linear Technology Magazine • September 2006 L DESIGN IDEAS controls the slew rate of the output voltage during start-up, which limits the inrush current of the input power supply. Since the LT3825 incorporates current-mode control, both shortcircuit behavior and ease of loop compensation are improved over voltage-mode controllers. The switching frequency can be set anywhere from 50kHz to 250kHz, making it possible to find the right balance of solution LTC4089, continued from page 22 High Voltage Buck Output Capacitor Selection All the ceramic capacitors used in the circuit are recommended to be X5R or better (X7R). However, be cautious about the claimed initial capacitance value (e.g., some 0805 size 22µF/6.3V X5R caps measure only 11µF at no bias) and derating with bias and temperature (some X5R caps derate to less than 20% of their initial values with full 6.3V voltage bias). It is critical to use a 22µF/16V X5R or better cap at the output of the LTC4089 buck regulator (connected to HVOUT), as low capacitance causes duty-jitter in certain conditions. The LTC4089-5 can operate with a 22µF/6.3V ceramic cap at the output. High Voltage Buck Current Limit As shown in Figure 7, the buck output current capability is a function of inductance and the input voltage. For most of the input range, the output current limit is 1A for a 10µH inductor size and efficiency for a specific application. The switching frequency can be synchronized to an external system clock for further flexibility. input voltage connected directly to the VCC pin, so several components are not needed to generate a bias supply, including D1, C6, R1, and R2. It Is Possible to Reduce the Parts Count Even More Conclusion and 1.1A for a 33µH inductor. When powered from the high voltage source, if the sum of the system load current at the OUT terminal and charge current (set by RPROG) exceeds the buck output current limit, the buck output voltage collapses to the battery voltage. connected to CLPROG pin. Figure 8 shows the schematic diagrams. 1.6 TYPICAL 1.5 L = 10µH IOUT (A) 1.4 1.3 1µF HVOUT 5V (NOM) FROM USB CABLE VBUS IN L = 10µH 0.9 5 10 15 20 VIN (V) 25 30 35 Figure 7. The high voltage switching regulator’s maximum output current for two different value inductors 32 HVPR LTC4089 4.7µF 1k 4.7µF OUT CLPROG IN-LMT 500mA 1000mA VOUT (TYP) VBAT +0.3V 5V 5V VBAT TIMER GND PROG 2k TO LDOs REGS, ETC. BAT 2k USB POWER 500mA ICHG 1.0 10µF HVEN 100k 0.1µF Li-Ion BATTERY + AVAILABLE INPUT HV INPUT (LTC4089) HV INPUT (LTC4089-5) USB ONLY BAT ONLY ICHG BAT D1 MINIMUM 1.1 10µH SW BOOST HVIN HIGH (6V-36V) VOLTAGE INPUT 5V WALL ADAPTER 850mA ICHG L = 33µH 1.2 The LTC4089 and LTC4089-5 combine a monolithic high voltage switching buck regulator, a full featured Li-ion battery charger, and a PowerPath controller in a tiny 3mm × 6mm DFN package. They solve many battery charging and power path problems and easily fits into handheld applications, such as portable GPS navigators and MP3 players, where a high voltage source and small PCB space are required. L 0.1µF ADPR LO HI L = 33µH Conclusion Accept USB and 5V Adaptor with Different Current Limits Like all other LTC PowerPath controllers, the LTC4089/LTC4089-5 can be configured to accept 5V adaptor/USB input in the same USB connector or different connectors with different current limits by changing the resistance ADPR (FROM SYS) 1.8 The LT3825 allows a designer to improve the performance of multioutput isolated flyback circuits while lowering parts count and simplifying implementation. L For lower input voltages (5V to 20V) and simpler designs, the LT3837 complements the LT3825. The LT3837 starts up and runs from the lower LTC4089 MP1 1k IN + PROG CLPROG MN1 2.87k 2k Li-Ion BATTERY 59k Figure 8. IN pin accepting USB and 5V Adaptor with different current limits Linear Technology Magazine • September 2006