Switching Regulator Design: Buck (Step-Down)
32
David Burgoon
Inverting DC/DC controller converts a positive input to a negative output with a single inductor
offers programmable soft-start or output tracking. Safety features include overvoltage, overcurrent and short-circuit protection, including frequency foldback.
− 5.2V, 1.7A converter operates from a 4.5V to 16V source
The circuit shown in Figure 32.1 produces a −5.2V, 1.7A out- put from a 4.5V–16V input. Operation is similar to a flyback converter, storing energy in the inductor when the switch is on and releasing it through the diode to the output when the switch is off, except that with the LTC3863, no transformer is required. To prevent excessive current that can result from minimum on-time when the output is short-circuited, the controller folds back the switching frequency when the out- put is less than half of nominal.
The LTC3863 can be programmed to enter either high effi- ciency Burst Mode operation or pulse-skipping at light loads.
In Burst Mode operation, the controller directs fewer, higher current pulses and then enters a low current quiescent state for a period of time depending on load. In pulse-skipping mode, the LTC3863 skips pulses at light loads. In this mode, There are several ways to produce a negative voltage from a
positive voltage source, including using a transformer or two inductors and/or multiple switches. However, none are as easy as using the LTC3863, which is elegant in its simplicity, has superior efficiency at light loads and reduces parts count compared to alternative solutions.
Advanced controller capabilities
The LTC3863 can produce a −0.4V to −150V negative out- put voltage from a positive input range of 3.5V to 60V. It uses a single-inductor topology with one active P-channel MOS- FET switch and one diode. The high level of integration yields a simple, low parts-count solution.
The LTC3863 offers excellent light load efficiency, draw- ing only 70μA quiescent current in user-programmable Burst Mode operation. Its peak current mode, constant frequency PWM architecture provides positive control of inductor cur- rent, easy loop compensation and superior loop dynamics.
The switching frequency can be programmed from 50kHz to 850kHz with an external resistor and can be synchronized to an external clock from 75kHz to 750kHz. The LTC3863
Figure 32.1 • Inverting Converter Produces −5.2V at 1.7A from a 4.5V to 16V Source Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00032-6
the modulation comparator may remain tripped for several cycles and force the external MOSFET to stay off, thereby skipping pulses. This mode offers the benefits of smaller out- put ripple, lower audible noise and reduced RF interference, at the expense of lower efficiency compared to Burst Mode operation. This circuit fits in about 0.5in2 (3.2cm2) with com- ponents on both sides of the board.
Figure 32.2 shows the switch node voltage, inductor cur- rent and ripple waveforms at 5V input and −5.2V output at 1.7A. The inductor is charged (current rises) when the PMOSFET is on, and discharges through the diode to the out- put when the PMOS turns off. Figure 32.3 shows the same waveforms at 70mA out in pulse-skipping mode. Notice how the switch node rings out around 0V when the inductor cur- rent reaches zero. The effective period stops when the cur- rent reaches zero. Figure 32.4 shows the same load condition with Burst Mode operation enabled. Power dissipation drops
by 31% at this operating point, and efficiency increases from 74% to 80.5%. At 12V input, the 45% reduction in dissipation is even more dramatic.
High efficiency
Figure 32.5 shows efficiency curves for both pulse-skipping and Burst Mode operation. Exceptional efficiency of 85.2% is achieved at 1.7A load and 12V input. Note how Burst Mode operation dramatically improves efficiency at loads less than 0.2A. Pulse-skipping efficiency at light loads is still much higher than that obtained from continuous conduction.
Conclusion
The LTC3863 simplifies the design of converters producing a negative output from a positive source. It is elegant in its sim- plicity, high in efficiency, and requires only a few inexpensive external components.
Figure 32.2 • Switch Node Voltage, Inductor Current and
Ripple Waveforms at 5V Input and −5.2V Output at 1.7A Figure 32.4 • Switch Node Voltage, Inductor Current and Ripple Waveforms at 5V Input and −5.2V Output at 70mA in Burst Mode Operation
33
Tom Gross
20V, 2.5A monolithic synchronous buck SWITCHER+ with input current, output current and temperature
sensing/limiting capabilities
Beyond its impressive regulator capabilities, the LTC3626’s current and temperature monitoring functions stand out.
They offer both monitoring and control capabilities with mini- mal additional components.
Introduction
The LTC3626 synchronous buck regulator with current and temperature monitoring is the first of Linear’s SWITCHER+ line of monolithic regulators. It is a high efficiency, mono- lithic synchronous step-down switching regulator capable of delivering a maximum output current of 2.5A from an input voltage ranging from 3.6V to 20V (circuit shown in Figure 33.1). The LTC3626 employs a unique controlled on- time/constant frequency current mode architecture, making it ideal for low duty cycle applications and high frequency operation, while yielding fast response to load transients (see Figure 33.2). It also features mode setting, tracking and synchronization capabilities. The LTC3626’s 3mm × 4mm package has such low thermal impedance that it can operate without an external heat sink even while delivering maximum power to the load.
Figure 33.1 • 20V Maximum Input, 2.5A, 2MHz Buck Regulator with Current and Temperature Monitoring Figure 33.2 • Load Step Response for Figure 33.1 Circuit
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00033-8
Output/input current sensing
The LTC3626 senses the output current through the synchro- nous switch during the switch’s on-time and generates a pro- portional current (scaled to 1/16000) at the IMONOUT pin.
Figure 33.3 shows the accuracy of the IMONOUT output by comparing the measured output of the IMONOUT pin with calculated values. Error remains less than 1% over most of the output current range.
Likewise, this same sense current signal is combined with the buck regulator’s duty cycle to produce a current pro- portional to the input current—again by 1/16000—at the IMONIN pin. A precision of better than 5% is achieved over a wide current range (see Figure 33.4).
Both current signals are connected to internal voltage amplifiers, referenced to 1.2V, that can shut down the part when tripped. So the input and output current limits are set by simply connecting a resistor to the IMONIN or IMONOUT
pins, respectively, as shown in Figure 33.1. The relationship between the current limit and the resistor is:
For example, a 10k resistor sets a current limit of approxi- mately 2A.
This simple scheme allows both monitoring and active con- trol of the input and output current limits—the latter can be implemented via external control circuitry, such as a DAC with a few passive components.
ILIM⋍1.2Vã16000 RLIM
Temperature sensing
The LTC3626 generates a voltage proportional to its own die temperature, which can be used to set a maximum tem- perature limit. The voltage at the temperature monitor pin (TMON) is typically 1.5V at room temperature. To calculate the die temperature, TJ, multiply the TMON voltage by the temperature monitor voltage-to-temperature conversion fac- tor of 200K/V, and subtract the 273°C offset. The LTC3626 also has a temperature limit comparator fed by the tempera- ture limit set pin, TSET, and the TMON pin. Hence, by apply- ing a voltage to the TSET pin, a maximum temperature limit can be set according to the following:
Choosing a maximum temperature limit of 125°C equates to an approximate 2V setting on the TSET pin—the IC will shut down once the die temperature TJ reaches this limit.
Conclusion
The LTC3626 combines current and temperature monitoring capabilities with a high performance buck regulator in a com- pact package. A microprocessor or other external control logic can supervise conditions via easy-to-use input and output cur- rent and temperature monitor pins, and it can shut itself down by setting a threshold voltage on the temperature set limit pin.
VTSET=
TJ+273 200◦K/V
34
Jeff Zhang
1.5A rail-to-rail output synchronous step-down regulator adjusts
with a single resistor
or synchronized to an external clock. The LTC3600 internally generates an accurate 50μA current source, allowing the use of a single external resistor to program the reference voltage from 0V to 0.5V below VIN. As shown in Figure 34.1, the output feeds directly back to the error amplifier with unity gain. The output equals the reference voltage at the ISET pin.
A capacitor can be paralleled with RSET for soft start or to improve noise while an external voltage applied to the ISET pin is tracked by the output.
Internal loop compensation stabilizes the output volt- age in most applications, though the design can be custom- ized with external RC components. The device also features a power good output, adjustable soft-start or voltage track- ing and selectable continuous/discontinuous mode operation.
These features, combined with less than 1μA supply current in shutdown, VIN overvoltage protection and output overcur- rent protection, make this regulator suitable for a wide range of power applications.
Introduction
A new regulator architecture the LTC3600 (first introduced with the LT3080 linear regulator) has wider output range and better regulation than traditional regulators. Using a precision 50μA current source and a voltage follower, the output is adjustable from “0V” to close to VIN. Normally, the lowest output voltage is limited to the reference voltage. However, this new regulator has a constant loop gain independent of the output voltage giving excellent regulation at any output and allowing multiple regula- tors to be paralleled for higher output currents.
Operation
The LTC3600 is a current mode monolithic step-down buck regulator with excellent line and load transient responses. The 200kHz to 4MHz operating frequency can be set by a resistor
Figure 34.1 • High Efficiency, 12V to 3.3V 1MHz Step-Down Regulator with Programmable Reference Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00034-X
Applications
Figure 34.1 shows the complete LTC3600 schematic in a typical application that generates a 3.3V output voltage from 12V input. Figure 34.2 shows the load step transient response
using internal compensation and with external compensation.
Figure 34.3 shows the efficiency in CCM and DCM modes.
Furthermore, the LTC3600 can be easily configured to be a current source, as shown in Figure 34.4. By changing the RSET resistance from 0Ω to 3kΩ, the output current can be programmed from 0A to 1.5A.
Conclusion
The LTC3600 uses an accurate internal current source to gen- erate a programmable reference, expanding the range of out- put voltages. This unique feature gives the LTC3600 great flexibility, making it possible to dynamically change the out- put voltage, generate current sources, and parallel regulators for applications that would be difficult to implement using a standard DC/DC regulator configuration.
Figure 34.2 • 0A to 1.5A Load Step Response of the Figure 34.1 Schematic
Figure 34.3 • Efficiency of 12 V Input to 3.3 V Output Regulator in CCM and DCM Mode
35
Hua (Walker) Bai
42V, 2.5A synchronous step-down regulator with 2.5 μ A quiescent current
Short-circuit robustness using small inductors
The LT8610 and LT8611 are specifically designed to minimize solution size by allowing inductor size to be selected based on the output load requirements of the application, rather than the maximum current limits of the IC. During overload or short-circuit conditions, the LT8610 and LT8611 safely tol- erate operation with saturated inductors through the use of a high speed peak current mode architecture and a robust switch design. For example, an application that requires a maximum of 1.5A should use an inductor that has an RMS rating of >1.5A and a saturation current rating of >1.9A.
This flexibility allows the user to avoid oversize inductors for applications requiring less than maximum output current.
Current sense and monitoring with the LT8611
The LT8611 includes a flexible current control and moni- tor loop using the ISN, ISP, IMON and ICTRL pins. The ISP and ISN pins connect to an external sense resistor that
Introduction
The LT8610 and LT8611 are 42V, 2.5A synchronous step- down regulators that meet the stringent high input voltage and low output voltage requirements of automotive, industrial and communications applications. To minimize external com- ponents and solution size, the top and bottom power switches are integrated in a synchronous regulator topology, including internal compensation. The regulator consumes only 2.5μA quiescent current from the input source even while regulating the output.
High efficiency synchronous operation
Replacing an external Schottky diode with an internal synchronous power switch not only minimizes the solution size, but also increases efficiency and reduces power dissipa- tion. The efficiency improvement is significant in low output voltage applications where the voltage drop of the Schottky diode represents a relatively large portion of the output volt- age. Figure 35.1 shows a 12V to 3.3V circuit. Figure 35.2 shows the efficiency of this circuit reaching 94%, which is 5% to 10% higher than a comparable nonsynchronous circuit.
Figure 35.1 • LT8610 12V to 3.3V Application Achieves
High Efficiency Figure 35.2 • Efficiency of the 12V to 3.3V Application (Circuit
Shown in Figure 35.1) Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00035-1
may be in series with the input or output of the LT8611 or in series with other system currents. The current limit loop functions by limiting the LT8611 output current such that the voltage between the ISP and ISN pins does not exceed 50mV. The ICTRL pin allows the user to control this limit between 0mV and 50mV by applying 0V to 1V to the ICTRL pin. The IMON pin outputs a ground-referenced voltage that is 20ã(ISP − ISN), which allows easy monitoring and may be used as an input to an A/D.
The LT8611 current sense and monitoring functionality may be used to limit short-circuit current or to create con- stant-current, constant-voltage (CCCV) supplies. Figure 35.3 shows well controlled current during a short-circuit event.
The LT8611 can also be combined with a microcontroller with A/D and D/A to create sophisticated power systems.
Typical apps include maximum power point tracking (MPPT) for solar charging and programmable LED current source.
Wide input range operation at 2MHz
It is well known that higher switching frequencies allow for smaller solution sizes. In fact, a 2MHz switching frequency is often used in automotive applications to avoid the AM band and minimize solution footprint.
High switching frequencies, though, come with some trade-offs, including reduced ability to handle wide input voltage range commonly found in automotive and industrial environments. However, the LT8610 and LT8611 minimize these restrictions by allowing both high switching frequen- cies and high conversion ratios. This is due to their low mini- mum on-times (50ns typical) and low dropout, resulting in a wide input range, even at 2MHz. Figure 35.4 shows a 5V, 2A, 2MHz circuit that can accept 5.4V to 42V inputs. The circuit has a 2A output current limit.
Low dropout operation
As the input voltage decreases toward the programmed out- put voltage, the LT8610 and LT8611 maintain regulation by skipping switch-off times and decreasing the switching fre- quency up to a maximum duty cycle of 99.8%. If the input voltage decreases further, the output voltage remains 450mV below the input voltage (at 2A load). The boost capaci- tor is charged during dropout conditions, maintaining high efficiency. Figure 35.5 shows the dropout performance.
Conclusion
LT8610 and LT8611 are 42V, 2.5A synchronous step-down regulators that offer 2.5μA quiescent current, high efficiency, fault robustness and constant current (LT8611 only), constant voltage operation in small packages. This combination of fea- tures makes them ideal for the harsh environment commonly found in automotive and industrial applications.
Figure 35.3 • Short-Circuit Current is Well Regulated at 42V with the LT8611
Figure 35.4 • LT8611 Running at 2MHz Reduces Solution Size, Avoids AM Band, and Still Allows High Duty Cycle
Figure 35.5 • LT8610/LT8611 Dropout Performance
36
Goran Perica Victor Khasiev
Bootstrap biasing of high input voltage step-down controller increases converter efficiency
Figure 36.1 shows a block diagram for this scheme. The output can be directly connected to the EX T VCC pin of the chip as long as the output voltage is above 4.7V. However, extra circuitry (described in the following section) is required for outputs below 4.7V.
Voltage doubler for output voltages below 4.7V
When the controller’s output is below 4.7V, it must be stepped up to allow the built-in LDO to work. A simple volt- age doubler solves this problem as long as the output is higher than 2.5V. Below 2.5V output, a multivibrator-based circuit can be used.
Introduction
High voltage buck DC/DC controllers such as the LTC3890 (dual output) and LTC3891 (single output) are popular in automotive applications due to their extremely wide 4V to 60V input voltage range, eliminating the need for a snubber and voltage suppression circuitry. These controllers are also well suited for 48V telecom applications where no galvanic isolation is required.
In a typical application for these controllers, the IC’s sup- ply voltage (INTVCC) is provided by the on-chip LDO. This LDO produces 5V from input voltages up to 60V to bias con- trol circuitry and provide power FET gate drive. Although simple, this built-in biasing scheme can be inefficient. Power losses can be significant in applications where the input volt- age is consistently high, such as in 48V telecom applications.
Reducing the power losses in the bias conversion can increase efficiency and also reduce the controller case operating temperature.
Employing EXTVCC to improve efficiency
One of the attractive features of the LTC3890 and LTC3891 controllers is the external power input (EX T VCC). This is a second on-chip LDO, which can be used to bias the chip.
When the input voltage is consistently high, it is more effi- cient to produce the biasing voltage by stepping down the converter’s output voltage, which is fed into EXTVCC, rather than generating 5V INTVCC from the high input voltage.
Figure 36.1 • Block Diagram Showing External Bias
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00036-3
Figure 36.2 shows a simple, low cost solution for output voltages between 2.5V and 4.7V. This is a voltage doubler scheme based on small P-channel and N-channel MOSFETs, Q1 and Q2. The gates of these transistors are controlled by the bottom gate driver, BG of the controller. When BG is high, Q2 is on, Q1 is off and capacitor C1 charges from out- put voltage VOUT through D1. When BG is low, Q2 is off, Q1 is on and capacitor C1 delivers a voltage close to 2 ã VOUT to EXTVCC.
Figure 36.3 shows a solution for voltages below 2.5V. The circuit consists of an astable multivibrator based on transistors Q1 and Q2, and a boost based on N-channel Q3 and inductor L1. Q1 and Q2 are biased from INTVCC and output voltage
VOUT is stepped up to 5V, which feeds EXTVCC. The multi- vibrator frequency is set at 50kHz to minimize the EMI sig- nature. The pulse width is defined by the ratio of resistors R1 and R2, as per the following expressions:
Conclusion
The efficiency of high input voltage DC/DC controllers can be significantly improved by using the controller’s output voltage to power the IC, instead of allowing the internal LDO to pro- duce the bias voltage. For input voltages above 30V, efficiency improvements of 2% to 3% are realized when a voltage doubler circuit is used for a 3.3V at 5A output (see Figure 36.4).
Similar efficiency improvements are shown for a 1.8V at 7A converter with a multivibrator-based circuit.
R1 =
Tã(1−D) 0.7ãC1 R2 =
TãD 0.7ãC2 D =
EX T VCC−VOUT EX T VCC T =
1 f
Figure 36.2 • Voltage Doubler Allows External Bias from VOUT
in the Range of 2.5V to 4.7V
Figure 36.3 • Boost Controlled by Astable Multivibrator Is
Used for VOUT Lower than 2.5V Figure 36.4 • LTC3890/LTC3891 Efficiency Improvement