Linear Regulator Design
163
Marty Merchant
High voltage inverting charge pump produces low noise positive and negative supplies
LDO−. The charge pump frequency can be adjusted between 50kHz to 500kHz by a single external resistor. The MODE pin is used to select between a high efficiency Burst Mode operation or constant frequency mode to satisfy low noise requirements.
Constant frequency mode
A single resistor at the RT pin sets the constant operating fre- quency of the charge pump. If the RT pin is grounded, the charge pump operates at 500kHz, where the open-loop out- put resistance (ROL) and the output ripple are optimized, allowing maximum available output power with only a few millivolts peak-to-peak output ripple.
Light load efficiency can be increased by reducing the oper- ating frequency, as shown in Figure 163.2, but at the expense of increased output ripple. The lower operating frequency produces a higher effective open-loop resistance (ROL), but the reduced switching rate also reduces the input current, resulting in increased efficiency at light loads. Furthermore, at relatively heavy loads, the increased ROL reduces the effective
Introduction
Dual-polarity supplies are commonly needed to operate electronics such as op amps, drivers, or sensors, but there is rarely a dual-polarity supply available at the point of load.
The LTC3260 is an inverting charge pump (inductorless) DC/
DC converter with dual low noise LDO regulators that can produce positive and negative supplies from a single wide input (4.5V to 32V) power source. It can switch between high efficiency Burst Mode operation and low noise constant frequency mode, making it attractive for both portable and noise-sensitive applications. The LTC3260 is available in a low profile 3mm × 4mm DFN or a thermally enhanced 16-lead MSOP, yielding compact solutions with minimal external components. Figure 163.1 shows a typical 12V to ±5V appli- cation featuring the LTC3260.
Inverting charge pump
The LTC3260 can supply up to 100mA from the inverted input voltage at its charge pump output, VOUT. VOUT also serves as the input supply to a negative LDO regulator,
Figure 163.1 • Typical 12V to ±5V Supply Figure 163.2 • LTC3260 VIN to VOUT and VIN to LDO– Efficiency vs Frequency for the Circuit in Figure 163.1
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00163-0
difference between VOUT and LDO− —decreasing the power dissipation in the negative LDO. The cumulative result is higher overall efficiency with high input voltages and/or light loads.
Reducing the frequency increases the output ripple as shown by the expression below and in Figure 163.3.
where tOFF=
1
fOSC −1às
In general, constant frequency mode is suitable for applica- tions requiring low output ripple even at light loads, but fur- ther gains in light load efficiency can be gained by using Burst Mode operation, described below.
Burst Mode operation
Figure 163.4 shows the light-load efficiency of the charge pump in Burst Mode operation. Burst Mode operation increases the output ripple over constant frequency mode, but the increase in ripple is only a small percentage of VIN, as shown in Figure 163.5.
VRIPPLE(PK
−PK)≈
IOUTãtOFF COUT
Burst Mode operation is implemented by charging VOUT
close to −VIN. The LTC3260 then enters a low quiescent current sleep state, about 100μA with both LDO regulators enabled, until the burst hysteresis is reached. Then the charge pump wakes up and the cycle repeats. The average VOUT is approximately −0.94VIN. As the load increases, the charge pump runs more often to keep the output in regulation. If the load increases enough, the charge pump automatically switches to constant frequency mode in order to maintain regulation.
Dual LDOs
Both of the LTC3260’s LDOs—the positive LDO regulator supplied from VIN, and the negative LDO regulator supplied from VOUT—are capable of supporting 50mA loads. Each LDO has a dropout voltage of 300mV with a 50mA output and has an adjust pin, allowing the output voltage to be set by a simple resistor divider. The LDO regulators can be individually ena- bled. The EN− pin enables both the inverting charge pump and LDO−. When both regulators are disabled, the part shuts down with only 2μA of quiescent current. The LDO references can be filtered by adding a capacitor on each of the bypass pins to further reduce noise at the LDO regulator outputs.
Conclusion
The LTC3260 produces low noise positive and negative supplies from a single positive power source. The LTC3260 features optional Burst Mode operation for light-load efficiency in battery-powered devices, or low noise constant frequency mode Figure 163.3 • VOUT Constant Frequency Ripple Comparison
at 500kHz, 200kHz and 50kHz at 20mA Load
Figure 163.5 • VOUT Ripple in Burst Mode Operation
164
Todd Owen
80V linear regulator is micropower
LT3010 is offered in both adjustable and fixed 5V output versions.
Figure 164.1 shows a typical application for the LT3010, illustrating how easy it is to design a low current supply running from a high voltage rail. The only external compo- nents required are input and output bypass capacitors, and the input bypass is not required if the device is located close enough to the main supply bypass capacitor. Internal fre- quency compensation on the LT3010 stabilizes the output for a wide range of capacitors. A minimum of 1μF output capaci- tance is required for stability, and almost any type of output capacitor can be used. Small ceramic capacitors with low ESR can be used without requiring extra series resistance as is sometimes required with other regulators.
Protection features are incorporated in the LT3010, safe- guarding itself and sensitive load circuits. If the input voltage reverses—from a backwards battery or fault on the line—no current flows into the device and no negative voltage is seen at the load. No external protection diodes are necessary when using the LT3010. With a reverse voltage from output to input, the LT3010 acts as though it has a diode in series with its out- put and prevents reverse current flow. For dual supply applica- tions where the regulator load is returned to a negative supply, the output can be pulled below ground by several volts while still allowing the device to start and operate. The LT3010 also provides current limiting and thermal limiting features.
Introduction
Industrial, automotive and telecom applications pose tough design challenges due to harsh operating environments and large voltage transients on already high (12V to 48V) rails.
Some switching power supplies are robust enough to provide localized low voltage/high current power from high voltage input rails, but switch-mode supplies are overly complex for low power keep-alive circuits that typically consume only a few milliamps of current. For most of these low power cir- cuits, a wide input range linear regulator is an ideal solution.
Introducing the LT3010 high voltage LDO
The LT3010 is a high voltage, micropower, low dropout linear regulator in a thermally enhanced 8-lead MSOP package. It can provide up to 50mA of output current from input sup- plies ranging from 3V to 80V. At 50mA of output current, dropout voltage on the LT3010 is only 300mV. The LT3010 operates normally on only 30μA of quiescent current. It can also be put into a low power shutdown state by pulling the SHDN pin low, bringing quiescent current down to just 1μA. For standard operation, the SHDN pin can be pulled as high as 80V (regardless of input voltage) or left floating. The
Figure 164.1 • LT3010 Typical Application
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00164-2
A versatile and rugged regulator
The LT3010 provides an optimum solution for harsh condi- tions. Long wire runs for high voltage rails can have transient voltage spikes as loads are switched on and off. Existing auto- motive applications run from 12V while some new systems are transitioning to 42V, but both can have transients greater than 60V. Telecom applications typically run from a 48V sup- ply that may extend as high as 72V. Industrial applications can span even wider input voltage ranges. Reverse input or reverse voltages from output to input are also possible.
Figure 164.2 shows a typical automotive or telecom appli- cation for the LT3010 that takes advantage of its micropower quiescent current. In an automotive application, this might be an always-on circuit that runs whether the ignition is on or not—common for many modern automotive subsystems. The total current consumed by all always-on subsystems must be no more than several milliamps to prevent excessive battery drain. The LT3010 can also be placed into the shutdown state whenever the subsystem is not needed.
Other features of the LT3010 make it ideal for automotive applications. The small size of the LT3010 and its associated external components keep board space and height to a mini- mum. Power connections to the LT3010 can come directly from a battery because input transients will not damage the regulator or the load. Even reversed battery connections pre- sent no worry since the LT3010 prevents reverse current from flowing and damaging sensitive load circuits. Above all, the LT3010’s input limit of 80V saves design time and costs by allowing subsystems to migrate directly from 12V to 42V (or anywhere in between) without redesign.
For telecom applications, the 48V rail powers a keep-alive circuit for monitoring or other purposes. The quiescent cur- rent is important, especially when battery back-up must kick in to keep the output alive when a fault occurs on the input.
Should a fault on the 48V rail occur, the battery back-up takes over and the internal protection of the LT3010 prevents current flow from the output back to the input, removing the need for protection diodes. Component size can still be a con- cern, depending upon application constraints. The 48V input rail in telecom applications can have transient voltages as high as 72V. The LT3010 can handle these transients without the need for preregulation or protection devices. Finally, the ther- mally enhanced 8-lead MSOP package provides a very com- pact, thermally efficient solution footprint. With a θJA of only 40°C/W, it is able to dissipate heat from high power transients found in these applications.
Conclusion
The LT3010 offers exceptional performance in a small pack- age. It can supply low power from high voltage rails in appli- cations that previously required external pre-regulation schemes or complex switching supplies. Low quiescent cur- rent minimizes the power consumption that can be dropped even further by placing the part into shutdown. Stable out- put voltage is available with a wide range of output capacitors, including small ceramics. Internal protection circuitry in the LT3010 eliminates the need for external protection diodes.
The thermally enhanced 8-lead MSOP package provides low thermal resistance making it easy to design the part into harsh environments.
Figure 164.2 • LT3010 Automotive or Telecom Application
165
Tom Gross
Very low dropout (VLDO) linear
regulators supply low voltage outputs
VLDO circuit descriptions
The LT1580 monolithic low dropout linear regulator, the LT1573 LDO PNP driver and the LT1575 LDO MOSFET controller/driver are devices well suited to deliver lower output voltages from a supply of 1.8V or lower. Each device offers excellent line/load regulation, temperature perfor- mance and transient load step response.
Figure 165.1 illustrates the LT1580 delivering 1.3V at 3A (maximum), from a 1.8V input supply. This particular config- uration requires a higher voltage supply to bias the control cir- cuitry. Specifically, the control voltage must be 1V above the
Introduction
With each new generation of computing systems, total power continues to increase while system voltages fall. CPU core voltages and logic supplies below 1.8V are now commonplace.
Power supplies must not only regulate low output voltages but they must also operate from low input voltages. A low voltage, low dropout linear regulator is an attractive conversion option for applications where output currents are in the several ampere range. Component count and cost are low in compari- son to switching regulator solutions, and with low input-to- output voltage differentials, efficiencies are comparable.
Figure 165.1 • LT1580 Fast Transient Response Low Dropout Linear Regulator
Figure 165.3 • LT1573 Low Dropout Linear Regulator with Low Output Voltage
Figure 165.2 • Load Transient Response for Circuit in Figure 165.1
Figure 165.4 • 3A Load Transient Response for Circuit in Figure 165.3
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00165-4
output voltage for proper operation or 2.3V minimum in this case. In current systems, a 2.5V supply is typically available and is used here as the control supply voltage. The dropout voltage from input-to-output is 300mV. The load step tran- sient response is shown in Figure 165.2. With a 3A load step, the output voltage deviation is less than 50mV and the output voltage recovers within 20 microseconds.
Figure 165.3 shows a circuit where the regulated output voltage is less than the feedback reference voltage. The cir- cuit consists of an LT1573 linear regulator generating 1.2V at 3A from a 1.8V supply. As in the previous circuit, a sec- ond 3.3V input voltage is required for the control circuitry. A resistor divider connected from the 1.8V supply to the output biases the feedback pin above the regulated output voltage by 65mV. This allows the feedback pin to regulate at 1.265V with a 1.2V output. R3’s value is chosen such that Q1 must be biased in order for the feedback pin to reach its regulated voltage. This method of generating the feedback voltage is acceptable when the input voltage is regulated. If necessary, an external voltage reference can be used to acquire a tighter output voltage tolerance. Figure165.4 shows the transient response for a 3A load step. Like the previous circuit, the minimum input voltage is 1.6V.
A linear regulator based on the LT1575 MOSFET driver can handle higher output power and very low dropout requirements. Figure 165.5 shows the LT1575 controller driving an external N-channel MOSFET. The regulator converts 1.8V to 1.5V, capable of delivering 4A maximum, using a logic-level Siliconix Si4410 MOSFET as the pass element. The LT1613 boost converter, which is able to operate with input voltages down to 1.1V, generates the
appropriate gate drive for the MOSFET. Note that the input capacitors used, Panasonic SP capacitors (part number EEFUE0E221R), were chosen because they represent a typi- cal output capacitor network of microprocessor power sup- plies. High frequency, low ESR tantalum capacitors, such as AVX TPS capacitors can be substituted for these capacitors.
Figure165.6 depicts the LT1575 load step transient response of less than 50mV output voltage deviation and under 100μs response. The efficiency of this linear regulator is 83%, primarily due to the low input-to-output voltage dif- ferential. An external MOSFET with lower RDS(ON) can make for even lower dropout performance and potentially higher efficiency. The LT1575 linear regulator controller offers the lowest dropout voltage performance of any commercially available linear regulator. For instance, the dropout voltage for this circuit is less than 100mV.
Conclusion
Low output voltage, low dropout voltage linear regulators are practical alternatives to switching regulators in the current and future generations of computer systems. All the circuits described above offer viable solutions for the power supply designer. For systems with bus voltages less than 2.5V, the LT1580 linear regulator circuit provides the necessary power conversion with the fewest external components. The LT1573 driver allows the use of a high current PNP pass transistor.
The LT1575 linear regulator combines very low dropout voltage performance with high output current capability.
166
Todd Owen Jim Williams
Lowest noise SOT-23 LDOs have 20μA quiescent current,
20μV RMS noise
Telecom and instrumentation applications often require a low noise voltage regulator. Frequently this requirement coincides with the need for low regulator dropout and small quiescent current. LTC recently introduced a fam- ily of devices to address this problem. Table 166.1 shows a variety of packages, power ranges and features in three basic regulator types. The SOT-23 packaged LT1761 has only 20μVRMS noise with 300mV dropout at 100mA. Qui- escent current is only 20μA.
Applying the regulators
Applying the regulators is simple. Figure 166.1 shows a minimum parts count, 3.3V output design. This circuit appears similar to conventional approaches with a notable exception: a bypass pin (BYP) is returned to the output via a 0.01μF capacitor. This path filters the internal reference’s output, minimizing regulator output noise. It is the key to the 20μVRMS noise performance. A shutdown pin (SHDN), when pulled low, turns off the regulator output while keeping
Figure 166.1 • Applying the Low Noise, Low Dropout, Micropower Regulator. Bypass Pin and Associated Capacitor Are Key to Low Noise Performance
Table 166.1 Low Noise LDO Family Short-Form Specifications. Quiescent Current Scales with Output Current Capability, Although Noise Performance Remains Constant
REGULATOR TYPE
OUTPUT CURRENT
RMS NOISE (10Hz TO 100kHz) CBYP=0.01μF
PACKAGE OPTIONS
FEATURES QUIESCENT
CURRENT
SHUTDOWN CURRENT
LT1761 100mA 20μV SOT-23 Shutdown, Reference Bypass,
Adjustable Output. SOT-23 Package Mandates Selecting Any Two Features
20μA <1μA
LT1762 150mA 20μV MS8 Shutdown, Reference Bypass,
Adjustable Output
25μA <1μA
LT1763 500mA 20μV SO-8 Shutdown, Reference Bypass,
Adjustable Output
30μA <1μA
current drain inside 1μA. Dropout characteristics appear in Figure 166.2. Dropout scales with output current, falling to less than 100mV at low currents.
Noise performance
Noise performance is displayed in Figure 166.3. This meas- urement was taken in a 10Hz to 100kHz bandwidth with a
“brick wall” multipole filter.1 The photo’s trace, applied to a
Note 1: Noise measurement and specification of regulators requires care and will be comprehensively treated in a forthcoming LTC Application Note.
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00166-6
thermally responding RMS voltmeter, contains less than 20μVRMS noise. Figure 166.4 shows noise in the frequency domain with noise power falling with increasing frequency.
Other advantages
The LT1761 family is stable (no output oscillation) even when used with low ESR ceramic output capacitors. This is in stark Figure 166.3 • LT1761 Output Voltage Noise in a 10Hz to 100kHz Bandwidth. 20μVRMS Noise Is the Lowest Available in an LDO
Figure 166.4 • Output Noise Spectral Density for Figure 166.1’s Circuit. Curves for Three Output Versions Show Dispersion Below 200Hz
Figure 166.5 • Transient Response with No Noise Bypass Capacitor
Figure 166.2 • Figure 166.1’s Dropout Voltage at Various Currents
Conclusion
These devices provide the lowest available output noise in a low dropout regulator without compromising other param- eters. Their performance, ease of use and versatility allow use in a variety of noise-sensitive applications.
167
Dave Dwelley Gary Maulding
High efficiency linear and switching solutions for splitting a digital supply
The LT1118 requires only two external components (Figure 167.2) and features a DC output impedance below 0.1Ω under all loading conditions, far better than any prac- tical resistor divider solution. The LT1118 draws only enough supply current to meet the demands of the load at the split supply, providing nearly 50% power efficiency over a wide range of load currents (Figure 167.3). Load transient response is excellent, with less than 5μs recovery time from a ±400mA current load step (Figure 167.4). At low current levels, the LT1118 is the optimum solution for splitting a digital supply.
It can be inconvenient to generate a split supply in a typical digital system. The classic solution is to use a pair of resis- tors between 5V and GND to create a 2.5V “ground” for analog circuitry (Figure 167.1). Unfortunately, the result- ant “ground” has a painfully high impedance and the resis- tors draw a large amount of supply current. The output can be buffered with an op amp to lower the impedance, but a specialized op amp is required to handle any significant bypass capacitance at the output. This Design Note presents two alternate methods of creating a split supply that can provide good transient response while conserving supply current.
The LT1118 is a specialized linear regulator designed to source or sink current as necessary to keep its output in regu- lation. It can handle output capacitors of arbitrarily large size, improving output transient response. Available with a fixed 2.5V output (ideal for splitting 5V supplies), it draws only 600μA quiescent current typically and can source 800mA or sink 400mA, enough to satisfy most analog subsystems.
Figure 167.1 • Resistor Divider Supply Splitter
Figure 167.2 • LT1118-2.5 Supply Splitter
Figure 167.3 • Effiency vs Load Current for Linear and Switching Circuits
Figure 167.4 • LT1118 Transient Response Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00167-8