“The circuit presented in this article can be very useful for designing compact power supplies and is extremely beneficial for system operation in the event of input power interruptions. The proposed solution can be embedded in an uninterruptible power supply system or a small stand-alone module. It can be successfully used in industrial and security applications, oil and gas exploration communication systems. The wide input voltage range makes it an excellent choice for the automotive industry, including electrical systems placed under the hood and in the cabin.
By Victor Khasiev and Gabino Alonso
The circuit presented in this article can be very useful for designing compact power supplies and is extremely beneficial for system operation in the event of input power interruptions. The proposed solution can be embedded in an uninterruptible power supply system or a small stand-alone module. It can be successfully used in industrial and security applications, oil and gas exploration communication systems. The wide input Voltage range makes it an excellent choice for the automotive industry, including electrical systems placed under the hood and in the cabin.
The purpose of this article is to describe the use of a 24V to 60V input voltage range (when input power is available) or a 14.4V battery pack (when input power is not available) to provide uninterrupted power to a 3.3V rail. When the input power is connected, the power supply automatically charges the battery and limits the input current during the charging process.
A pair of high voltage controllers, the LTC3890 and LTC4000, provide a complete DC voltage power supply solution with battery backup and a very wide operating voltage range. The LTC3890’s input voltage range is 4.5V to 60V, and the LTC4000 can charge batteries rated at 3V to 60V. The LTC3890 is a dual, two-phase synchronous step-down DC/DC controller. An advantage of the LTC3890 is its extremely low 50µA no-load quiescent current. Low quiescent current and very low dropout operation (99% duty cycle) make the LTC3890 extremely useful in battery powered systems. The logic-level mosfets used in this solution reduce gate-related losses and improve overall system efficiency. The circuit described in this article uses one LTC3890 output to provide a fixed and accurate voltage to power the customer load, and it uses a second output as a variable voltage supply for battery charging (which is controlled by the LTC4000). The LTC4000 is a controller designed for converting DC/DC power supplies, typically as a voltage supply for battery chargers. The LTC4000 is a full-featured controller for battery charging and power management. In addition, the device has the ability to limit the system input current and reduce the stress ON the input line. It is of great importance in certain applications when the power supply has to simultaneously charge the battery and supply energy to high power-hungry loads.
Figure 1: Power Block Diagram
The block diagram of the proposed circuit is shown in Figure 1, which has a wide input voltage range: from 24V up to 60V. The circuit includes the following components: a high voltage DC/DC converter (HVDC) based on one output LTC3890 converter, a practical battery charger based on the LTC4000, and a low voltage DC/DC converter (LVDC) based on another output LTC3890 converter . The HVDC converter can deliver up to 10A over an output voltage range of 15V to 22V. The battery charger can provide a maximum charging current (Ich) of 4A at a charging voltage of 16.8V (Vfl). LVDC is set to deliver 2A at 3.3V. The NL2044 Smart Li-Ion battery pack was chosen as the backup battery. The battery pack specifications are at VMAX=16.8V, VNOM = 14.4V and VCUTOFF = 9.6V and has a 6.6Ah capacity.
Figure 2: Power Supply Schematic
The detailed electrical schematic is shown in Figure 2. It is based on a high voltage step-down switching Regulator LTC3890. The first output, the LTC3890, is controlled by the LTC4000 and distributed to power two loads: the battery charger and the LVDC. High priority is given to the control signal from the LTC4000, which sets the voltage level of the first output of the LTC3890 to ensure accurate battery charging. The voltage level on this output is not fixed, it follows the battery charge cycle. The second output of the LTC3890 is LVDC, which is supplied by the first output and provides a fixed 3.3V voltage to the load. The voltage level on this output is independent of system voltage, battery charging process, or power source (input voltage or battery). The solution presented enables seamless switching between different power sources.
The LTC4000 charging circuit is responsible for the following functions:
• Complete charge cycle of the battery. This charge cycle includes:
Battery status detection
Provides programmed charge current and battery voltage control
charge cycle terminated
• Input diode to isolate reverse current from battery to high voltage DC/DC converter
• Disconnect the fully charged battery from the input voltage source
• Limit the input current of the entire system to the programmed value.This feature is important in systems using fuses and circuit breakers
The voltage rails are labeled similar to the LTC4000 demo circuit 1830A, which is also recommended for prototyping and simulation board testing. The following is a brief description of the voltage rails and functions of the power chain components:
V from unregulated raw voltage supply (24V to 60V)IN+ input voltage.
V to HVDCIN Input voltage: Q3, Q4, L1. Current sense Resistor RS1 is responsible for limiting the input current to the system. ideal diode. Q1 at VIN Disconnect when the voltage is interrupted and disconnect the HVDC from the battery pack.
VOUTThe -SYS rail is generated from the source of PMOS Q1, and it is responsible for feeding the battery (during charging) and the LVDC.
PMOS Q2 is the LTC4000 PowerPath™ part of the controller. LVDC by VOUTThe -SYS rail supplies power and it is responsible for supplying power to the end load, Q5, Q6, L2 form the power chain of the converter.
Common cathode diode D1 uses the input voltage or battery to maintain the bias on the LTC3782.
When the input voltage is applied, it activates the HVDC and battery charger. The LTC3890 begins to make VOUT The voltage on the rail ramps up. VOUT The rise is controlled by the voltage level on the TRACK/SS2 pin until the voltage on this pin reaches 0.8V. At that point, the LTC4000 battery charger is running and it begins to control V through its own ITH pin (which is hardwired to the LTC3890’s ITH2 pin)OUT and VOUT-SYS level. The HVDC output voltage (and the corresponding feedback signal on the LTC3890 VFB2 pin) is set higher than the battery float voltage (or the manufacturer’s recommended charging voltage). This will ensure that under normal operating conditions only the LTC4000 battery charger (and not the LTC3890 buck controller) will regulate the output. The LTC3890 IC is responsible for controlling the switch NMOS Q1, while Q2 is controlled by the LTC4000. Since the LTC3890’s voltage setting exceeds the actual (floating) voltage set by the LTC4000, the LTC3890’s error Amplifier (EA) will supply current in an attempt to increase the voltage on its ITH pin. The LTC4000 will then sink current, keeping the ITH voltage in a steady state. The floating voltage is set by Resistor divider RB1, RB2.
If the battery voltage drops below the float voltage, the LTC4000 will analyze the condition of the battery. Provided the battery is not shorted or over-discharged, it will provide the programmed charge current to the battery. The charge current value is set by current sense resistor RS2 and resistor RCL. The LTC4000 regulates the charge current until the battery voltage reaches a floating value. Once the battery voltage reaches a floating value, the LTC4000 will switch from constant current mode to constant voltage mode, providing a constant voltage during charging. As the charging cycle progresses, the charging current value gradually decreases, as shown in Figure 5. On the schematic shown in Figure 2, the TMR pin is connected to the BIAS node, which means that the charge cycle will terminate as soon as the charge current decreases to the programmed C/X value.
In addition, the LTC4000 monitors the input current value. If the input current level exceeds the programmed value, the LTC4000 will reduce the charging current and voltage, allowing the load connected to the LVDC to continue to operate without interruption. The input current limit is set by the current sense resistor RS1 and RIL (not shown).
When the charging current decreases below the C/X set limit, the battery is disconnected from the charging circuit and the PMOS Q2 is turned off. At this point, the LTC4000 regulates the output voltage above the float value to ensure that the body diode of Q2 is reverse biased and current does not flow from the battery to the load.
Circuit description and setup of two controllers
In this article, the battery used for charging and discharging is the NL2044HD22. This is a Li-Ion battery pack that combines 12 18650 size cells assembled in a (4S3P) configuration. Battery manufacturers recommend a charge voltage of 16.8V ± 50mV and a maximum charge current of 4A.
Setting up the LTC4000
Battery float voltage setting, BFB pin. Note: The FBG pin is the ground return pin for the resistor divider connected to the BFB and OFB pins. Assuming RB1 is 499k, RB2 is calculated to be 36.5k required to provide the 16.8V float voltage. Battery output voltage setting, OFB pin. This voltage is set to 18V by selecting RO1 and RO2 to be 499k and 35.7k respectively.
Resistor RS1, pins CSP and CSN, is chosen to be 12mΩ to set the charge current limit to 4.1A.
Resistor RCL is set to 19.1k, which sets the battery charging current to 4.0A.
Resistor RS2, pins IN and CLN, is chosen to be 5mΩ to set the input current limit to 10.0A.
Resistor RCX is chosen to be 21.0k. It uses the corresponding formula in the LTC4000 product manual to set the charge termination current to 0.4A.
Pin IL is left open, which defines the 50mV maximum voltage that can be used to sense the input current.
The same 30-V Si7135DP PMOS was chosen for Q1 and Q2.
Detailed instructions and recommendations for charging circuit component selection can be found in the LTC4000 data sheet.
Setting up the LTC3890
There are four different versions of the LTC3890 controller (LTC3890, -1, -2 and -3), the differences between the versions are described in Table 1 of the LTC3890-3 data sheet. The LTC3890-3 controller was chosen for this solution: the device does not permanently turn on the lower mosfet in the event of an overvoltage, which is very important in battery powered applications. However, any of the four LTC3890 versions can be used if specific functionality is required and important.
The LTC3890’s OUT2 (the “battery input” bus) is set to 22V by voltage dividers RF1, RF2, however, as mentioned above, the actual output voltage will never climb that high. The switching frequency is set at 200kHz by selecting a 37.4k resistor. Resistor dividers RO1 and RO2 set VOUT1 to 3.3V.
To select power chain components, the LTC3890-3 data sheet and LTspice and LTpowerCAD simulation and design tools are available.
HVDC power budget and converter component selection
The power budget of HVDC (PHVDC) combines the power P required to charge the batteryBAT and the power P drawn by the low voltage DC/DC converterLVDC, VLOAD, ILOAD. The power delivered by the LVDC to the load is determined by the nominal battery voltage VNOM Sure. Assuming this voltage will exist at maximum current and load conditions:
PHVDC= (PBAT +PLVDC / ηl) / ηh; PHVDC= (VNOM *ICH +VLOAD *ILOAD / ηl) / ηh
where ηl and ηh are the efficiencies of the LVDC and HVDC converters.
Figure 3: Input power switching, 0.5V per division
Ch4, red, battery current
Ch3, purple, input voltage
Ch2, green, load voltage (3.3V/2A)
Figure 3 shows the seamless switching of the load power supply from the input voltage to the battery. Ch 4 (red trace) shows the battery current. When the input voltage is present, the battery draws current during charging. Once the input voltage is disconnected, current is supplied (discharged) by the battery. The output Ch 2 (green trace) of the LVDC is unchanged, the circuit can safely supply 3.3V to the load at 2.0A, independent of the power supply.
Figure 4: Power Supply Efficiency vs. Input Voltage, ICH 4.0A, convection air cooling
Efficiency curves are shown in Figure 4. Measurements were made with a constant charge current of 4.0A and a constant float voltage of 16.8V with convection air cooling (no forced air). The charger showed very high efficiency (about 97%).
Figure 5: Power supply charging voltage and current versus time
The changes in charging current and battery voltage during charging are shown in Figure 5.
The LTC3890 and LTC4000 are highly integrated high voltage, high performance controllers. With these two devices, a multifunctional power supply with battery backup can be designed. This article provides block diagrams, detailed electrical schematics, and calculation guidelines for this type of power supply.
The LTC3890-3 is a high performance, dual, step-down switching regulator DC/DC controller for driving an all N-channel synchronous power MOSFET stage. The device utilizes a constant frequency current mode architecture, which provides a phase lockable frequency of up to 850kHz. Power loss and power supply noise are minimized by operating the two controller output stages out of phase. 50µA of no-load quiescent current extends operating life in battery-powered systems. OPTI-LOOP® The use of compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The wide input supply range of 4V to 60V covers a wide range of intermediate bus voltages and battery chemistries. A separate TRACK/SS pin for each controller is responsible for ramping the output voltage during startup. Current-mode control limits the Inductor current during short-circuit conditions. The PLLIN/MODE pin is used to select Burst Mode operation, Pulse Skipping Mode or Continuous Conduction Mode under light load conditions. For device versions with different and/or additional features, see “Table 1” in the LTC3890 data sheet.
The LTC4000 is a high voltage, high performance controller that converts many externally compensated DC/DC sources into a fully functional battery charger. LTC4000 battery charger features include: accurate (±0.25%) programmable float voltage, selectable timer or current charge termination, temperature-friendly charging with NTC thermistor, automatic recharge, for deep discharge C/10 trickle charging of batteries, dead battery detection, and status indicator output functions. In addition, the battery charger has accurate current sensing capability, which provides lower sensing voltage for high current applications. The LTC4000 supports intelligent PowerPath control. An external PFET is used to provide low loss reverse current protection. Another external PFET is responsible for providing low-loss charging or discharging of the battery. This second PFET also facilitates “instant-on” functionality, which provides immediate downstream system power even when connected to a severely discharged or short-circuited battery.
Victor Khasiev was a senior applications engineer at Analog Devices and has extensive experience in power electronics for AC/DC and DC/DC conversion. He holds two patents and has authored several articles. These articles cover the use of ADI semiconductor devices in automotive and industrial applications, covering boost, buck, SEPIC, positive-negative, negative-negative, flyback, forward converters, and bidirectional backup power supplies. He holds patents related to high efficiency power factor correction solutions and advanced gate drivers. Victor is happy to provide technical support to Analog Devices customers with questions about ADI products, power schematic design and verification, printed circuit board layout, troubleshooting, and final system testing.
Gabino Alonso is currently the Director of Strategic Marketing for the Power by Linear™ division. Before joining ADI, Gabino held various positions in Marketing, Engineering, Operations and Instructor at Linear Technology, Texas Instruments, and Cal Poly State University. He holds a master’s degree in electrical and computer engineering from the University of California, Santa Barbara.