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LTC3425EUH Datasheet(PDF) 11 Page - Linear Technology |
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LTC3425EUH Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC3425 11 3425f OPERATIO DETAILED DESCRIPTION The LTC3425 provides high efficiency, low noise power for high current boost applications such as cellular phones and PDAs. The true output disconnect feature eliminates inrush current and allows VOUT to go to zero during shutdown. The current mode architecture with adaptive slope compensation provides ease of loop compensation with excellent transient load response. The low RDS(ON), low gate charge synchronous switches eliminate the need for an external Schottky rectifier, and provide efficient high frequency pulse width modulation (PWM) control. High efficiency is achieved at light loads when Burst Mode operation is entered, where the IC’s quiescent current is a low 12 µA typical on VOUT. MULTIPHASE OPERATION The LTC3425 uses a 4-phase architecture, rather than the conventional single phase of other boost converters. By having multiple phases equally spaced (90 ° apart), not only is the output ripple frequency increased by a factor of four, but the output capacitor ripple current is greatly reduced. Although this architecture requires four induc- tors, rather than a single inductor, there are a number of important advantages. • Much lower peak inductor current allows the use of smaller, lower cost inductors. • Greatly reduced output ripple current minimizes output capacitance requirement. • Higher frequency output ripple is easier to filter for low noise applications. • Input ripple current is also reduced for lower noise on VIN. The peak boost inductor current is given by: I I DN di LPEAK O =+ (– ) • 12 Where IO is the average load current, D is the PWM duty cycle, N is the number of phases and di is the inductor ripple current. This relationship is shown graphically in Figure 1 using a single phase and a 4-phase example. Example: The following example, operating at 50% duty cycle, illustrates the advantages of multiphase operation over a conventional single-phase design. VIN = 1.9V, VOUT = 3.6V, Efficiency = 90% (approx), IOUT = 2A, Frequency = 1MHz, L = 2.2µH Table 1 SINGLE FOUR CHANGE FROM PARAMETER PHASE PHASE 1 TO 4 PHASE Peak-Peak Output 4.227A 0.450A Reduced by 89% Ripple Current RMS Output Ripple Current 2.00A 0.184A Reduced by 91% Peak Inductor Current 4.227A 1.227A Reduced by 71% Output Ripple Frequency 1MHz 4MHz Increased by 4 × With 4-phase operation, at least one of the phases will be delivering current to the load whenever VIN is greater than one quarter VOUT (duty cycles less than 75%). For lower duty cycles, there can be as many as two or three phases delivering load current simultaneously. This greatly re- duces both the output ripple current and the peak current in each inductor, compared with a single-phase converter. This is illustrated in the waveforms of Figures 2 and 3. Operation Using Only Two or Three Phases The LTC3425 can operate as a 2- or 3-phase converter by simply eliminating the inductor from the unused phase(s). TIME ( µs) 0 2 FOUR PHASE SINGLE PHASE 3 3425 F01 1 0 0.5 1 1.5 5 4 Figure 1. Comparison of Output Ripple Current with Single Phase and 4-Phase Boost Converter in a 2A Load Application Operating at 50% Duty Cycle |
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