DESIGN FEATURES Micropower SOT-23 Buck Regulator by Jeff Witt Accepts Inputs to 34V Introduction D2 The LT1934 is a micropower buck regulator with an internal 400mA power switch. It provides up to 300mA output current, while consuming just 12µA of quiescent current, an important feature for always-on battery-powered applications such as laptop computer standby supplies or remote instrumentation that must operate for years from primary batteries. Its wide input voltage range of 3.4V to 34V makes it applicable to a variety of power sources including 24V industrial supplies, automotive batteries and unregulated wall transformers. 3.3V 250mA Supply from 24V Input Consumes Just 15µA Figure 1 shows a 3.3V/250mA supply that accepts inputs from 4.5V to 34V. With the output in regulation and with no load, the input current with VIN = 24V is less than 15µA. Input current falls to less than 1µA when the LT1934 is placed in shutdown mode by pulling SHDN to ground. The SHDN pin can be tied to VIN if the shutdown mode is not used. The capacitor and diode tied to the BOOST pin provide a bias voltage above VIN in order to fully saturate the internal NPN power switch, thereby maintaining high efficiency over the entire input voltage range. 100 90 LT1934 VOUT = 3.3V L = 47µH EFFICIENCY (%) VIN = 5V 80 70 VIN = 24V VIN = 12V 60 50 0.1 1 10 100 LOAD CURRENT (mA) Figure 1b. Efficiency of Figure 1’s ciruit 6 0.22µF BOOST VIN 4.5V TO 34V ON OFF VOUT 3.3V 250mA SW VIN C2 2.2µF L1 47µH D1 LT1934 SHDN 10pF FB 1M + C1 100µF 604k GND C1: SANYO 4TPB100M C2: TAIYO YUDEN GMK325BJ225MN D1: ON SEMI MBR0540 D2: CENTRAL CMDSH-3 L1: SUMIDA CDRH4D28-470 (619) 661-6835 (408)573-4150 (602) 244-6600 (516) 435-1110 (847) 956-0667 Figure 1a. This buck regulator supplies 250mA at 3.3V from an input voltage as high as 34V. Input current with no load applied is less than 15µA. The LT1934 uses Burst Mode® operation, with low quiescent current, to achieve high efficiency across a wide range of load currents, as shown in Figure 1b. An internal comparator monitors the voltage at the FB pin. When the voltage at this pin is above 1.25V, the LT1934 is in its sleep mode with only its reference and feedback comparator biased. In this state the VIN pin current is 12µA. As the load current discharges the output capacitor, the FB pin voltage falls below 1.25V and the comparator enables the internal oscillator and drive circuits. The LT1934 switches with a fixed off time of 1.8µs, limiting the peak inductor current to 400mA as it delivers power to the output. Figure 2 shows the operating waveforms of the circuit in Figure 1a. Output voltage ripple is less than 50mVP–P. Figure 3a shows a circuit that generates 5V from a 6.5V–34V input. Figure 3b shows the efficiency of this circuit. The accurate, fast, cycle-by-cycle current limit of the LT1934 keeps the switch and inductor currents under control at all times. In addition, the 1.8µs switch off time is extended during fault conditions when the output voltage is pulled below the programmed output voltage. This ensures that the LT1934 can handle a shorted output. The LT1934-1 for Low Current Supplies The LT1934-1 is identical to the LT1934 except that the current limit is 120mA. The LT1934-1 can be used in applications with a maximum load current of 60mA. This lower current limit allows a better match between the power components (the inductor and the input and output capacitors) and the application, resulting in a smaller circuit size. Input ripple is also lower, reducing noise and simplifying input filtering. The LT1934-1 may be a better choice when the power source has a high output impedance, such as 4mA-20mA loops, long-life primary batteries with high internal resistance VOUT 50mV/ DIV VSW 10V/DIV ISW 0.5A/ DIV IL1 0.5A/ DIV 5µs/DIV Figure 2. The operating waveforms of the circuit in Figure 1 as it delivers 180mA from a 10V input. The fast, accurate, cycle-by-cycle current limit results in a robust circuit that can withstand a shorted output. Linear Technology Magazine • March 2003 DESIGN FEATURES D2 100 BOOST VIN 6.5V TO 34V VOUT 5V 250mA SW VIN C2 2.2µF D1 LT1934 ON OFF L1 68µH 10pF 1M FB SHDN + VIN = 12V C1 68µF 332k GND C1: SANYO 6TPB68M C2: TAIYO YUDEN GMK325BJ225MN D1: ON SEMI MBR0540 D2: CENTRAL CMPD914 L1: SUMIDA CDRH5D28-680 LT1934 VOUT = 5V 90 EFFICIENCY (%) 0.1µF 80 VIN = 24V 70 60 (619) 661-6835 (408) 573-4150 (602) 244-6600 (516) 435-1110 (847) 956-0667 50 0.1 Figure 3a. The LT1934 provides 250mA at 5V. 1 10 100 LOAD CURRENT (mA) Figure 3b. Efficiency of Figure 3’s circuit D2 0.1µF BOOST VIN 4.5V TO 34V VIN C2 1µF D1 SHDN 10pF Li-Ion Battery Charger with Industrial Input Range VOUT 3.3V 45mA SW LT1934-1 ON OFF L1 100µH 1M FB + The circuit in Figure 5 is a complete, single-cell Li-Ion battery charger that operates from an input of 7V to 28V and provides a charge current of 350mA. This charger requires no microcontroller. A charge cycle is initiated when power is applied to the circuit, and end-of-charge is indicated when the battery voltage reaches 4.2V and charge current has fallen to one tenth of the nominal charge current. The LT1934 acts as a currentlimited 5V pre-regulator for the LTC4052-4.2 pulse charger. The LTC4052 monitors the battery voltage and pulls the CHRG pin low to indicate that the battery is being charged. C1 22µF 604k GND C1: TAIYO YUDEN JMK316BJ226ML C2: TAIYO YUDEN GMK316BJ105ML D1: ON SEMI MBR0540 D2: CENTRAL CMDSH-3 L1: COILCRAFT DO1608C-104 OR WURTH ELECTRONICS WE-PD4 TYPE S (408)573-4150 (602) 244-6600 (516) 435-1110 (847) 639-6400 Figure 4. Smaller components can be used with the lower current LT1934-1. or remote supplies with long cables. Low maximum switch current is also helpful in designing intrinsically safe systems, which have limits on stored energy and require current limiting resistors in series with external connections. Figure 4 shows a 3.3V supply implemented with the LT1934-1. D2 VIN 7V TO 28V 0.1µF L1 47µH BOOST D3 VIN SW SHDN GATE 0.022µF CHRG FB 332k + C1 47µF ACPR SENSE BAT TIMER C5 10µF C1: SANYO 6TPB47M C2: TAIYO YUDEN GMK316BJ105ML D1, D3: ON SEMICONDUCTOR MBR0540 D2: CENTRAL CMDSH-3 L1: SUMIDA CR43-470 0.047µF LTC4052 1.00M GND + 1k D1 LT1934 C2 1µF 10k VIN 1k GND CTIMER 0.1µF (619) 661-6835 (408) 573-4150 (602) 244-6600 (516) 435-1110 (847) 956-0667 350mA 1-CELL 4.2V Li-Ion BATTERY CHARGE STATUS AC PRESENT Figure 5. This standalone 350mA Li-Ion battery charger accepts inputs to 28V and requires no heat sinks. Linear Technology Magazine • March 2003 7 DESIGN FEATURES 500 VIN = 24V CHARGE CURRENT (mA) 400 VIN = 8V 300 VIN = 12V 200 100 0 2.5 3.0 3.5 4.0 BATTERY VOLTAGE (V) 4.5 Figure 6. Charging current as a function of battery voltage and input voltage If this voltage is less than 2.5V, the LTC4052 applies a 24mA trickle charge, with the LT1934 maintaining 5V across C1. When the battery voltage rises above 2.5V, the LTC4052 enters its fast charge mode, turning on an internal N-channel FET switch between its SENSE and BAT pins. This connects the LT1934’s output to the battery. As the voltage across C1 falls to the battery voltage, the LT1934 limits its output current, charging the battery at 350mA. Because the FET is fully on, very little power is dissipated, and the charging circuit maintains high efficiency during the entire charge cycle. When the battery voltage reaches 4.2V, the LTC4052 turns off the switch for 100ms. If the battery voltage falls below 4.2V during this period, the switch is turned back on for at least 400ms. In this manner, the LTC4052 modulates the duty cycle of the 350mA current source. When this duty cycle falls to 10%, the LTC4052 indicates that the battery is nearly charged by switching the CHRG pin to a weak 40µA pull down. Pulse charging continues until a timer in the LTC4052 stops the charge cycle after three hours, when the CHRG pin goes to a high impedance state. If the battery voltage is pulled below 4.05V due to a load current, the LTC4052 reenters its fast charge mode. If input power is removed, the LTC4052 isolates the LT1934’s circuitry and reduces battery load to less than 1µA. Diode D3 provides reverse input protection. 8 The LT1934 regulates its output current by limiting the peak current in L1 to 400mA. The charge current equals the average current in L1, and depends on the input and output voltages, as shown in Figure 6. In addition to these variations, expect approximately ±15% variation in the nominal charge current due to variations in the current limit and switch off time of the LT1934 and the value of L1. In addition to indicating charge status, the LTC4052 also provides an AC-present flag that indicates that power has been applied to the charger. CLOSING SWITCH SIMULATES HOT PLUG IIN + LOW IMPEDANCE ENERGIZED 24V SUPPLY VIN A similar device, the LTC1730, can be used to provide additional features, including a thermistor interface for monitoring the temperature of the battery and a shutdown pin that can be used to reinitialize the charge cycle timer. This Li-Ion charger uses only surface mount devices, requires no heat sinks, and its wide input voltage range accepts a variety of power sources including unregulated wall transformers, car batteries and 24V industrial supplies. The charger is completely standalone, requiring no continued on page 27 VIN 10V/DIV LT1934 2.2µF IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 10µF 35V AI.EI. a 10µs/DIV LT1934 + 2.2µF b 1Ω LT1934 0.1µF 2.2µF c LT1934-1 uh oh… 1µF d 4.7Ω LT1934-1 0.1µF ahh… 1µF e Figure 7. A well chosen input network prevents input voltage overshoot and ensures reliable operation when the LT1934 is connected to a live supply. Linear Technology Magazine • March 2003 DESIGN FEATURES uses a P-channel MOSFET located in series with the wall adapter power path. This MOSFET functions like a lossless ideal diode—replacing the large series diode that is used to prevent battery current from going back to the power adapter. The MOSFET is actively driven in a linear mode to maintain a constant 25mV forward voltage, which makes the MOSFET act as an ideal diode when forward current flows yet allows fast (10µs) cutoff when a current reversal is sensed. Small PCB Footprint Traditionally, high current chargers require a large number of external support components, but the LTC4006, LTC4007 and LTC4008 offer features to push solution size down. For instance, N-channel MOSFETs are traditionally used in high current applications because of their lower RDS(ON). Today, P-channel MOSFETs offer the performance that N-channel LT1934, continued from page 8 microcontroller coding. On the other hand, its charge status and AC-present pins provide useful information to the user through LEDs or to a microcontroller through a few digital I/O pins, making it simple to integrate into a battery powered system. Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1934 and LT1934-1 circuits. However, these capacitors can cause problems if the LT1934 is plugged into a live supply.1 The low-loss ceramic capacitor combined with stray inductance in series with the power source forms an underdamped tank circuit, and the voltage at the VIN pin of the LT1934 can ring to twice the nominal input voltage, possibly exceeding the LT1934’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT1934 into an energized supply, the input network should be designed to prevent this overshoot. Linear Technology Magazine • March 2003 MOSFETs were offering only a couple of years ago. Moving from an N-channel to a P-channel MOSFET drastically simplifies the design of the charger circuit. There are no boosted topside gate drive supplies to deal with, saving components and IC pins. Increasing the switching frequency reduces the inductor size and output capacitance requirements. Loop response is also improved, further reducing the output capacitance such that small ceramic capacitors can be used. Using a 100mV regulation point for current sensing, small 1206 sized sense resistors can be used. The INFET circuit is also used to generate the AC present flag without requiring an extra comparator to detect the presence of a power adapter. Finally, improved internal circuit design leads to further reduction in both pin count, part count and the size of each external component needed to make the IC work. These features add up to an overall system solution that meets the needs of today’s smaller products. Figure 7 shows the waveforms that result when an LT1934 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2µF ceramic capacitor at the input. The input voltage rings as high as 35V, and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 7b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 7c. A 1Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1µF capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V. Voltage overshoot gets worse with reduced input capacitance. Figure 7d shows the hot plug response with a 1µF ceramic input capacitor, with the input ringing above 40V. The LT1934-1 can tolerate a larger input resistance, such as shown in Figure 7e where a 4.7Ω resistor damps the voltage transient and greatly reduces the input current glitch on the 24V supply. Conclusion The LTC4006, LTC4007 and LTC4008 integrate more functions in smaller circuit footprints than any high power battery charger IC available today. The LTC4006 and LTC4007 provide simple standalone solutions for complete Li-Ion battery care without the need to write software programs and/or design a complex battery charger control system. The LTC4006 is targeted for applications where small circuit size is most important. If complete monitoring or limited host control of the charge process is desired, the LTC4007 offers a complete set of feedback, status and control signals. Finally, the LTC4008 is a general purpose charger that works with multiple battery chemistries by offering direct control over the entire charge process. Conclusion The LT1934’s 34V input range and short circuit robustness make it a great choice for small industrial systems. It delivers up to 300mA output with an all-surface-mount circuit that requires no heat sinks. The integrated power switch, SOT-23 package and high frequency operation result in a very small regulator, requiring less than one square centimeter of PCB area. Efficiency is high over the entire load range, with the 12µA quiescent current extending battery life. Notes 1.See Linear Technology Application Note 88 for a complete discussion. 27