Automotive and Industrial Power Design
309
Martin Merchant
Versatile industrial power supply takes high voltage input and yields from eight 1A to two 4A outputs
pushbutton interface to supply input power to the LTC3375 buck regulators.
External VCC LDO and external input power supply start-up control
The LTC3375 can control an external LDO pass device to sup- ply its VCC power and any other low current electronics such as an RTC. The VCC powers the internal pushbutton circuitry, WDT, internal registers and open-drain pull-ups. The external LDO in Figure 309.1 creates a 3.3V supply from the 24V rail.
Introduction
Today’s industrial electronic systems contain many of the same components as consumer electronics—microcontrollers, FPGAs, system-on-chip ASICs and other electronics—requir- ing multiple low voltage rails at widely varied load currents.
Industrial applications can also demand a pushbutton inter- face, an always-on supply for a real-time clock (RTC) or memory and the ability to take input power from a high volt- age supply. Other required features may be a watchdog timer (WDT), a kill or reset button, software adjustable voltage lev- els and error reporting of low input/output voltages and high die temperature.
The LTC3375 is a highly configurable multioutput step- down power converter that offers the features often required by industrial electronics while providing the flexibility to configure various outputs with maximum currents ranging from 1A to 4A.
Configurable maximum output current
The LTC3375’s eight 1A channels can be combined to pro- duce various combinations of 1A, 2A, 3A and 4A buck regulators, as shown by the 15 different output current con- figurations in Table 309.1.
Connecting the feedback pin of a given channel to its VIN pin configures that channel as a slave to the adjacent channel.
The switch pins of the two channels are connected together to share a single inductor and output capacitor. Master/slave channels are enabled via the master’s enable pin and regulate to the master’s feedback network.
Output current can be increased to 3A or 4A by connect- ing additional adjacent channels. The circuit in Figure 309.1 shows the LTC3375 configured with a 3A output, a 1A out- put, two 2A outputs and an always-on LDO. It also illustrates how the LTC3375 can be connected to control the start- up of an upstream external buck controller via the on-chip
Table 309.1 LTC3375 Maximum Current Configurations NUMBER OF BUCKS OUTPUT CONFIGURATION
8 1A, 1A, 1A, 1A, 1A, 1A, 1A, 1A
7 1A, 1A, 1A, 1A, 1A, 1A, 2A
6 1A, 1A, 1A, 1A, 1A, 3A
6 1A, 1A, 1A, 1A, 2A, 2A
5 1A, 1A, 1A, 1A, 4A
5 1A, 1A, 1A, 2A, 3A
5 1A, 1A, 2A, 2A, 2A
4 1A, 1A, 2A, 4A
4 1A, 1A, 3A, 3A
4 1A, 2A, 2A, 3A
4 2A, 2A, 2A, 2A
3 1A, 3A, 4A
3 2A, 2A, 4A
3 2A, 3A, 3A
2 4A, 4A
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00309-4
When the pushbutton is pressed, the ON pin is released and the RUN pin is pulled high on the LTC3891, supply- ing input power to the buck regulators of the LTC3375.
When the LTC3891 achieves regulation, the PGOOD pin is released, enabling EN1 of the LTC3375 and turning on the 2A regulator. The remaining regulators can be enabled with the precision threshold enable pins or via software-controlled I2C commands. Pressing the pushbutton again for 10 seconds or more, or pulling KILL low for 50ms or more, causes the ON pin to be pulled low, disabling all of the buck regulators.
Unique power control and features
The I2C interface allows extensive control of regulator opera- tion. Each regulator may be set to a high efficiency Burst Mode operation to save power at light loads or set to forced continuous mode for lower output ripple voltage. Each regula- tor can also have the switching cycle phase shifted by 0°, 90°, 180°, or 270° with respect to the reference clock to allow a lower input ripple current when multiple outputs are supply- ing large loads. Another feature is the ability to manipulate each output voltage up or down by adjusting the feedback ref- erence voltage from the default 725mV setting in 25mV steps (ranging from 425mV to 800mV). The I2C interface is also used for reporting error conditions for each regulator.
The LTC3375 has a reset (RST) pin and an interrupt request (IRQ) pin, which can be programmed to report when
any regulator’s output voltage has dropped below 92.5% of the regulation point. The IRQ pin can also be programmed to report when the input voltage drops below the undervolt- age lockout (UVLO) threshold or when the die temperature has reached a set temperature threshold. The regulator’s PGOOD and UVLO status, the die temperature warning and the measured die temperature can be monitored by the microprocessor via the I2C interface.
One problem with microprocessors is that a software bug can cause the program to hang. The LTC3375 includes a watchdog timer input (WDI) pin to monitor the SCL pin or some other pin to determine if the software is still running.
If the software has stopped running, the watchdog timer out- put (WDO) pin can be used to reset the microprocessor or power down the HV buck and the LTC3375 buck regulators.
Connecting the WDO pin to the RST pin of a microprocessor causes the microprocessor to reset when the WDT is not satis- fied. Connecting the WDO pin to the KILL pin causes the ON pin to go low, disabling the HV buck and all LTC3375 regula- tors. The KILL pin can be pulled low by a pushbutton “paper clip” switch to power down all the regulators as a last resort.
Conclusion
The LTC3375 can be configured with multiple regulated 1A to 4A outputs totaling up to 8A, and includes many features required by today’s industrial electronics.
310
Charlie Zhao
65V, 500mA step-down converter fits easily into automotive and industrial applications
possible by paralleling multiple LTC3630s together and con- necting the FBO of the master device to the VFB pin of a slave device. An adjustable soft-start is included. A precision RUN pin threshold voltage can be used for an undervoltage lockout function.
Introduction
The trend in automobiles and industrial systems is to replace mechanical functions with electronics, thus multiplying the number of microcontrollers, signal processors, sensors and other electronic devices throughout. The issue is that 24V truck electrical systems and industrial equipment use rela- tively high voltages for motors and solenoids while the micro- controllers and other electronics require much lower voltages.
As a result, there is a clear need for compact, high efficiency step-down converters that can produce very low voltages from the high input voltages.
65V input, 500mA DC/DC converter with an adjustable output down to 800mV
The LTC3630 is a versatile Burst Mode synchronous step- down DC/DC converter that includes three pin-selectable preset output voltages. Alternatively, the output can be set via feedback resistors down to 800mV. An adjustable output or input current limit from 50mA to 500mA can be set via a single resistor. The hysteretic nature of this topology provides inherent short-circuit protection. Higher output currents are
Figure 310.1 • High Efficiency 24V Regulator with Undervoltage Lockout and 300mA Current Limit
Figure 310.2 • Efficiency of Circuit in Figure 310.1
Figure 310.3 • Input Voltage Sweep vs Output Voltage Showing Undervoltage Lockout Threshold Levels Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00310-0
24V regulator with 300mA output current limit and input undervoltage lockout
Figure 310.1 shows a 48V to 24V application that showcases several of the LTC3630’s features, including the undervoltage lockout and output current limit. Operational efficiencies are shown in Figure 310.2.
The RUN pin is programmed for VIN undervoltage lockout threshold levels of 27V rising and 24V falling. Figure 310.3 shows VOUT vs VIN. This feature assures that VOUT is in regu- lation only when sufficient input voltage is available.
The 24V output voltage can be programmed using the 800mV 1% reference or one of the preset voltages. This cir- cuit uses the 5V preset option along with feedback resistors to program the output voltage. This increases circuit noise immunity and allows lower value feedback resistors to be used.
Although the LTC3630 can supply up to 500mA of out- put current, the circuit in Figure 310.1 is programmed for a maximum of 300mA. An internally generated 5μA bias out of the ISET pin produces a voltage across an ISET resistor, which determines the maximum output current. Figure 310.4 shows the output voltage as a resistive load is varied from approxi- mately 100Ω down to 8Ω while maintaining the output cur- rent near the programmed value of 300mA. In addition, the hysteretic topology used in this DC/DC converter provides inherent short-circuit protection.
Input current limit
on the ISET pin that tracks VIN. This allows VIN to control output current which determines input current.
An increased voltage on ISET increases the converter’s current limit. Figure 310.6 shows the steady-state input current vs input voltage and the available output current before the output voltage begins to drop out of regulation.
For the values shown in Figure 310.5, the input current is limited to approximately 55mA over a 10V to 60V input voltage range.
Conclusion
The LTC3630 offers a mixture of features useful in high Figure 310.4 • Resistive Load Sweep vs Output Current vs
Output Voltage with Output Current Limit Set to 300mA
Figure 310.5 • 5V Regulator with 55mA Input Current Limit
Figure 310.6 • Input Voltage vs Load Current and Input Current with Input Current Limit Circuit Shown in Figure 310.5
311
Goran Perica
2-phase, dual output synchronous boost converter solves thermal problems in harsh environments
this current level, diode D1’s voltage drop is 0.57V, resulting in 1.6W of power lost as heat. Dissipating 1.6W in an 85°C (or higher) automotive operating environment is not trivial.
To keep the circuit cool, heat sinks, cooling fans and multi- layer printed circuit boards must be used. This, of course, adds complexity, cost and size to an ostensibly simple boost converter.
A far better solution (featured in a dual output configura- tion) is shown in Figure 311.2, where a synchronous power MOSFET rectifier replaces the output diode. Under the same conditions, the voltage drop across output synchronous MOSFET Q2 is only 42mV or 7.4% of the voltage drop in the diode D1. The resulting power dissipation of 115mW in Q2 is relatively trivial. Another advantage of using a MOS- FET as the output rectifier is the elimination of leakage cur- rent, about 10mA in the case of the MBR2545 diode—an additional 240mW of power dissipation in the application of Figure 311.1.
Dual output automotive boost converter
Figure 311.2 illustrates a typical automotive boost application with a 5V to 36V input voltage range. Here, the converter produces a 12V output for generic automotive loads such as entertainment systems, and a 24V output for circuits such as high power audio amplifiers. The two outputs are completely independent and can be controlled separately.
Because the circuit in Figure 311.2 is a boost converter, the output voltage can be regulated only for input voltages that are lower than the output voltage. The output voltage regula- tion versus input voltage is shown in Figure 311.3. When the input voltage is higher than the preset output voltage, syn- chronous MOSFETs Q2 and Q4 are turned continuously ON and boost MOSFETs are not switching. This feature allows the converter to be used in applications that require boost- ing only during load transients such as cold-cranking of a car
Introduction
Boost converters are regularly used in automotive and industrial applications to produce higher output voltages from lower input voltages. A simple boost converter using a Schottky boost diode (Figure 311.1) is often sufficient for low current applications. However, in high current or space-con- strained applications, the power dissipated by the boost diode can be a problem especially in high ambient temperature envi- ronments. Heat sinks and fans may be needed to keep the cir- cuit cool, resulting in high cost and complexity.
To solve this problem, the Schottky output rectifier can be replaced by a synchronous MOSFET rectifier (Figure 311.2).
If MOSFETs with very low RDS(ON) are used, the power dis- sipation can be reduced to the point where no heat sinks or active cooling is required, thus reducing costs and saving space.
Advantages of synchronous rectification
Consider the power dissipation of the single output circuit in Figure 311.1. The output diode D1 carries 6.7A of RMS current to produce 3A of output current from a 5V input. At
Figure 311.1 • Although This Simple Circuit Is Capable of 3A of Output Current, Beware of Power Dissipation in the Output Diode D1
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00311-2
engine. In this case, the LTC3788 circuit’s input voltage could be as low as 2.5V.
The efficiency of this converter (Figure 311.4) is high
output ripple currents are greatly reduced—nearly canceling out at 50% duty cycle. Thus, smaller output capacitors can be used with lower output ripple currents and voltages.
Figure 311.2 • The LTC3788 Converter Is over 95% Efficient Even under Worst-Case Conditions. When VIN > VOUT(SET), Efficiency Approaches 100% as Shown in Figure 311.4
Figure 311.3 • The Output Voltage Follows the Input Voltage When VIN > VOUT(SET)
Figure 311.4 • The Converter in Figure 311.2 Peaks at 95%
Efficiency When Operating from a 5V Input
312
George H. Barbehenn
High efficiency USB power management system safely
charges Li-Ion/Polymer batteries from automotive supplies
Complete USB/battery charging solution for use in large transient environments
Figure 312.1 shows such a design. This complete Power- Path manager and battery charger system seamlessly charges the Li-Ion battery from a wide ranging high voltage or USB source.
In this circuit, the LTC4098 USB power manager/Li-Ion battery charger controls an LT3480 HV step-down regulator.
The LTC4098’s Bat-Track feature provides a high efficiency,
Introduction
Automotive power systems are unforgiving electronic environments. Transients to 90V can occur when the nomi- nal voltage range is 10V to 15V (ISO7637), along with battery reversal in some cases. It is fairly straightforward to build automotive electronics around this system, but increasingly end-users want to operate portable electronics, such as GPS systems or music/video players, and to charge their Li-Ion batteries from the automotive battery. To do so requires a compact, robust, efficient and easy-to-design charging system.
Figure 312.1 • LTC4098 USB Power Manager/Li-Ion Battery Charger Works with an LT3480 HV Buck Regulator to Accept Power from an Automotive Environment or FireWire System. Overvoltage Protection Protects Both ICs and Downstream Circuits
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00312-4
low power dissipation battery charger from low and high volt- ages alike. The Bat-Track feature controls an internal input current-limited switching regulator to regulate VOUT to approximately VBAT + 0.3V which maximizes battery charger efficiency, and thus minimizes power dissipation by operating the battery charger with minimal headroom. Furthermore, the Bat-Track feature reduces charge time by allowing a charge current greater than the USB input current limit—the switch- ing regulator behaves like a transformer exchanging output voltage for output current.
The LTC4098 can extend the Bat-Track concept to an aux- iliary regulator via the WALL and VC pins. When sufficient voltage is present on WALL, Bat-Track takes control of the auxiliary regulator’s output via the VC pin, maintaining the regulator’s output at VBAT + 0.3V.
The LTC4098 also includes an overvoltage protection function—important in volatile supply voltage environments.
Overvoltage protection shuts off a protection N- channel MOSFET (M2) when the voltage at the OVSENS pin exceeds approximately 6V. The upper limit of voltage pro- tection is limited only by the breakdown voltage of the MOSFET, and by the current flowing into the OVSENS pin.
Overvoltage protection covers the entire battery charger/power manager system
The overvoltage protection function of the LTC4098 can pro- tect any part of the circuit. In Figure 312.1, the protection has been extended to the LT3480 VIN input. The overvolt- age shutdown threshold has been set to 24V. This threshold provides ample margin against destructive overvoltage events without interfering with normal operation.
In Figure 312.1, M1 is a P-channel MOSFET that provides reverse voltage protection, whereas M2 is the overvoltage pro- tection MOSFET, and M3 level-shifts the OVGATE output of the LTC4098.
If the HVIN voltage is less than zero, the gate and source voltages of both M1 and M2 are held at ground through R3, R4, and R5, ensuring that they are off. If the HVIN voltage is between 8V and approximately 24V, the gate of M3 is driven high via the LTC4098’s OVGATE pin. This turns on M1 and M2 by pulling their gates 7V to 10V below their sources via M3, D1, R1 and R5. With M1 and M2 on current flows from HVIN to VIN and the system operates normally.
If the HVIN input exceeds approximately 24V, the LTC4098 drives the gate of M3 to ground, which allows R5 to reduce the VGS of M1 and M2 to zero, shutting them off and disconnecting HVIN from VIN.
M1, M2 and M3 have a BVDSS of 100V, so that this circuit can tolerate voltages of approximately −30V to 100V. It will operate normally from 8V to approximately 24V. This combi- nation is ideal for the harsh automotive environment, provid- ing a robust, low cost and effective solution for Li-Ion battery charging from an automotive power system.
Finally, setting the OVSENS resistor divider requires some care. For an OVSENS voltage between approximately 2V and 6V, VOVGATE = 1.9 ã VOVSENS. OVSENS is clamped at 6V and the current into (or out of ) OVSENS should not exceed 10mA.
The chosen resistor divider attenuates HVIN by a factor of 4, so M3 has sufficient gate voltage to turn on when HVIN exceeds approximately 8V. When HVIN = 100V, the current into OVS- ENS is just 2.25mA—well below the 10mA limit.
As shown in Figure 312.2, VIN is only present when HVIN is in the 8V to 24V region. Figure 312.3 shows a close-up cen- tered on the load dump ramp. The ISO7637 test ramp rises from 13.2V to 90V in 5ms. There is a 220μs turn-off delay—
OVGATE going low to the gates of M1 and M2—which results in an overshoot on VIN. The maximum value of this overshoot is 3.5V (VVIN(MAX) ≈ 27.5V). The magnitude of this overshoot can be calculated for different ramp rates, such that
where ΔV = (90V−13.6V), Δt = 5ms, and tDELAY = 220μs, so, VOVERSHOOT = 3.36V.
If less delay, and thus less overshoot, is desired, an active turn-off circuit can reduce the delay from OVGATE to the gates of M1 and M2 to a few microseconds.
Conclusion
The LT3480 high voltage step-down regulator and LTC4098 Li-Ion/Polymer battery charger, combined with a few exter- nal components, produce a robust high performance Li-Ion charger suitable for portable electronics plugged into an auto- motive power source and maintain compatibility with USB power. The circuit provides all the functionality that custom- ers expect, along with voltage protection from battery reversal and load dump transients.
VOVERSHOOT=�V/�tãtDELAY
313
Victor Khasiev
Low profile synchronous, 2-phase boost converter produces 200W with 98% efficiency
A 24V output boost converter at 8.5A (continuous), 10.5A (peak) from a car battery
Figure 313.1 shows a boost converter that generates 24V from an input voltage range of 8.5V to 18V. Output power is 200W continuous and 250W for short pulse loads, corresponding to 8.5A continuous current and 10.5A pulsed current.
Introduction
Automotive audio amplifiers require a high power boost con- verter that is both efficient and compact. High efficiency is essential to keep dissipated heat low and avoid bulky and expensive heat sinks. The LT3782A is a 2-phase synchronous PWM controller, making it possible to produce a low profile, high power boost supply that achieves 98% efficiency.
Figure 313.1 • Synchronous Boost Converter Based on the LT3782A (VOUT = 24V at 8.5A, VIN = 8.5V to 18V) Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00313-6