DESIGN IDEAS Positive Buck Regulator Makes Negative Boost DC/DC Converter by Keith Szolusha Power supply designers can choose from a plethora of available positive buck regulators that can also be used as negative boost DC/DC converters. Some buck regulators have a negative feedback reference voltage expressly for this purpose, but they are far outnumbered by the variety of ICs that have positive reference feedback voltages. A designer can take advantage of this greater variety of devices by using a positive buck switch-mode regulator to create an excellent negative boost converter—all that is needed are a few small modifications to the typical buck converter configuration. Figure 1a shows a –5V input to –9V output at 1.2A negative boost converter using the LT1765EFE positive buck converter switch-mode regulator. The LT1765EFE operates with a 3V to 25V input, uses a 1.2V feedback voltage, and has an internal 3A power switch. The fast 1.25MHz switching frequency of the LT1765EFE helps reduce the size of the inductor and input and output capacitors. Figure 1b shows a typical positive buck converter application for the LT1765EFE, a 12V in to 3.3V out at 2.2A DC/DC converter. In Figure 1a the VIN pin is connected to system ground and the GND pin of the IC is connected to the negative voltage output. This makes the negative boost converter configuration provide a positive voltage at the VFB pin with respect to the GND pin of the IC. In this topology the maximum input voltage CMDSH-3 0.22µF VSW U1 SYNC LT1765EFE FB SHDN VC GND COUT 22µF 16V X5R CERAMIC OUTPUT –9V AT 1.2A L1 2.5µH BOOST VIN CC 4700pF RC 6.8k INPUT –5V CIN 2.2µF 6.3V X5R CERAMIC UPS120 64.9k 10.0k CF 100pF L1: SUMIDA CDRH5D28-2R5 (847) 956-0667 HIGH ∆I/∆t PATH IS IN BOLD FOR LAYOUT CONSIDERATIONS rating of the IC has to be greater than the magnitude of output voltage for the negative boost converter. The IC must also have a minimum input voltage rating that is less than the magnitude of the input voltage in order for the circuit to turn-on upon power-up, since the output voltage can have an initial state of 0V. Notice that the maximum output current for the negative boost converter in Figure 1a is less than the maximum output current of the positive buck converter Figure 1b, even though they use the same 3A internal power switch. Inductor Selection The inductor is chosen based on maximum output current, peak switch current, and desired ripple current. First calculate the duty cycle (DC), and then either calculate the ripple current (IP–P) based on the chosen inductor (L), or the inductor value based on the desired ripple current. It is generally good practice to choose the inductor value so that the peakto-peak ripple current is about 40% of the input current. These calculations are approximate and ignore the effect of switch, inductor, and Schottky diode power losses. continued on page 35 Figure 1a. –5V input to –9V output at 1.2A DC/DC converter 90 CMDSH-3 OUTPUT 3.3V AT 2.2A 0.22µF VIN BOOST VSW U1 SYNC LT1765EFE FB SHDN VC GND CIN 2.2µF 25V X5R CERAMIC CC 1000pF RC 4.7k 17.4k B220A 62pF CF 100pF 10.0k COUT 10µF 6.3V X5R CERAMIC L1: SUMIDA CDRH5D28-2R5 (847) 956-0667 fSWITCH = 1.25MHz HIGH ∆I/∆t PATH IS IN BOLD FOR LAYOUT CONSIDERATIONS Figure 1b. 12V input to 3.3Voutput at 2.2A DC/DC converter Linear Technology Magazine • March 2003 85 EFFICIENCY (%) INPUT 12V L1 2.5µH 80 75 70 65 60 0 200 400 600 800 1000 1200 1400 LOAD CURRENT (mA) Figure 2. Efficiency of the negative boost converter in Figure 1a is as high as 85% and typically greater than 80%. 33 DESIGN IDEAS because they can increase the ambient temperature (TA) and reduce the maximum charge current. 800mA Charger Circuit Another method of maximizing charge current is to dissipate some of the power in an external component, thus reducing the power dissipation on the die. Figure 1 shows how the LTC4054 can provide a complete standalone lithium-ion charger solution using few external components. The external resistor Rcc is used to dissipate 160mW of the charger’s total power dissipation, enabling the LTC4054 to thermally regulate at higher charge currents. Because the power is dissipated in an external component that also uses the PC board as its heat sink, the temperature of the die is reduced. When this circuit is programmed to charge at 800mA, the voltage on the VCC pin drops to 4.8V. With a nominal battery voltage of 3.7V and an ambient temperature of 25°C, the LTC4054 enters thermal regulation when (see sidebar): θJA ≤ 95°C = 108°C/W 1.1V • 800mA The thermal resistance of the LTC4054 can now be as high as 108°C/W before thermal regulation limits the charge current. Dissipating power in an external component is a useful technique, especially when a high input supply voltage is used. However, the designer should avoid dropping the VCC pin voltage low enough to put the LTC4054 into dropout, which could increase the time spent charging in constant-voltage mode. This occurs when the voltage across the internal MOSFET drops low enough to cause the FET to enter the linear region. The transistor does not enter the linear region as long as the following condition is met: VCC – VBAT ≥ IBAT • R DS(ON) The RDS(ON) of the LTC4054 FET is nominally 600mΩ. Since Li-Ion battery voltages do not typically exceed 4.2V, an LTC4054 programmed with 800mA will not enter dropout as long as the VCC pin stays above 4.68V. Conclusion The LTC4054 standalone Li-Ion battery charger provides a simple, compact solution for charging single cell Li-Ion batteries using very few external components. Its thermal regulation feature allows the designer to eliminate the need for thermal overdesign, maximize charge current, and shorten charge times. LT1765, continued from page 33 DC = IIN ~ (VOUT – VIN ) Maximum output current (I OUT(MAX)) is an approximation derived from the maximum allowable input current given the ripple current. VOUT VOUT • IOUT VIN • η where η is the overall efficiency IOUT(MAX) = IP –P ISW(MAX) – • VIN • η 2 VOUT IP –P = IIN • 40% IP –P = Input and Output Capacitors DC • VIN f •L where f is the switching frequency or L = DC • VIN f • IP –P Maximum inductor current (IL(MAX)) is equal to peak switch current in this configuration. The IC has a maximum switch current (ISW(MAX)) of 3A, so the maximum inductor current must remain below 3A. To keep switch current below the maximum, more inductance might be needed to keep the ripple current low enough. IL(MAX) = ISW(MAX) = IIN + IP –P 2 Linear Technology Magazine • March 2003 Like a typical boost converter, the input capacitor in the negative boost topology has low ripple current and the output capacitor has high discontinuous ripple current. The size of the output capacitor is typically bigger than the input capacitor in order to handle the greater RMS ripple current. ICIN(RMS) = IP –P ICOUT(RMS) = 12 2 2 I 1 – DC • IIN + P –P 12 ( ) The output capacitor ESR has a direct effect on the output voltage ripple of the DC/DC converter. Choosing higher frequency switch-mode regulators reduces the need for excessive RMS ripple current rating. Regardless, a low-ESR output capacitor, such as a ceramic, can minimize the output voltage ripple of the negative boost converter. ∆VOUT(P –P) = ISW(MAX) • ESR COUT Layout Figures 1a and 1b show the high ΔI/Δt switching paths of the negative boost and positive buck DC/DC converters. This loop must be kept as small as possible, by minimizing trace lengths, in order to minimize trace inductance. The discontinuous currents in this path create very high ΔI/Δt values. Any trace inductance in this loop results in voltage spikes that can render a circuit noisy or uncontrollable. For this reason, circuit layout can be just as important as component selection. Note that the layout of the negative boost is similar to the positive buck regulator, with the locations of the input and output swapped. 35