Datasheet LTC3772B (Analog Devices) - 9
| Fabricante | Analog Devices |
| Descripción | Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller |
| Páginas / Página | 20 / 9 — OPERATIO. (Refer to the Functional Diagram). Light Load Current … |
| Formato / tamaño de archivo | PDF / 331 Kb |
| Idioma del documento | Inglés |
OPERATIO. (Refer to the Functional Diagram). Light Load Current Operation. Soft-Start
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LTC3772B
U OPERATIO (Refer to the Functional Diagram)
However, once the controller’s duty cycle exceeds 20%, voltage to rise smoothly from 0V to its final value, while slope compensation begins and effectively reduces the maintaining control of the inductor current. After the peak sense voltage by an amount given by the curve in soft-start is timed out, it is disabled until the part is put in Figure 1. shutdown again or the input supply is cycled. The peak inductor current is determined by the peak sense
Light Load Current Operation
voltage and the on-resistance of the external P-channel Under very light load current conditions, the I MOSFET: TH/RUN pin voltage will be very close to the zero current level of 0.85V. ∆V As the load current decreases further, an internal offset at I SENSE MAX ( ) PEAK = the current comparator input will assure that the current RDS ON ( ) comparator remains tripped (even at zero load current) and the regulator will start to skip cycles, as it must, in
Soft-Start
order to maintain regulation. This behavior allows the The start-up of V regulator to maintain constant frequency down to very OUT is controlled by the LTC3772B inter- nal soft-start. During soft-start, the error amplifier com- light loads, resulting in low output ripple as well as low pares the feedback signal V audio noise and reduced RF interference, while providing FB to the internal soft-start ramp (instead of the 0.8V reference), which rises linearly high light load efficiency. from 0V to 0.8V in about 0.6ms. This allows the output 100 90 80 70 60 50 40 30 20 10 SF = REDUCTION IN SENSE VOLTAGE (mV) 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 3772B F01
Figure 1. Reduction in Sense Voltage Due to Slope Compensation vs Duty Cycle
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U OPERATIO (Refer to the Functional Diagram)
However, once the controller’s duty cycle exceeds 20%, voltage to rise smoothly from 0V to its final value, while slope compensation begins and effectively reduces the maintaining control of the inductor current. After the peak sense voltage by an amount given by the curve in soft-start is timed out, it is disabled until the part is put in Figure 1. shutdown again or the input supply is cycled. The peak inductor current is determined by the peak sense
Light Load Current Operation
voltage and the on-resistance of the external P-channel Under very light load current conditions, the I MOSFET: TH/RUN pin voltage will be very close to the zero current level of 0.85V. ∆V As the load current decreases further, an internal offset at I SENSE MAX ( ) PEAK = the current comparator input will assure that the current RDS ON ( ) comparator remains tripped (even at zero load current) and the regulator will start to skip cycles, as it must, in
Soft-Start
order to maintain regulation. This behavior allows the The start-up of V regulator to maintain constant frequency down to very OUT is controlled by the LTC3772B inter- nal soft-start. During soft-start, the error amplifier com- light loads, resulting in low output ripple as well as low pares the feedback signal V audio noise and reduced RF interference, while providing FB to the internal soft-start ramp (instead of the 0.8V reference), which rises linearly high light load efficiency. from 0V to 0.8V in about 0.6ms. This allows the output 100 90 80 70 60 50 40 30 20 10 SF = REDUCTION IN SENSE VOLTAGE (mV) 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 3772B F01
Figure 1. Reduction in Sense Voltage Due to Slope Compensation vs Duty Cycle
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