L DESIGN FEATURES Reduce the Size and Cost of High Current, High-Power-Density Systems with Tracking Dual, 2-Phase Constant Frequency Step-Down Regulator by Wilson Zhou and Henry Zhang Introduction PolyPhase® power converters offer several advantages over single phase converters, including higher efficiency, smaller size and lower capacitor ripple currents. The LTC3828 is a 2-phase, constant frequency, dual synchronous buck controller that minimizes both the size and total cost of the power system. 4.7µF The LTC3828 offers voltage tracking with flexibility of start-up control, a fixed internal start-up time and an adjustable external soft-start. Multiple LTC3828s can be daisy-chained in applications that require more than two tracked and synchronized voltages. The LTC3828 is well suited to high current and high density power + VIN PGOOD INTVCC TG2 0.1µF BOOST1 SW1 CMDSH-3 500kHz BG2 PGND SENSE1+ SENSE2 + – SENSE1 VOSENSE1 – SENSE2 VOSENSE2 TRCKSS2 TRCKSS1 ITH2 1000pF 63.4k 1% + FCB/PLLIN 63.4k 1% 20k 20k 1% 1% 47µF 4V SP ITH1 220pF RUN1 SGND RUN2 0.01Ω TRCKSS1 220pF 15k 0.1µF 3.2µH 0.1µF SW2 1000pF 0.01Ω 3.3V 5A BOOST2 LTC3828 BG1 4.5V TO 28V 22µF 50V CERAMIC 1µF CERAMIC CMDSH-3 TG1 3.2µH solutions. Its adjustable operating frequency (260kHz–550kHz) allows the use of small surface mount inductors and capacitors, and powerful gate drivers and short dead time make it possible to achieve high efficiency over a wide range of load currents. Figure 1 shows a typical application and Figure 2 shows its efficiency. 15k 2.5V 5A 42.2k 1% 20k 1% 56µF 4V SP 0.1µF + Figure 1. The two outputs of this dual 2.5V/3.3V step-down converter operate out of phase to increase efficiency and reduce size. In this configuration, the channels track each other. 100 Burst Mode 90 OPERATION 70 60 OUTPUT VOLTAGE EFFICIENCY (%) 80 CONSTANT FREQUENCY (BURST DISABLE) 50 40 VOUT2 30 20 10 0 0.001 FORCED CONTINUOUS MODE VIN =15V VOUT = 5V F = 260kHz 0.01 1 0.1 OUTPUT CURRENT (A) 10 Figure 2. Efficiency of circuit in Figure 1 VOUT2 VOUT1 OUTPUT VOLTAGE VOUT1 TIME (a) Coincident tracking TIME (b) Ratiometric tracking Figure 3. Two different methods of output voltage tracking Linear Technology Magazine • September 2006 DESIGN FEATURES L VOUT1 VOUT2 TO TRCKSS2 PIN R3 R1 R4 R2 TO TO VOSENSE1 VOSENSE2 PIN PIN R3 VOUT1 TO TRCKSS2 PIN R4 VOUT2 R1 R2 TO TO VOSENSE2 VOSENSE1 PIN PIN R3 R4 Figure 4. Setups for coincident and ratiometric tracking Tracking and Soft-Start I I S1 TRCKSS D1 + D2 EA – 0.8V internal soft-start voltage higher than the voltage on TRCKSS pin. During start-up, if the voltage on TRCKSS pin is lower than 60mV, there is no switching action to force the output to follow the tacking pin voltage. This is helpful to prevent the power supply from mis-tracking noise and potentially damaging the system. When the TRCKSS pin voltage ramps up and is lower than 0.6V, no negative inductor current is allowed and the converter stays in pulse skip mode. Figure 6 shows the output voltage and inductor current waveforms during start-up. Figure 5 shows the tracking function. At the input stage of the error amplifier, three diodes are used to clamp the equivalent reference voltage. The top two current sources are of the same value. When TRCKSS voltage is low, switch S1 is on and VOSENSE follows the TRCKSS voltage. When the TRCKSS voltage is close to the reference voltage, 0.8V, switch S1 is off and VOSENSE follows the reference voltage. Regulation of the channels’ outputs is not affected by the tracking mode. In the ratiometric tracking mode, the two channels do not exhibit cross talk. When LTC3828 is configured to soft-start by itself, an external softstart capacitor should be connected to the TRCKSS pin. A soft-start current of 1.2µA charges the soft-start capacitor. If during start-up, the voltage at TRCKSS pin is already higher than 0.8V, the soft-start for this particular channel is provided by an internal soft-start timer (about 450µs). If the voltage at TRCKSS pin is already high, but lower than 0.8V, this channel’s VOSENSE follows the internal soft-start voltage until the The two supply channels are ramped according to the voltages on the TRCKSS pins. The channel outputs track either coincidentally or ratiometrically, as shown in Figure 3. To implement coincident tracking (Figure 3a), connect a resistive divider to the output of the master channel (for coincident tracking, the master must be the higher output volatge) and connect its midpoint to the slave channel’s TRCKSS pin. The ratio of this divider should be the same as the slave channel’s feedback divider (Figure 4a). To implement ratiometric tracking (Figure 3b), no extra divider is needed; simply connect one of the TRCKSS pins to the other channel’s VFB pin (Figure 4b). D3 VOSENSE Figure 5. Equivalent input circuit of error amplifier to implement start-up tracking Power Good Monitor and Fault Protection Two window comparators monitor the feedback voltages and the open-drain PGOOD output is pulled low when the feedback is not within the 7.5% of the 0.8V reference voltage. The PGOOD only monitors the channel when the channel is enabled (This channel’s RUN pin voltage is high). For example, if channel 1 is shutdown and channel 2 is enabled, PGOOD is high only continued on page 28 VIN 2V/DIV VOUT 1V/DIV VSW 10V/DIV IL 2A/DIV 1ms/DIV VSW 10V/DIV VSW 10V/DIV IL 2A/DIV IL 2A/DIV 2µs/DIV 2µs/DIV Figure 6. Soft-start waveforms (no negative current) Linear Technology Magazine • September 2006 L DESIGN IDEAS ternal clock from 200kHz to 600kHz. Synchronization facilitates integration into applications using other switching regulators. Essential Soft-Start, Short Circuit and Overvoltage Protection VOUT 12V SW The LTC3827 includes a programmable soft-start time, which requires only a single external capacitor between the SS pin and ground. At high input voltages, a relatively large capacitor prevents inrush currents during start-up. This in turn prevents output overvoltage and sudden drops in VIN, which in the extreme case could force the LTC3824 below its 4V undervoltage lockout. During soft-start, the voltage on the SS pin, VSS, acts as the reference voltage that controls the output voltage ramp-up. The effective range of VSS during ramp-up is 0V to 0.8V. The typical time for the output to reach the programmed level is determined by the selected soft-start capacitor and the SS pin’s 7µA pull-up current: TSS = (C • 0.8V)/7µA. Short circuit and overvoltage protection are designed to keep the LTC3824 operating normally even LTC3828, continued from page when the channel 2 feedback voltage is within ±7.5% window. The LTC3828 incorporates protection features such as current limit, short circuit current foldback limit, input undervoltage lockout and output overvoltage protection. The current comparators have a maximum sense voltage of 75mV resulting in a maximum MOSFET current of 75mV/ RSENSE. If the output falls below 70% of its nominal output level, then the maximum sense voltage is progressively lowered from 75mV to 25mV. Table 1. The phase relationships of the two output channels and the clock out (CLKOUT) pin depend on the voltage at the PHSMD pin. VPHSMD GND OPEN INTVCC Controller 1 0° 0° 0° Controller 2 180° 180° 240° CLKOUT 60° 90° 120° 28 VIN 20V TO 12V 400µs/DIV Figure 4. When the input voltage drops to below the programmed output voltage, the output voltage gracefully tracks the input voltage. under extreme conditions. In normal operation, the feedback voltage VFB is regulated to 0.8V. If VFB drops below 0.5V, the LTC3824’s switching frequency folds back to 50kHz on the assumption that inductor current is ramping up too quickly during the MOSFET’s on-time. Runaway is avoided by providing extra time for the inductor current to discharge. An overvoltage comparator monitors the voltage at VFB, and in the event of an overshoot adjusts the VC voltage downward, keeping the MOSFET off. The overvoltage protection (OVP) threshold is lowered during light load Burst Mode operation, which causes cycles to be skipped. The OVP threshold A comparator monitors the output for overvoltage condition. When the comparator detects the feedback voltage higher than 7.5% of reference voltage, the top MOSFET is turned off and the bottom MOSFET is turned on. Phase-Locked Loop and Phase Mode Selection The LTC3828 includes a phase-locked loop comprising an internal voltage controlled oscillator and phase detector. This allows the top MOSFET turn-on to be locked to the rising edge of an external source, where the frequency range of the voltage controlled oscillator is ±50% around the center frequency. A voltage applied to the PLLFLTR pin of 1.2V corresponds to a frequency of approximately 400kHz. The nominal operating frequency range is 260kHz to 550kHz. In the LTC3828, there is an internal master oscillator running at a frequency twelve times that of each Figure 5. The LTC3824 comes in a small, thermally enhanced MSE package. goes up when load current increases. This scheme maintains protection yet ensures the tightest possible output voltage regulation. Conclusion LTC 3824 is a high voltage step-down controller with essential features for many sophisticated industrial and automotive systems. It comes in a tiny thermally enhanced 10-pin MSE package (Figure 5) to save space, and is highly configurable, including the ability to synchronize with external frequency sources, two modes of light load operation, and programmable soft-start and current limit. L controller’s frequency. The PHSMD pin (UH package only) determines the relative phases between the internal controllers as well as the CLKOUT signal as shown in Table 1. The phases tabulated are relative to zero phase being defined as the rising edge of the top gate (TG1) driver output of controller 1. The CLKOUT signal can be used to synchronize additional power stages in a multiphase (3-, 4-, or 6-phase) power supply solution feeding a single, high current output or separate outputs. In the G28 package, CLKOUT is 90° out of phase with channel 1 and channel 2. Conclusion The LTC3828 is a constant-frequency dual high performance step-down switching regulator controller. Its high efficiency, high power density, current mode architecture make this product ideal for automotive, telecom and battery systems. L Linear Technology Magazine • September 2006