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L6995D Scheda tecnica(PDF) 8 Page - STMicroelectronics |
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L6995D Scheda tecnica(HTML) 8 Page - STMicroelectronics |
8 / 25 page L6995 8/25 Eq 6 Where ∆VOUT is the output ripple and ∆VINT is the ripple wanted at the INT pin (100mV typ). Figure 4. Integrator loop block diagram Respect to a traditional PWM controller, that has an internal oscillator setting the switching frequency, in a hys- teretic system the frequency can change with some parameters (input voltage, output current). In L6995 is im- plemented the voltage feed-forward circuit that allows constant switching frequency during steady-sate operation with the input voltage variation. There are many factors affecting switching frequency accuracy in steady-state operation. Some of these are internal as dead times, which depend on high side MOSFET driver. Others related to the external components as high side MOSFET gate charge and gate resistance, voltage drops on supply and ground rails, low side and high side RDSON and inductor parasitic resistance. During a positive load transient, (the output current increases), the converter switches at its maximum frequency (the period is TON+TOFFmin) to recover the output voltage drop. During a negative load transient, (the output current decreases), the device stops to switch (high side MOSFET remains off). 1.3 Transition from PWM to PFM/PSK To achieve high efficiency at light load conditions, PFM mode is provided. The PFM mode differs from the PWM mode essentially for the off section; the on section is the same. In PFM after a turn-on cycle the system turns- on the low side MOSFET, until the inductor current reaches the zero A value, when the zero-crossing compar- ator turns off the low side MOSFET. In this way the energy stored in the output capacitor will not flow to ground, through the low side MOSFET, but it will flow to the load. In PWM mode, after a turn on cycle, the system keeps the low side MOSFET on until the next turn-on cycle, so the energy stored in the output capacitor will flow through the low side MOSFET to ground. The PFM mode is naturally implemented in hysteretic controller, in fact in PFM mode the system reads the output voltage with a comparator and then turns on the high side MOS- FET when the output voltage goes down a reference value. The device works in discontinuous mode at light load and in continuous mode at high load. The transition from PFM to PWM occurs when load current is around half the inductor current ripple. This threshold value depends on VIN, L, and VOUT. Note that the higher the in- C IN T 2 C IN T 1 ---------------- V OUT ∆ V IN T ------------------ = PCB TRACES LOAD From Vsense Q Cint1 R1 INT Vref FB DS R S Vout Vin HGATE LGATE Q OSC Vref One-shot generator FFSR PWM comparator Vsense Gndsense R2 Cint2 HS LS Integrator amplifier + - - + + - |
Codice articolo simile - L6995D |
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Descrizione simile - L6995D |
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