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LTC3414MPFE Datasheet(PDF) 10 Page - Linear Technology |
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LTC3414MPFE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3414 10 3414fb Burst Clamp Programming If the voltage on the SYNC/MODE pin is less than VIN by 1V, Burst Mode operation is enabled. During Burst Mode Operation, the voltage on the SYNC/MODE pin determines the burst clamp level, which sets the minimum peak inductor current, IBURST, for each switching cycle accord- ing to the following equation: I A V VV BURST BURST = ⎛ ⎝⎜ ⎞ ⎠⎟() 69 06 0 383 . . –. VBURST is the voltage on the SYNC/MODE pin. IBURST can only be programmed in the range of 0A to 7A. For values of VBURST greater than 1V, IBURST is set at 7A. For values of VBURST less than 0.4V, IBURST is set at 0A. As the output load current drops, the peak inductor currents decrease to keep the output voltage in regulation. When the output load current demands a peak inductor current that is less than IBURST, the burst clamp will force the peak inductor current to remain equal to IBURST regardless of further reductions in the load current. Since the average inductor current is greater than the output load current, the voltage on the ITH pin will decrease. When the ITH voltage drops to 150mV, sleep mode is enabled in which both power MOSFETs are shut off along with most of the circuitry to minimize power consumption. All circuitry is turned back on and the power MOSFETs begin switching again when the output voltage drops out of regulation. The value for IBURST is determined by the desired amount of output voltage ripple. As the value of IBURST increases, the sleep period between pulses and the output voltage ripple in- crease. The burst clamp voltage, VBURST, can be set by a resistor divider from the VFB pin to the SGND pin as shown in Figure 1. Pulse skipping, which is a compromise between low output voltage ripple and efficiency, can be implemented by connecting pin SYNC/MODEto ground. This sets IBURST to 0A. In this condition, the peak inductor current is limited by the minimum on-time of the current comparator. The lowest output voltage ripple is achieved while still operat- ing discontinuously. During very light output loads, pulse skipping allows only a few switching cycles to be skipped while maintaining the output voltage in regulation. Frequency Synchronization The LTC3414’s internal oscillator can be synchronized to an external clock signal. During synchronization, the top MOSFET turn-on is locked to the falling edge of the external frequency source. The synchronization frequency range is 300kHz to 4MHz. Synchronization only occurs if the external frequency is greater than the frequency set by the external resistor. Because slope compensation is generated by the oscillator’s RC circuit, the external frequency should be set 25% higher than the frequency set by the external resistor to ensure that adequate slope compensation is present. Soft-Start The RUN/SS pin provides a means to shut down the LTC3414 as well as a timer for soft-start. Pulling the RUN/SS pin below 0.5V places the LTC3414 in a low quiescent current shutdown state (IQ < 1μA). The LTC3414 contains an internal soft-start clamp that gradually raises the clamp on ITH after the RUN/SS pin is pulled above 2V. The full current range becomes available on ITH after 1024 switching cycles. If a longer soft-start period is desired, the clamp on ITH can be set externally with a resistor and capacitor on the RUN/SS pin as shown in Figure 1. The soft-start duration can be calculated by using the following formula: tR C V VV SECONDS SS SS SS IN IN = ⎛ ⎝⎜ ⎞ ⎠⎟ ln –. () 18 Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses: VIN quiescent current and I2R losses. APPLICATIO S I FOR ATIO |
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