DESIGN FEATURES L 36V Dual 1.4A Monolithic Step-Down Converter has Start-Up Tracking and by Keith Szolusha Sequencing Introduction The LT3508 simplifies the design of dual output, wide-input-range power converters—especially those that require power supply tracking and sequencing. It is a dual output current mode PWM step-down DC/DC converter with internal power switches capable of generating a pair of 1.4A outputs. Its wide 3.6V to 36V input range makes it suitable for regulating power from a wide variety of sources, including automotive batteries, 24V industrial supplies and unregulated wall adaptors. Both converters are synchronized to a single oscillator programmable from 250kHz up to 2.5MHz and run with opposite phases, reducing input ripple current. The high operating frequency allows the use of small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. Each regulator has independent tracking and soft-start circuits and generates a power good signal when its output is in regulation, making power supply sequencing and VIN 6V TO 36V 95 SHDN 1N4448W VIN1 BOOST1 VIN2 80 75 65 0 0.5 1 LOAD CURRENT (A) Figure 2. Efficiency for circuit of Figure 1 1N4448W 10µH SW2 LT3508EFE B240A 35.7k FB1 VC1 VC2 TRACK/SS1 PG1 RT/SYNC 1000pF B240A CDRH6D28 56.2k 10µF 10V OUT2 5V 1.4A FB2 TRACK/SS2 20k 1000pF 1.5 0.22µF SW1 CDRH5D28 1500pF VOUT1 = 3.3V 85 BOOST2 0.22µF 11.5k VOUT2 = 5V 70 6.2µH 10µF 6.3V VIN = 12V 90 10µF 50V SHDN OUT1 3.3V 1.4A interfacing with microcontrollers and DSPs easy. Cycle-by-cycle current limit, frequency foldback and thermal shutdown provide protection against shorted outputs, and soft-start eliminates input current surge during start-up. The low current (<2µA) shutdown mode enables easy power management in battery-powered systems. EFFICIENCY (%) The latest DSPs and microcontrollers found in automotive electronics, industrial supplies, and even walltransformers typically require power supplies with output voltages of both 1.8V and 3.3V and output current capability of 1A or greater. DSL and cable modems also require multiple supplies, usually a combination of a single 5V supply rail and either a 3.3V or 1.8V rail. PCI Express and motherboard interconnect devices supply 3.3V or 5V in addition to a 12V intermediary source. In all of these cases, the supplies must follow a specific start-up sequence or track each other to avoid system latch up or worse. One common challenge in these applications is producing well-regulated outputs from wide ranging inputs. For instance, a 12V automotive battery produces a voltage range from a low of 4V to a high of 36V. 24V industrial supplies and rectified 12V wall transformers produce similarly wide voltage ranges. 52.3k PG2 43k 10.7k GND 330pF f = 700kHz Figure 1. Dual 1.4A monolithic step-down converter with 3.3V and 5V outputs Linear Technology Magazine • March 2007 21 L DESIGN FEATURES VOUT2 200mV/DIV VOUT1 200mV/DIV ILOAD1 500mA/DIV ILOAD2 500mA/DIV VIN = 6.8V 20µs/DIV VOUT2 = 5V LOAD STEP = 700mA TO 1400mA VIN = 12V 20µs/DIV VOUT1 = 3.3V LOAD STEP = 700mA TO 1400mA (a) (b) Figure 3. Dual step-down 5V (a) and 3.3V (b) output voltage transient response VIN 5V TO 16V 4.7µF 25V SHDN SHDN 1N4448W VIN1 VIN2 BOOST1 0.1µF 0.1µF OUT1 1.8V 1A 1N4448W BOOST2 3.0µH 22µF 6.3V 4.7µH CDRH3D18 SW1 SW2 DFLS130 LT3508EUF CDRH5D18C DFLS130 12.4k 35.7k FB1 10.0k FB2 VC1 VC2 TRACK/SS1 PG1 TRACK/SS2 9.1k RT/SYNC 1200pF 1000pF 1000pF 10µF 6.3V OUT2 3.3V 1A PG2 11k 11.5k GND 16.9k 680pF f = 1.6MHz Figure 4. Small dual step-down 1.8V and 3.3V schematic with output sequencing Versatility Comes from Independent Control of Two 1.4A Channels Each channel has its own power good, track/soft-start and, unlike most dual channel converters, each has its own VIN pin (more about this below). The boost pin for each channel can be tied to the higher of the two outputs, one to each output (if the channels are turned on and off separately), the input, or an external source. The boost pin voltage must be at least 3V above the switch pin voltage for saturation of the internal power switch. Individual track/soft-start and power good pins offer a variety of supply tracking and sequencing options. The channels can track each other coincidentally or ratiometrically. The power good pins can be used for sequencing the two channels or 22 simply interfacing with an external microcontroller. The unique, separate VIN pins for each channel offer uncommon design flexibility. For instance, the converter can satisfy high VIN/VOUT ratio applications that might be otherwise limited by a single converter’s typical minimum duty cycle constraints. Simply cascade the two converters by attaching the output of one channel to the input of the other channel. This allows the input voltage to be twice as high for a given output voltage and switching frequency without violating minimum duty cycle constraints. In some cases, VOUT1 500mV/DIV VIN 2V/DIV VOUT2 500mV/DIV VTRACK/SS2 500mV/DIV VIN1 = VIN2 = 12V VOUT1 = 1.8V VOUT2 = 3.3V ILOAD1 = ILOAD2 = 1A 500µs/DIV Figure 5. Dual step-down 1.8V and 3.3V start-up with output sequencing Linear Technology Magazine • March 2007 DESIGN FEATURES L VIN 8.5V TO 28V CIN1 2.2µF 35V CIN2 1µF 10V SHDN SHDN 1N4448W VIN1 VIN2 BOOST1 1N4448W BOOST2 0.1µF COUT1 4.7µF 10V 0.1µF 33µH 7.7V 2.4µH SW1 CDRH3D18LP SW2 LT3508EUF DFLS130 86.6k FB1 10.0k 1% 1000pF 12.4k VC2 TRACK/SS1 PG1 OUT2 1.8V 1.4A PG2 10.0k 18k GND 680pF RT 9.76k 2200pF COUT2 22µF 6.3V FB2 RT/SYNC 2200pF DFLS220L VC1 TRACK/SS2 30k CDRH3D14/HP f = 2.2MHz Figure 6. 2.2MHz 28V to 1.8V step-down with cascaded channels and output sequencing VIN 2V/DIV VOUT1 2V/DIV VOUT2 500mV/DIV VTRACK/SS2 500mV/DIV VIN1 = 12V VIN2 = VOUT1 = 7.7V VOUT2 = 1.8V ILOAD2 = 1.4A 500µs/DIV Figure 7. 2.2MHz 28V to 1.8V step-down start-up with output sequencing the separate VIN pins also allow the two channels to be run from two separate current-limited sources that may not have enough power alone to provide full power to both channels’ outputs. The LT3508’s two channels run 180° out of phase to minimize input current ripple and voltage ripple, thus limiting EMI and reducing the required size of the input capacitor. High VIN, Low VOUT and Adjustable Switching Frequency The wide input range of 3.6V to 36V makes the LT3508 suitable for regulating power from a wide variety of sources, including automotive batteries, 24V industrial supplies and unregulated wall adaptors. The operating frequency for the converters can be programmed by a single resistor Linear Technology Magazine • March 2007 or synchronized to an external clock ranging from 250kHz to 2.5MHz. High operating frequency allows the use of VIN 12V C1 4.7µF 40.2k OUT1 5V 0.9A VIN1 C3 0.1µF 6.8µH BOOST1 C6 10µF BOOST2 SW1 D3 52.3k 10.0k VIN2 C2 4.7µF D2 VIN2 3.3V SHDN D1 14.7k small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. However, selecting SW2 100pF VC1 VC2 TRACK/SS1 TRACK/SS2 PG1 PG2 C1 TO C6: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B140 D4: DIODES INC. B120 47k 33.2k C5 47µF 15.0k RT/SYNC 0.1µF OUT2 1.8V 1.4A 18.7k FB2 GND 3.3µH D4 LT3508 FB1 43k C4 0.1µF 100k 330pF fSW = 1MHz POWER GOOD Figure 8. PCI Express power supply with separate inputs 23 L DESIGN FEATURES a low operating frequency makes it possible to produce high input voltage, low output voltage applications by reducing the duty cycle. The LT3508’s low minimum switch on time of 130ns offers the benefits of high frequency even in high input-output ratio applications. For instance, a frequency of 700kHz is low enough to provide 6V to 36V input voltage range for both 5V and 3.3V outputs at full 1.4A load current (see Figure 1). The output voltage for the LT3508 can be set as low as the 0.8V refer- ence voltage. With 130ns minimum on-time, the maximum input voltage is calculated by: VIN(MAX ) = ( VOUT + VF ) t ON(MIN) • fOSC – VF + VSW VF is the forward voltage of the catch diode, VSW is the internal switch saturation voltage, and fOSC is the oscillator frequency. For 36VIN to 3.3VOUT, fOSC must be below 790kHz. To achieve 36VIN to 1.8VOUT, fOSC must be 470kHz or less. Likewise, a simple 12V to Independent Start-Up Ratiometric Start-Up Coincident Start-Up VOUT1 VOUT1 VOUT2 VOUT1 VOUT2 1V/DIV VOUT2 1V/DIV 1V/DIV 20ms/DIV 0.1µF TRACK/SS1 VOUT1 20ms/DIV 5V 0.22µF TRACK/SS2 VOUT2 20ms/DIV 5V TRACK/SS1 VOUT1 LT3508 0.047µF 3.3V step-down ratio is possible with a switching frequency of 2.3MHz. An application converting 12VIN to 5VOUT and 3.3VOUT can take advantage of a high switching frequency of 2.2MHz and remain above the AM band for automotive electronics. In cases where both a high switching frequency and a high step down ratio are required (as in the case of an automotive power supply that requires a 2.2MHz switching frequency to keep interference outside of the AM band), a cascaded solution can 0.1µF LT3508 3.3V 5V TRACK/SS1 VOUT1 LT3508 3.3V TRACK/SS2 VOUT2 3.3V TRACK/SS2 VOUT2 R1 28.7k (9a) R2 10.0k (9b) Output Sequencing (9c) Controlled Power Up and Down VOUT1 VOUT1 VOUT2 VOUT2 1V/DIV 1V/DIV EXTERNAL SOURCE 20ms/DIV 0.1µF 20ms/DIV TRACK/SS1 VOUT1 5V EXTERNAL SOURCE LT3508 PG1 0.047µF TRACK/SS2 VOUT2 + – 3.3V TRACK/SS1 VOUT1 5V LT3508 TRACK/SS2 VOUT2 3.3V R1 28.7k R2 10.0k (9d) (9e) Figure 9. Tracking and soft-start options 24 Linear Technology Magazine • March 2007 DESIGN FEATURES L be used. As shown in Figure 6, 28VIN to 1.8VIN is possible if one output is set for 7.7V and tied to the VIN pin of the 1.8V channel. Higher switching frequency reduces inductor and capacitor sizes and achieves faster transient response. Fast Transient Response The current mode architecture of the LT3508 control loop yields fast transient response with small, ceramic output capacitors and simple compensation. Small 0805 and 1206 case size 10µF and 22µF 6.3V ceramic output capacitors are typical for up to 1.4A output applications. High temperature coefficient capacitors such as X5R and X7R ceramics are recommended for most designs. Figure 3 shows the transient response for a typical LT3508 application. Transient response ripple is about 200mVP–P for both the 3.3V output and the 5V output. The response time is about 20µs to 40µs, excellent for 1.4A outputs. This is an important feature when the power supply is used with DSPs and microcontrollers that are sensitive to voltage ripple. Low Dropout The LT3508 features low dropout for output voltages above 3V. The minimum operating voltage of the device is determined either by the LT3508’s undervoltage lockout or by its maximum duty cycle. If VIN1 and VIN2 are tied together, the undervoltage lockout is at 3.7V or below. If the two inputs are used separately, then VIN1 has an undervoltage lockout of 3.7V or below and VIN2 has an undervoltage lockout of 3V or below. Because the internal supply runs off VIN1, channel 2 will not operate unless VIN1 is above its undervoltage lockout. The dropout of the 5VOUT circuit shown in Figure 1 is less than 1V, with start-up occurring at a minimum of 5.9V and the converter running down to 5.5V before dropout occurs. Unlike many fixed frequency regulators, the LT3508 can extend its duty cycle by turning on for multiple cycles. The LT3508 will not switch off at the end of each clock cycle if there is sufLinear Technology Magazine • March 2007 ficient voltage on the boost capacitor. Eventually, the voltage on the boost capacitor falls and requires refreshing. A bigger boost capacitor allows for higher maximum duty cycle. Circuitry detects a depleted boost capacitor and forces the switch to turn off, allowing the inductor current to charge up the boost capacitor. This places a limitation on the maximum duty cycle. The minimum input voltage can be calculated as: VIN(MIN) = – VF + VSW SW ( VOUT + VF ) 1+ β 1 βSW is the switch current to boost current ratio. Refer to the data sheet section “Minimum Operating Voltage” for details. Track/Soft-Start and Power Good Pins Simplify Supply Sequencing DSPs and microcontrollers require power supply sequencing and tracking. Both LT3508 channels have independent tracking and soft-start circuits and each generates a power good signal when its output is in regulation. Most start-up/shut-down scenarios are possible by combining the function of the track/soft-start (TRACK/SS) with the power good (PG) pins. Figure 9 shows how easy it is to implement independent channel softstart, ratiometric start-up, coincident start-up, output sequencing, and externally controlled power up and power down. Soft-start prevents inrush current spikes, which can drag down the source voltage upon start-up and cause other system problems. Simple soft-start of each channel requires only a capacitor on the pin (Figure 9a). The rate of soft-start is determined by the size of capacitor and by the capabilities of the power source. As the name suggests, the TRACK/ SS pins also facilitate supply tracking, including ratiometric, coincident and externally controlled start-up and shut-down. Figure 9d shows how to connect the PG pin of one channel to the track pin of another channel to sequence the two—one channel is held off until the other channel is good. The track/soft-start function can also be used to power a channel down, but to minimize current draw, shut down the regulator via the shutdown (SHDN) pin as described below. Low Shutdown Current When the shutdown pin is pulled low, both channels turn off and the part consumes a very low quiescent current (<2µA), saving battery energy and extending lifetime. The shutdown pin can also be used as a 2.63V accurate undervoltage lockout (UVLO) with a resistor divider from VIN. In shutdown, the power good comparator is disabled and not valid and the soft-start capacitors are reset. TSSOP-16 and QFN Packages The LT3508 is available in two types of thermally-enhanced packages. The UF package is a 4mm × 4mm 24-pin QFN. The FE package is a 16-pin thermally-enhanced TSSOP surface mount with an exposed thermal pad. Both packages have equally low 40°C/W junction-to-ambient thermal impedance and 10°C/W junction-to-case impedance, important for applications that require a high input voltage, high switching frequency and high load current, all of which raise the junction temperature. Conclusion The LT3508 is a wide input voltage 36V dual 1.4A monolithic step-down converter with tracking/soft-start pins and power good pins for power supply sequencing and simple diagnostic interface with DSPs and microcontrollers. It has adjustable switching frequency from 250kHz to 2.5MHz, either set by a resistor or synchronized to an external source. Its thermally enhanced packages and Eand I-grade temperature ratings allow it to be used in thermally demanding environments. Separate VIN pins for each channel provide the capability of cascading channels and achieving extreme VIN to VOUT ratios by using the output of one channel as the input for the other. L 25