July 2013 I N T H I S I S S U E straighten out your power source priorities 8 µModule DC/DC combines Volume 23 Number 2 Minimize Standby Current in Automotive DDR Supplies David Gilbert high efficiency switching with low noise linear regulation 18 reference clock distribution for a 325MHz IF sampling system with over 30MHz bandwidth, 64dB SNR and 80dB SFDR 26 near noiseless ADC drivers for imaging 31 When you turn on a laptop or a smart phone, you expect to wait for it to boot up, but you are less patient when you turn on your car. With a car, consumers expect immediate access to computer electronics, including navigation and infotainment systems, and automobile manufacturers strive to meet this desire with design strategies that shorten start-up time. One such strategy is to keep dynamic memory (RAM) active at all times, even during the ignition-off state. The DDR3 memory used in automobiles operates on a 1.5V rail with peak load currents over 2A—preferably produced by a high efficiency DC/DC converter to minimize heat dissipation. In these applications, light load efficiency is just as important to preserve battery life when the automobile is not running. DDR memory can consume 1m A–10m A from the 1.5V rail in standby, but drawing 10m A from the battery is unacceptable when the car is parked for long periods. This constraint rules out the use of a linear regulator, where input and output current are equal. On the other hand, a switching step-down (buck) regulator draws less input current than load current in proportion to the step-down ratio: IIN = 1 VOUT • IOUT • VIN η where h is the efficiency factor (0 to 1). Figure 1 shows that the LT®8610AB synchronous stepdown regulator achieves ~83% efficiency at a 1m A load. For a battery voltage of 12V and load current of 1m A at 1.5V, calculated input current is only 151µ A. The LT8610 keeps automotive electronics running. Caption w w w. li n ea r.com (continued on page 4) The LT8610A and LT8610AB have a low component count, low minimum input voltage, low quiescent current and high efficiency across a wide load range. These features make them the preferred solution for providing standby power to DDR memory in automotive applications. (LT8610A/AB continued from page 1) 100 DIRECT DC/DC CONVERSION FROM CAR BATTERY TO 1.5V DDR MEMORY 95 EFFICIENCY (%) 90 LT8610A and LT8610AB are monolithic, synchronous step-down regulators designed specifically for automotive systems. They deliver 3.5A while consuming only 2.5µ A quiescent current. Building a circuit around them is easy. No additional semiconductors are required, they work with inexpensive ceramic capacitors, and the MSOP package has leads that are easy to solder and inspect. With a typical minimum on-time of 30ns (45ns guaranteed maximum), one can design compact, high switching-frequency buck regulators with large step-down ratios. Figure 2 shows an application that delivers 3.5A at 1.5V. The operating frequency is 475kHz to optimize efficiency and remain below the AM radio band. Figure 2. This LT8610A or LT8610AB step-down converter circuit accepts automotive battery and generates 1.5V at 3.5A. Low quiescent current and synchronous rectification result in high efficiency across the entire load range. 4 | July 2013 : LT Journal of Analog Innovation 4.7µF 80 75 70 65 60 55 50 1 0.1 1k 10k Figure 1. LT8610AB efficiency versus load input is worst-case 3.4V, the maximum duty cycle is above 99% and the dropout voltage is typically 200mV at 1A, all of which keep the output in regulation through cold-crank. The typical minimum input voltage is plotted in Figure 3. The difference between the LT8610A and LT8610AB is that the LT8610AB features higher efficiency at light loads. This is achieved by using an increased Burst Mode current limit, allowing more energy to be delivered during each switch cycle and lowering the switching frequency for a given load. Because a fixed amount of energy is required to SAVE THE BATTERY WITH LOW RIPPLE Burst Mode OPERATION AND MINIMAL QUIESCENT CURRENT VIN BST EN/UV LT8610A/ PG SW LT8610AB BIAS SYNC FB TR/SS INTVCC RT 93.1k fSW = 475kHz GND 0.1µF 3.3µH 549k 4.7pF 1M 3.6 3.4 3.2 INPUT VOLTAGE (V) The LT8610A and LT8610AB are designed to minimize output voltage ripple over the entire load range. At light loads, they 10nF 1µF 10 100 ILOAD (mA) VIN = 12V VOUT = 1.5V RT = 93.1k (475kHz) L = COILCRAFT XAL6030-332ME 3.3µH Both parts feature excellent fault tolerance to automotive environments. A maximum input of 42V handles load dumps. A robust switch design and high speed current comparator protect the device during output shorts. The minimum VIN 12V 85 maintain efficiency by reducing their operating frequency and entering Burst Mode® operation. Fast transient response is maintained even at very low loads. This feature combined with the very low quiescent current of 2.5µ A means that, even at a few µ A of load, the LT8610A and LT8610AB are more efficient than a linear regulator with zero quiescent current. For systems where low frequency operation must be avoided, Burst Mode operation can be turned off by applying either a logic high signal or clock signal to the SYNC pin. 3.0 2.8 2.6 2.4 VOUT 1.5V 47µF 3.5A ×3 2.2 2.0 –55 –25 95 65 35 TEMPERATURE (°C) 5 125 155 Figure 3. Keeping the memory alive in cold-crank or start-stop events. The LT8610A and LT8610AB operate down to a typical minimum input voltage of 2.9V at 25°C, with 3.4V maximum guaranteed over temperature. design features The LT8610A and LT8610AB are designed to minimize output voltage ripple over the entire load range. At light loads, they maintain efficiency by reducing their operating frequency and entering Burst Mode operation. Fast transient response is maintained even at very low loads. EFFICIENCY (%) 90 OUTPUT VOLTAGE RIPPLE (mVP–P) 95 LT8610AB 85 80 LT8610A 75 70 65 60 55 50 1 0.1 10 100 ILOAD (mA) 1k 10k 70 70 60 60 50 40 30 20 10 0 VIN = 12V VOUT = 1.5V RT = 93.1k (475kHz) L = COILCRAFT XAL6030-332ME 3.3µH switch the MOSFET on and off, a lower switching frequency reduces gatecharge losses and increases efficiency. Figure 4 shows the efficiency difference between the LT8610A and LT8610AB. For loads between 1m A and 100m A, the LT8610AB improves efficiency by more than 10% compared to the LT8610A. The trade-off to the increased Burst Mode current limit is that more energy is delivered in each switch cycle, so VIN 4.7µF BST EN/UV LT8610A/ PG SW LT8610AB BIAS SYNC 10nF FB TR/SS 1µF INTVCC RT 18.2k fSW = 2MHz GND LT8610A 2 1 3 NUMBER OF 1210 47µF OUTPUT CAPACITORS VIN = 12V VOUT = 1.5V RT = 93.1k (475kHz) L = COILCRAFT XAL6030-332ME 3.3µH Figure 4. An increased Burst Mode current limit on the LT8610AB results in substantial efficiency gains at light load compared to the LT8610A. VIN 12V LT8610AB 0.1µF 1µH 549k (a) 50 40 30 20 LT8610AB 10 0 LT8610A 2 1 3 NUMBER OF 1210 47µF OUTPUT CAPACITORS VIN = 12V VOUT = 1.5V RT = 18.2k (2MHz) L = COILCRAFT XAL4020-102ME 1µH (b) Figure 5. Output voltage ripple versus number of 1210 size 47µF output capacitors for two inductor values, at 10mA load. (a) Ripple for the 475kHz application in Figure 2. (b) Ripple for the 2MHz application in Figure 6. more output capacitance is required to keep output voltage ripple low. Figure 5 compares the output ripple for the LT8610A and LT8610AB as a function of the output capacitance for two inductor values, at 10m A load. one. If high efficiency at light loads is paramount, then the inductor value can be increased beyond the starting value recommended in the data sheet. In addition to the current limit, the inductor choice affects the efficiency and switching frequency in Burst Mode operation. This is because for a fixed current limit, a larger inductor value can store more energy than a smaller For most automotive systems, 9V to 16V is the typical input voltage, so application circuits are usually optimized for this range. The 475kHz application in Figure 2 operates at the designed frequency over the entire input range of 3.5V to 42V. However, if we restrict the normal operating voltage to 16V (42V transient), the operating frequency can be increased and the value and size of the inductor reduced. With a worst-case minimum on-time of 45ns, the LT8610A and LT8610AB can be programmed to 2MHz as shown in Figure 6. VOUT 1.5V 47µF 3.5A ×3 4.7pF 1M OUTPUT VOLTAGE RIPPLE (mVP–P) 100 Figure 6. Similar 12V to 1.5V application as in Figure 2, but the operating frequency of the LT8610A and LT8610AB is increased to 2MHz for reduced inductor value and size. GO FASTER FOR A SMALLER SOLUTION July 2013 : LT Journal of Analog Innovation | 5 100 100 95 95 90 90 COILCRAFT XAL5030-222ME 2.2µH 85 EFFICIENCY (%) EFFICIENCY (%) An important feature is that this internal regulator can draw current from either the VIN pin or the BIAS pin. If a voltage of 3.1V or higher is tied to the BIAS pin, gate drive current is drawn from BIAS. If the BIAS voltage is lower than VIN, the internal linear regulator will run more efficiently using the lower voltage supply, thus increasing overall efficiency. 80 75 COILCRAFT XAL4020-102ME 1µH 70 65 80 75 65 55 55 1 10 100 ILOAD (mA) 1k 50 10k COILCRAFT XAL4020-102ME 1µH 70 60 0.1 The LT8610A and LT8610AB use two internal nMOSFETs specifically optimized for automotive applications. In particular, the gate drive circuitry requires less than 3V to fully enhance the FETs. To generate the gate drive supply, the LT8610A/AB includes an internal linear voltage regulator, the output of which is the INTVCC pin (do not load INTVCC with external circuitry). 85 60 50 BIAS PIN OPTIMIZES EFFICIENCY LT8610AB VIN = 12V VOUT = 1.5V RT = 18.2k (2MHz) COILCRAFT XAL5030-222ME 2.2µH 0.1 1 10 100 ILOAD (mA) 1k 10k An important feature is that this internal regulator can draw current from either the VIN pin or the BIAS pin. If the BIAS pin is left open, then gate drive current is drawn from VIN . However, if a voltage of 3.1V or higher is tied to the BIAS pin, gate drive current is drawn from BIAS. If the BIAS voltage is lower than VIN, the internal linear regulator will run more efficiently using the lower voltage supply, thus increasing overall efficiency. LT8610A VIN = 12V VOUT = 1.5V RT = 18.2k (2MHz) Figure 7. LT8610A and LT8610AB efficiency versus load at 2MHz with two inductor values 100 100 95 95 90 BIAS = 3.3V GATE DRIVE DRAWN FROM 85% EFFICIENT SUPPLY 85 80 75 EFFICIENCY (%) EFFICIENCY (%) 90 BIAS = 0V GATE DRIVE DRAWN FROM VIN 70 65 85 80 70 65 60 60 55 55 50 0.1 1 10 100 ILOAD (mA) 1k 10k LT8610AB VIN = 12V VOUT = 1.5V RT = 18.2k (2MHz) L = COILCRAFT XAL4020-102ME 1µH BIAS = 3.3V GATE DRIVE DRAWN FROM 85% EFFICIENT SUPPLY 75 50 BIAS = 0V GATE DRIVE DRAWN FROM VIN 0.1 1 10 100 ILOAD (mA) 1k 10k LT8610A VIN = 12V VOUT = 1.5V RT = 18.2k (2MHz) L = COILCRAFT XAL4020-102ME 1µH Figure 8. Efficiency can be increased if the BIAS pin is tied to an external 3.3V supply. (External supply efficiency of 85% is assumed and factored into overall efficiency shown here.) Note that when the input voltage goes above 16V, the output remains in regulation although the switching frequency decreases to maintain safe operation. The 2MHz solution is identical to the circuit in Figure 2, except for the RT resistor 6 | July 2013 : LT Journal of Analog Innovation changed to 18.2kΩ and the inductor value and size reduced in order to save space. Figure 7 shows efficiency versus load for two inductor choices. The efficiency data in Figures 1, 4 and 7 was recorded with the BIAS pin open. After all, if the 1.5V output is the only rail alive, then there is likely no good place to tie the BIAS pin. However, if there is a 3.3V or 5V supply, tie it to the BIAS pin, even if the supply is not available in standby or ignition-off conditions. Figure 8 shows the efficiency with and without a 3.3V supply connected to BIAS. In calculating the total efficiency, we have included the power drawn from the 3.3V rail and assumed that it was generated with 85% efficiency. Note that the benefit to externally powering BIAS is greater at higher operating frequencies because the gate drive current is higher. The LT8610A also benefits design features An important consideration for automotive applications is the behavior of the power supply during cold crank and idle-stop transients, when the voltage from the 12V battery may drop below 4V. The LT8610AB operates up to 99% duty cycle, providing output regulation at the lowest possible input voltage. more from external bias compared to the LT8610AB—the AB’s increased Burst Mode current limit results in a lower operating frequency for a given load. and reliable output voltage as a function of input voltage. Figure 10(b) shows the output voltage as the input supply is ramped from zero to 10V and back to zero. NOT JUST FOR MEMORY CONCLUSION The LT8610AB is an excellent regulator for other automotive supplies, including 3.3V and 5V supplies, with efficiency above 90%, as shown in Figure 9. The LT8610A and LT8610AB have a low component count, low minimum input voltage, low quiescent current and high efficiency across a wide load range. These features make them the preferred solutions for providing standby PARAMETER LT8610 LT8610A LT8610AB Max Load Current (A) 2.5 3.5 3.5 Minimum On-Time (ns) (Typ) 50 30 30 V IN = 12V, V OUT = 1.5V, I LOAD = 10mA, f SW=475kHz, L = 3.3µH 73.9 73.9 85.5 Output Voltage Ripple (mV P-P) I LOAD = 10mA, C OUT = 47µF, L = 3.3µH 8.4 8.4 52.5 800 90 700 80 600 60 50 40 30 fSW = 700kHz VIN = 12V L = 4.7µH 20 0.001 0.1 VOUT = 3.3V VOUT = 5V 1 10 100 LOAD CURRENT (mA) 1000 Figure 9. Efficiency for 3.3V and 5V outputs is above 90%, reducing total power dissipation and keeping temperature under control. CONDITIONS Efficiency (%) 100 70 Visit www.linear.com/LT8610 for data sheets, demo boards and other applications information. n Table 1. Comparison of features for the LT8610 family of monolithic, synchronous buck converters DROPOUT VOLTAGE (V) EFFICIENCY (%) An important consideration for automotive applications is the behavior of the power supply during cold crank and idle-stop transients, when the voltage from the 12V battery may drop below 4V. The LT8610AB operates up to 99% duty cycle, providing output regulation at the lowest possible input voltage. Figure 10(a) shows the dropout voltage. This is the difference between VIN and VOUT as the input voltage decreases towards the intended output regulation voltage. The LT8610AB also has excellent start-up and dropout behavior, resulting in predictable power to DDR memory in automotive applications. Table 1 summarizes the performance of the LT8610 family. VIN 2V/DIV VIN 500 400 VOUT 2V/DIV 300 VOUT 200 100 0 0 0.5 1.5 2 2.5 1 LOAD CURRENT (A) (a) 3 3.5 100ms/DIV 2.5Ω LOAD (2A IN REGULATION) (b) Figure 10. The LT8610AB operates to 99% duty cycle, providing smooth start-up and low dropout voltage. July 2013 : LT Journal of Analog Innovation | 7