2MHz Dual Buck Regulator Operates Outside of AM Radio Band When Delivering 3.3V and 1.8V from 16V Input Pit-Leong Wong The number of microprocessor-based control units continues to grow in both automobiles and industrial systems. Because the amount of required processing power is also increasing, even modest processors require a low voltage core supply in addition to 3.3V or 5V memory, I/O and analog supplies. In automotive systems, power comes from the battery, with its voltage typically between 9V and 16V. Including cold crank and double battery jump-starts, the minimum input voltage may be as low as 4V and the maximum up to 36V, with even higher transient voltages. Likewise, a 24V industrial supply may be as high as 32V. With these high input voltages, linear regulators cannot be used for supply currents greater than 200mA without overheating the regulator. Instead, high efficiency switching regulators must be used to minimize thermal dissipation. There are challenges in applying switching regulators in these systems. A small circuit is desired, and certain operating frequencies may be unacceptable. At high step-down ratios, switching regulators typically operate at frequencies in the AM radio band. One solution is to dynamically move the power converter switching frequency (and harmonics) away from the tuned AM frequency, but moving the switching frequency can lead to unexpected EMI problems. A cleaner solution is to simply set the switching frequency higher than the top of the AM radio band, which is at 1.8MHz. This is easier said than done, since most existing buck converters cannot meet the low (<100ns over temperature) minimum 20 | April 2010 : LT Journal of Analog Innovation timer to supervise microprocessors. The LT3640 is offered in 4mm × 5mm QFN and 28-lead FE packages. DUAL BUCK REGULATOR The LT3640 is a dual channel, constant frequency, current mode monolithic buck switching regulator with power-on reset and watchdog timer. Both channels are synchronized to a single oscillator with frequency set by RT. The adjustable frequency ranges from 350kHz to 2.5MHz. The internal oscillator of the LT3640 can be synchronized to an external clock signal on the SYNC pin. on-time required to produce the step-down ratio from a 16V input to a 3.3V output. The LT3640 solves this problem with a fast non-synchronous high voltage buck converter and a high efficiency synchronous low voltage buck converter. With a typical minimum on time of about 60ns over temperature, the high voltage channel in the LT3640 can deliver 3.3V from 16VIN at 2MHz with comfortable margin. The synchronous low voltage channel in the LT3640 can be cascaded from the 3.3V channel to generate the other lower voltage buses such as 2.5V, 1.8V or 1.2V. The high voltage channel is a non-synchronous buck with an internal 1.7A NPN top switch that operates from an input of 4V to 35V. Above 35V, an internal overvoltage lockout circuit suspends switching, protecting the LT3640 and downstream circuits from input faults as high as 55V. The low voltage channel is a synchronous buck with internal CMOS power switches The LT3640 also includes a programmable power-on reset timer and watchdog 0.22µF Figure 1. 2MHz 3.3V/0.8A and 1.8V/0.8A step-down regulators VIN 5V TO 35V CIN 4.7µF D2 L1,3.3µH EN/UVLO VIN SYNC WDE PGOOD SW BST SW1 D1 80.6k DA FB1 49.9k VOUT1 100k 100k LT3640 RST1 RST2 WDO WDI µP CIN: TAIYO YUDEN UMK316BJ475KL COUT1, COUT2: TAIYO YUDEN JMK212BJ226MD L1: VISHAY IHLP2020BZER3R3 L2: VISHAY IHLP1616BZER1R0 D1, D2: DIODES INC. B240A D2: CENTRAL SEMICONDUCTOR CMDSH-4E VOUT1 3.3V/0.8A COUT1 22µF VIN2 EN2 L2,1µH SW2 100k CWDT CPOR 1.5nF RT GNDSS2 1.5nF 32.4k FB2 SS1 1nF 1nF 49.9k VOUT2 1.8V/0.8A COUT2 22µF design features The high voltage buck regulator in the LT3640 has a very fast minimum on time of about 60ns. This enables the LT3640 to operate at a high step-down ratio while maintaining high switching frequency. 90 90 VIN2 = 3.3V 85 VIN = 16V 80 VIN = 24V 75 80 75 fSW = 2MHz 3.3V CHANNEL 70 0 0.2 0.4 0.6 0.8 1.0 VOUT1 CURRENT (A) FAST, HIGH VOLTAGE BUCK REGULATOR The high voltage buck regulator in the LT3640 has a very fast minimum on time of about 60ns. This enables the LT3640 to operate at a high step down ratio while maintaining high switching frequency. Figure 3 shows the waveform of the LT3640 operating from input voltage of 35V to regulate a 3.3V output at 2MHz. The on time of the top power switch is about 60ns, which is also flat over temperature. 85 VIN = 12V EFFICIENCY (%) EFFICIENCY (%) providing high efficiency without the need of an external Schottky diode and accepts an input of 2.5V to 5.5V. Typically, the low voltage channel can operate from the output of the high voltage channel to form a cascaded structure as shown in Figure 1, but it can also operate from a separate supply source. The low voltage channel only switches when the high voltage channel output is within regulation. 1.2 fSW = 2MHz 1.8V CHANNEL 70 0 0.2 0.4 0.6 VOUT2 CURRENT (A) 0.8 1 Figure 2. Efficiency of the circuit in Figure 1 Besides the fast minimum on time, the high voltage buck regulator has fast switching edges to minimize switching losses and improve conversion efficiency at high frequency. Figure 4 shows the efficiency of the high voltage buck regulator at 2MHz operation for different input voltages. For 5V output voltage, the high voltage buck maintains an efficiency of higher than 86% for input voltage up to 24V. OUTPUT SHORT-CIRCUIT ROBUSTNESS The LT3640 monitors the catch diode current to guarantee the output short-circuit robustness for the high voltage buck converter. The LT3640 waits for the catch diode current to fall below its limit before starting a new cycle. The top NPN does not turn on until the catch diode current is below its limit. This control scheme 90 85 EFFICIENCY (%) VSW1 10V/DIV SW1 10V/DIV 80 75 IL1 0.5A/DIV fSW = 2MHz VOUT1 = 5V 100ns/DIV Figure 3. Fast high voltage buck regulator 70 0 0.2 VIN = 12V VIN = 16V VIN = 20V VIN = 24V 0.4 0.6 0.8 1.0 VOUT1 CURRENT (A) IL1 0.5A/DIV 1.2 Figure 4. Efficiency of the high voltage buck regulator 1µs/DIV VIN = 30V VOUT1 = SHORT RT SET = 2MHz Figure 5. Output shorted robustness, high voltage channel April 2010 : LT Journal of Analog Innovation | 21 ensures cycle-by-cycle current limit, providing protection against shorted output. The switching waveform for VIN = 30V and VOUT = 0V is shown in Figure 5. LOW VOLTAGE SYNCHRONOUS BUCK REGULATORS The low voltage channel is a synchronous buck with internal CMOS power switches providing high efficiency without the need of external Schottky diode. This channel only switches when the high voltage channel output is within regulation. The output can be programmed as low as 0.6V, covering any core voltage in modern microprocessors. The low voltage buck has a similar scheme as the high voltage buck of monitoring the current in the bottom NMOS to guarantee shorted output robustness. However, when the bottom NMOS current exceeds its limit, the oscillator frequency is not affected. The low voltage buck simply skips one cycle to avoid interference with the high voltage buck. At light load the low voltage buck also operates in low ripple Burst Mode operation to minimize output ripple and power loss. Although the two channels in the LT3640 share a common oscillator, they may require different light load operation frequencies to optimize efficiencies at their respective loads. In this case, the oscillator always runs at the higher frequency, with the channel requiring the lower frequency skipping cycles. Figures 6 and 7 show the light load switching waveforms of two channels running at same reduced frequency and at different frequencies, respectively. Output ripple for both channels remains below 10mVP–P. No-load quiescent current from the input is only 300µA with both outputs in regulation. BENEFITS OF CASCADING As described above, there are clear advantages of cascading two switchers to generate I/O and core supplies. The higher operating frequency reduces circuit size 22 | April 2010 : LT Journal of Analog Innovation SW1 10V/DIV SW1 10V/DIV IL1 0.5A/DIV IL1 0.5A/DIV SW2 5V/DIV SW2 5V/DIV IL2 0.5A/DIV IL2 0.5A/DIV 500ns/DIV VIN = 12V VOUT1 = 3.3V/25mA VIN2 = VOUT1 VOUT2 = 1.8V/30mA 2µs/DIV VIN = 12V VOUT1 = 3.3V/0mA VIN2 = VOUT1 VOUT2 = 1.8V/30mA Figure 6. Two channels running in discontinuous mode at light load remain synchronized. Figure 7. At still lighter loads, the two channels switch at different frequencies to maintain high efficiency and low output ripple. and provides faster transient response for better regulation. The low voltage technology used for the core supply switcher further reduces solution size. Figure 1 against two non-synchronous bucks operating from VIN, the overall efficiency is nearly identical. If the core voltage is reduced from 1.8V to 1.2V, the LT3640 circuit is actually more efficient. Although the core supply is generated via two conversions, with two efficiency hits, keep in mind that the core supply is often low power, even if it is high current, so total power loss is minimal. Also, a buck converter generating the core voltage directly from the input does not typically operate in an efficient region anyway, and it would be larger and slower. Comparing the circuit in Figure 8. Power-on reset and watchdog timing POWER-ON RESET AND WATCHDOG TIMER In high reliability systems, a supervisor monitors the activity of the microprocessor. If the processor appears to stop, due to either hardware or software faults, the supervisor resets the microprocessor in an attempt to restore the system to a functional state. While some FB tRST tUV RST a. Power-on reset timing WDI WDO tDLY t < tWDL tRST b. Watchdog timing t < tWDU tWDL < t < tWDU tWDU tRST design features modern processors include internal supervisor functions, it is better practice to separate the two. Typical supervisor functions are voltage monitors with power-on resets to qualify supply voltages and watchdog timers to monitor software and hardware functions. function for higher reliability. If the falling edges on the WDI pin are grouped too close together or too far apart, the WDO pin is pulled down for a period the same as the power-on reset timeout period before the watchdog timer is started again. The timing diagrams of the power-on reset and watchdog timer are shown in Figure 8. The LT3640 includes one power-on reset timer for each buck regulator and one common watchdog timer. Power-on reset and watchdog timers are both adjustable using external capacitors. LOW NOISE DATA ACQUISITION SUPPLY Figure 9 shows a 4-output supply that generates low noise 5V and 3.3V rails for analog circuits, along with 3.3V I/O and 1.5V core supplies for digital circuits. The high voltage channel of the LT3640 converts the input to a 5.7V intermediate bus, the low voltage channel bucks to the 1.5V core, and a few LDOs regulate the 5V and 3.3V outputs. The 5.7V bus voltage gives the 5V regulator suitable headroom for good PSSR and transient performance. Once the high voltage buck output voltage reaches 90% of its regulation target, the high voltage channel reset timer is started and the RST1 pin is released after the reset timeout period. The low voltage channel reset timer is started once the low voltage buck output voltage reaches 92% of its regulation voltage, and releases RST2 after the reset timeout period. Diode D2 performs two functions. First, it lowers the intermediate voltage to a value below the 5.5V maximum operating voltage of the synchronous buck regulator. Second, it isolates high frequency ripple current flowing into the VIN2 and The watchdog circuit monitors a microprocessor’s activity. As soon as both RST1 and RST2 are released and an additional delay has expired, the watchdog starts monitoring the signal at the WDI pin. The LT3640 implements windowed watchdog Figure 9. This circuit generates two low noise analog rails plus I/O and core supplies for a microprocessor. BST pins of the LT3640 from the inputs of the LDOs regulating the analog supplies, resulting in quiet analog rails. Total current draw from the 5.7V rail is 820mA, so about 30% more power is available from this output. RST1 and RST2 indicate power is good when the 1.5V and 5.7V rails are in regulation. The watchdog function is not used here, and is disabled by tying WDE to VIN2. The associated pins, not shown on this schematic, should be left floating. CONCLUSION The LT3640 is a dual channel buck regulator. The high voltage channel buck is capable of converting 16V input voltage to 3.3V output at 2MHz with comfortable margin. The high voltage buck maintains efficiency above 86% for delivering 5V from up to 24V input at 2MHz switching frequency. The low voltage channel buck input ranges from 2.5V to 5.5V. The LT3640 also includes power-on reset and watchdog timer to monitors a microprocessor’s activity. The high frequency high efficient buck converters and the programmable timers make the LT3640 ideal for automotive applications. n LOW NOISE, LDO MICROPOWER REGULATORS IN OUT LT1962-5 SHDN BYP GND D3 VIN 7V TO 35V CIN1 10µF VIN BST EN/UVLO SW SW1 226k 49.9k DA FB1 POR 1nF CIN1: TAIYO YUDEN UMK325BJ106MM COUTSW1: TAIYO YUDEN JMK212BJ226MD CIN2, COUT1, COUT2, COUT3 : TAIYO YUDEN JMK212B7106KG COUT4 : TAIYO YUDEN JMK212BJ476MG L1: VISHAY IHLP2525CZER8R2 L2: VISHAY IHLP1616BZER2R2 D1: UPS140 D2: CMMR1-02 D3: PMEG4005 D1 178k 1nF 1nF 82.5k VIN2 RST1 RST2 WDE CPOR EN2 SHDN BYP GND RT SYNC GND FB2 VOUT2 3.3V/100mA COUT2 10µF 10nF VOUT3 3.3V/300mA COUT3 10µF VOUT4 1.5V/800mA SW2 SS2 SHDN BYP GND IN OUT LT1763-3.3 CIN2 10µF 75k 10nF D2 L2 2.2µH SS1 IN OUT LT1761-3.3 COUTSW1 22µF LT3640 100k VOUT1 5V/100mA COUT1 10µF 1µF L1,8.2µH 49.9k VOUT3 10nF COUT4 47µF 49.9k April 2010 : LT Journal of Analog Innovation | 23