Switching Regulator Design: DC/DC Controllers
142
Mike Shriver
Dual controller provides 2μs step response and 92% efficiency for 1.5V rails
The LTC3838 accepts a wide input range, 4.5V to 38V, and can produce 0.6V to 5.5V outputs.
The remotely sensed VOUT1 has a voltage regulation accu- racy of 0.67%, from 0°C to 85°C, even with a voltage differ- ence of ±0.5V between local ground and remote ground. The current sense comparators are designed to sense the inductor current with either a sense resistor for high accuracy or with the inductor DCR directly for reduced power losses and circuit size.
Introduction
The LTC3838 is a dual output, dual phase buck control- ler that employs a controlled constant on-time, valley cur- rent mode architecture to provide fast load step response, high switching frequency and low duty cycle capability. The switching frequency range is 200kHz to 2MHz—its phase- locked loop keeps the frequency constant during steady- state operation and can be synchronized to an external clock.
Figure 142.1 • Dual Output, 1.5V/25A and 1.2V/25A Buck Converter Operating at FSW= 300kHz Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00142-3
1.5V/25A and 1.2V/25A buck converter
Figure 142.1 shows a dual 25A output buck converter syn- chronized to an external 300kHz clock. The controlled con- stant on-time valley current mode architecture allows the switch node pulses to temporarily compress when a 5A to 25A load step is applied to the 1.2V rail, resulting in a voltage undershoot of only 58mV (see Figure 142.2).
The full load efficiency for the 1.5V and 1.2V rails is 91.8% and 90.8%, respectively, as shown in Figure 142.3. The high efficiency is realized by the strong gate drivers, optimized dead time and DCR sensing.
Detect transient feature further speeds up transient response
An innovative feature of the LTC3838 is the load release transient detect feature. The DTR pin indirectly monitors the output voltage by looking at the AC-coupled ITH signal. If the inferred overshoot exceeds a user set value, the bottom FET turns off. This allows the inductor current to slew down at a faster rate, which in turn reduces the overshoot. Per Figure 142.4, a 32% reduction in the overshoot is realized on the 1.2V rail. Greater improvements occur at lower output voltages.
Conclusion
The LTC3838 is a dual output buck controller ideal for appli- cations that require a fast load step response, high switch- ing frequency, high efficiency and accurate output voltages.
Other features include selectable operating modes: forced continuous mode (FCM) for fixed frequency operation or dis- continuous mode (DCM) for higher efficiency at light load, programmable current limit thresholds, soft-start, rail tracking and individual PGOOD and RUN pins. The LTC3838 comes in a 5mm × 7mm QFN package or a thermally enhanced 38-lead TSSOP package.
Figure 142.2 • 20% to 100% Step Load Response of the 1.2V Rail at VIN= 12V, FSW= 300kHz, Mode = FCM
Figure 142.3 • Efficiency and Power Loss of the 1.5V/25A and 1.2V/25A Converter
Figure 142.4a • Implementation of the Detect Transient Feature on the 1.2V Rail
143
Ding Li
Dual DC/DC controller for DDR power with differential V DDQ sensing
and ± 50mA V TT reference
200kHz and 2MHz. It also features voltage-tracking soft-start, PGOOD and fault protection.
High efficiency, 4.5V to 14V input, dual output DDR power supply
Figure 143.1 shows a DDR3 power supply that operates from a 4.5V to 14V input. Figure 143.2 shows efficiency curves for discontinuous and forced continuous modes of operation.
Load-release transient detection
As output voltages drop, a major challenge for switching regu- lators is to limit the overshoot in VOUT during a load-release transient. The LTC3876 uses the DTR pin to monitor the
Introduction
The LTC3876 is a complete DDR power solution, compat- ible with DDR1, DDR2, DDR3 and DDR4 lower voltage standards. The IC includes VDDQ and VTT DC/DC control- lers and a precision linear VTT reference. A differential output sense amplifier and precision internal reference combine to offer an accurate VDDQ supply. The VTT controller tracks the precision VTTR linear reference with less than 20mV total error. The precision VTTR reference maintains 1.2% regula- tion accuracy, tracking one-half VDDQ over temperature for a
±50mA reference load.
The LTC3876 features controlled on-time, valley current mode control, allowing it to accept a wide 4.5V to 38V input range, while supporting VDDQ outputs from 1.0V to 2.5V, and VTT and VTTR outputs from 0.5V to 1.25V. Its phase-locked loop (PLL) can be synchronized to an external clock between
Figure 143.1 • 1.5V VDDQ/20A 0.75V VTT/10A DDR3 Power Supply Figure 143.2 • Efficiency of Circuit in Figure 143.1 (VDDQ= 1.5V, fSW= 400kHz, L = 470nH)
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00143-5
first derivative of the ITH voltage to detect load release tran- sients. Figure 143.3 shows how this pin is used for transient detection.
The two RITH resistors establish a voltage divider from INTVCC to SGND, and bias the DC voltage on the DTR pin (at steady-state load or ITH voltage) slightly above half of INTVCC. For a given CITH1, this divider does not change com- pensation performance as long as RITH1/RITH2 equals RITH
that would normally be used in conventional single-resistor OPTI-LOOP compensation.
The divider sets the RC time constant needed for the DTR duration. The DTR sensitivity can be adjusted by the DC bias voltage difference between DTR and half INTVCC. This dif- ference could be set as low as 100mV, as long as the ITH rip- ple voltage with DC load current does not trigger the DTR. If the load transient is fast enough that the DTR voltage drops below half of INTVCC, a load release event is detected. The bottom gate (BG) is turned off, so that the inductor current flows through the body diode in the bottom MOSFET.
Note that the DTR feature causes additional losses on the bottom MOSFET, due to its body diode conduction. The bot- tom FET temperature may be higher with a load of frequent and large load steps—an important design consideration. Test results show a 20°C increase when a continuous 100%-to- 50% load step pulse chain with 50% duty cycle and 100kHz frequency is applied to the output.
VTT reference (VTTR)
The linear VTT reference, VTTR, is specifically designed for large DDR memory systems by providing superior accuracy
recommended for most typical applications. It is suggested to use no less than 1μF and no more than 47μF on the VTTR output. The VTTR power comes from the VTTRVCC pin.
The typical recommended input VTTRVCC RC decoupling filter is 2.2μF and 1Ω. When VDDQSNS is tied to INTVCC, the VTTR linear reference output is 3-stated and VTTR becomes a reference input pin, with voltage from another LTC3876 in a multiphase application (see Figure 143.4).
VTT supply
The VTT supply reference is connected internally to the out- put of the VTTR VTT reference output. The VTT supply oper- ates in forced continuous mode and tracks VDDQ in start-up and in normal operation regardless of the MODE/PLLIN set- tings. In start-up, the VTT supply is enabled coincident with the VDDQ supply. Operating the VTT supply in forced contin- uous mode allows accurate tracking in start-up and under all operating conditions.
Conclusion
The LTC3876 is a complete high efficiency and high accuracy Figure 143.3 • Functional Diagram of DTR Connection
for Load Transient Detection
Figure 143.4 • Load Release Comparison
144
Jesus Rosales
Single resistor sets positive or
negative output for DC/DC converter
Adjustable/synchronizable switching frequency
It is often necessary to operate a converter at a particular fre- quency, especially if the converter is used in an RF commu- nications product that is sensitive to spectral noise in certain frequency bands. Also, if the area available for a converter is limited, operating at higher frequencies allows the use of tiny component sizes, reducing the real estate required and the output ripple. If power loss is a concern, switching at a lower frequency reduces switching losses, improving efficiency. The switching frequency can be set from 200kHz to 2.5MHz via a single resistor from the RT pin to ground. The device can also be synchronized to an external clock via the SYNC pin.
Soft-start and undervoltage lockout
To alleviate high inrush current levels during start-up, the LT3580 includes a soft-start feature which controls the ramp rate of the switch current by the use of a capacitor from SS to ground.
Introduction
Many electronic subsystems, such as VFD (vacuum fluorescent display), TFT-LCD, GPS or DSL applications, require more than just a simple step-down or step-up DC/
DC converter. They may require inverting, noninvert- ing converters or both. Designers usually resort to differ- ent regulator ICs to control various polarity outputs, thus increasing the inventory list. The LT3580 solves this prob- lem by controlling either positive or negative outputs using the same feedback configuration. It contains an integrated 2A, 42V switch and packs many popular features such as soft-start, adjustable frequency, synchronization and a wide input range into a small footprint. The LT3580 comes in an 8-pin 3mm × 3mm DFN or MSOP packages and can be used in multiple configurations such as boost, SEPIC, fly- back and Cuk topologies.
Sensing output voltage has never been easier
The LT3580 has a novel FB pin architecture that simplifies the design of inverting and noninverting topologies. Namely, there are two internal error amplifiers; one senses positive outputs and the other negative. Additionally, the LT3580 has integrated the ground side feedback resistor to minimize component count. To illustrate the benefits, notice how the schematics in Figures 144.1, 144.3 and 144.5 need only one feedback resistor.
A single sense resistor simply connects to the FB pin on one side and to the output on the other regardless of the out- put polarity, eliminating the confusion associated with positive or negative output sensing and simplifying the board layout.
A user decides the output polarity he needs, the topology he
wants to use and the LT3580 does the rest. Figure 144.1 • 3V–10V to 12V, 300mA Boost Converter
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00144-7
The SHDN pin in the LT3580 serves two purposes. Tying it high or low turns the converter on or off. In situations where the input supply is current limited, has a high source impedance or ramps up/down slowly, the SHDN pin can be configured to provide undervoltage lockout through a simple
Cuk converter
Figure 144.3 shows a schematic for a Cuk converter, which produces a negative output with no DC path to the source.
The output can be either higher or lower in amplitude than the input. The Cuk converter has output short-circuit protec- tion, which is made more robust by the frequency foldback feature in the LT3580. Figure 144.4 shows the efficiency graph for the Cuk converter in Figure 144.3 at a 5V input.
SEPIC converters
Figure 144.5 shows a SEPIC converter. A SEPIC converter is similar to the Cuk in that it can step up or step down the input; it offers output disconnect and short-circuit protection but produces a positive output. Figure 144.6 shows the switch waveform of the SEPIC converter during an output short-cir- cuit event. Notice how the switching frequency folds back to one-fourth of the regular frequency as soon as the output volt- age is shorted to ground. This feature enhances short-circuit performance for both Cuk and SEPIC converters.
Conclusion
Figure 144.3 • 5V–24V to −12V, 350mA Cuk Converter
Figure 144.5 • 9V–24V to 10.5V, 600mA SEPIC Converter Figure 144.2 • Efficiency for the Figure 144.1 Converter
at 4.2VIN
Figure 144.4 • Efficiency for the Figure 144.3 Converter at 5VIN
Figure 144.6 • Short-Circuit Event for the Figure 144.5 Converter at 24VIN
145
Tick Houk
Multiphase DC/DC controller pushes accuracy and bandwidth limits
In addition to high accuracy, the LTC3811’s low minimum on-time (typically 65ns) allows users to convert a 12V input to a 1V output at switching frequencies up to 750kHz, opti- mizing load transient response and reducing the solution size.
A dual output, 2-phase supply with differential remote sensing and inductor DCR sensing
Figure 145.1 illustrates a dual output supply using the LTC3811. The 1.5V, 15A output is regulated using the inte- grated differential remote sense amplifier and tracks the output of channel 1 during start-up. Both outputs use DCR
Introduction
Speed and accuracy do not always go hand-in-hand in DC/DC converter systems—that is, until now. The LTC3811 is a dual output, fixed frequency current mode DC/DC switching reg- ulator controller designed for one of today’s most demanding power supply applications: high current, low voltage processor core supplies.
With supply current requirements in excess of 100A and supply voltages as low as 1V, every milliohm of PCB resistance and every millivolt of IR drop count. The LTC3811 has an output voltage tolerance of ±0.5% over temperature, giving power supply designers unprecedented flexibility when mak- ing component value and board layout choices.
Figure 145.1 • Dual Output, 2-Phase Supply with Differential Remote Sensing and Inductor DCR Sensing Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00145-9
sensing in order to maximize efficiency and operate 180° out of phase in order to reduce the size of the input capacitor.
Figure 145.2 illustrates the load step response for channel 2.
Figure 145.3 illustrates low duty cycle waveforms for a 20V input, 1.2V output application.
For noise-sensitive applications, where the switching fre- quency needs to be synchronized to an external clock, the LTC3811 contains a PLL with an input range of 150kHz to 900kHz. In addition, the MODE/SYNC, PHASEMODE and CLKOUT pins allow multiple LTC3811s to be daisy- chained in order to produce a single high current output.
The LTC3811 can be configured for 2-, 3-, 4-, 6- or 12-phase operation, extending the load current range to beyond 200A.
A tried-and-true architecture
The fixed frequency, peak current mode control architec- ture was chosen for its excellent channel-to-channel current matching and its robust cycle-by-cycle current limit. Current sensing can be done using either a resistor in series with the inductor or by sensing the DCR of the inductor with an RC filter. This gives the user a choice between optimum control of the maximum inductor current and maximum efficiency.
In order to accommodate the use of low DCR induc- tors and still maintain good control over the maximum output current, the current sense voltage for each channel is programmable from 24mV to 85mV using the RNG pins.
The LTC3811 has a 4.5V to 30V input voltage range and is available in two package options: a 38-pin 5mm×7mm QFN and a 36-pin SSOP.
Load step improvement with voltage positioning
For single-output multiphase applications, the LTC3811 con- tains an amplifier for voltage positioning purposes. The cur- rent sense input voltages are converted to an output current by a multiple-input, single-output transconductance amplifier, so that an error voltage proportional to the load current can be introduced at the input of the differential amplifier. This transconductance amplifier allows the user to program an out- put load line, improving the DC and AC output accuracy in the presence of load steps. Figure 145.4 illustrates the load step response for a 2-phase, single output power supply using the LTC3811.
Conclusion
The LTC3811 is a versatile, high performance synchronous buck controller optimized for low voltage, high current sup- ply applications. With an output accuracy of ±0.5% and a remote sensing differential amplifier, it represents a new benchmark for DC/DC converters. It can easily be configured for either single or dual output supplies, inductor DCR sens- ing or a sense resistor, and it takes advantage of Linear Tech- nology’s proprietary PolyPhase current sharing architecture.
The combination of a very low minimum on-time and a fixed frequency peak current mode control architecture no longer force the power supply designer to trade off performance for protection.
Figure 145.2 • Load Step Response
146
David Chen
2-phase DC/DC controller makes fast, efficient and compact
power supplies
simultaneously achieve all three. Figure 146.1 show the LTC3708 in a compact, high efficiency dual-output power supply that has excellent transient response.
High efficiency, fast transient response and small size are often at odds in power supply designs. Fortunately, the 2-phase LTC3708 PWM controller makes it possible to
Figure 146.1a • Dual Power Supply with VOUT2 Tracking VOUT1
Figure 146.1b • Load Step on VOUT1 Figure 146.1c • Load Step on VOUT2 Figure 146.1d • Efficiency Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00146-0
entire output range at both ramp-up and ramp-down transi- tions (Figure 146.2). Table 146.1 explains these features in more detail.
The LTC3708 hosts other features that make it an ideal controller for high performance power management applications. The input voltage can be as high as 36V and the output regulates down to 0.6V. The current limit is user programmable to accommodate the variation of MOSFET RDS(ON) values. Protection functions include cycle-to-cycle current limit, an overvoltage crowbar and an optional short- circuit timer. When either output is out of regulation, a Power Good indicator falls low after 100μs masking—a tech- nique that prevents transitory glitches and noise from falsely triggering system protection.
Figure 146.2a • Coincident Tracking (R1 = R2 = 12.1k) Figure 146.2b • Ratiometric Tracking (R1 = 19.1k, R2 = 6.04k)
Table 146.1 LTC3708 Design Features
FEATURES FUNCTIONS BENEFITS
Output Tracking Various Modes of Tracking and Sequencing can be Programmed: Coincident, Ratiometric, etc
Simplifies the Timing Design of Multiple Supply Systems
No RSENSE Output Current is Sensed through the Synchronous
MOSFET
Improves the Efficiency of Low Output Applications (VOUT ≤ 5V)
2-Phase Operation Two Output Channels Operate at Same Frequency with 180° Phase Shift
Reduces Input RMS Current and EMI Noises; Minimizes Input Capacitance
Constant On-Time Control Architecture
The Top MOSFETs can be Turned on Immediately Without Clock Latency
Expedites Transient Response and Reduces Output Capacitance
Minimum tON < 85ns This is the Minimum Duration that the Top MOSFETs Need to be On
Expedites Transient Response and Enables High Frequency Designs
The device uses a combination of design features to make all of this possible. High efficiency is attained through a comb- ination of features: a No RSENSE current sensing technique, 2-phase operation mode, onboard high current synchronous MOSFET gate drivers and a pulse skipping function that reduces the switching and gate driving losses at light loads.
Fast transient response comes from the constant on-time con- trol architecture with a very narrow pulse width (minimum tON < 85ns). Compact solution size is achieved because of the LTC3708’s high frequency capability, minimized input and output capacitance requirements and high levels of circuit integration. All control circuitry and MOSFET gate drivers are incorporated within a small 5mm × 5mm QFN package.
The LTC3708 also provides accurate voltage tracking over the
147
Wei Chen
High performance 3-phase power supply delivers 65A and high
efficiency over the entire load range
energy for maximum battery run-time is also a priority.
Unfortunately, traditional PolyPhase converters, although exceptional performers at heavy loads, do not yield compara- ble efficiencies at light load. Linear Technology Corporation offers a solution to this problem with a new family of Poly- Phase controllers that allow the design of converters that are efficient over the entire CPU load range.
These new 3-phase controllers, the LTC3730, LTC3731 and LTC3732, operate efficiently at both heavy loads and light loads. These new controllers introduce Stage Shedding operation to improve light load efficiency. Like LTC’s 2-phase controllers, these new 3-phase controllers provide true
Introduction
CPUs used in notebook computers and other mobile applica- tions now draw more than 30A of current and may draw as much as 65A in the near future. For these heavy loads, high efficiency power supplies are required to protect the system from excessive thermal stress. With this in mind, PolyPhase switching DC/DC converters have become the standard for CPU power supplies because they are extremely efficient at these heavy loads. Nevertheless, for portable applications that spend much of the time in sleep or standby modes, light load efficiency has also grown in importance, where conserving
Figure 147.1 • Schematic Diagram of a 3-Phase LTC3732 65A VRM9.x Power Supply Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00147-2