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PM6680 数据表(PDF) 23 Page - STMicroelectronics |
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PM6680 数据表(HTML) 23 Page - STMicroelectronics |
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23 / 49 page  PM6680 Device description 23/49 7.2 Constant on time architecture Figure 29 shows the simplified block diagram of a Constant On Time controller. A minimum off-time constrain (300 ns typ.) is introduced to allow inductor valley current sensing on synchronous switch. A minimum on-time (70 ns) is also introduced to assure the start-up switching sequence. PM6680 has a one-shot generator for each power section that turns on the high side MOSFET when the following conditions are satisfied simultaneously: the PWM comparator is high, the synchronous rectifier current is below the current limit threshold, and the minimum off-time has timed out. Once the on-time has timed out, the high side switch is turned off, while the synchronous switch is turned on according to the anti-cross conduction circuitry management. When the negative input voltage at the PWM comparator (Figure 29), which is a scaled- down replica of the output voltage (see the external R1/R2 divider in Figure 29), reaches the valley limit (determined by internal reference Vr = 0.9 V), the low-side MOSFET is turned off according to the anti-cross conduction logic once again, and a new cycle begins. Figure 29. Constant on-time block diagram + - HS HS driver driver Level Level shifte shifte rr LS LS driver driver Q Q R S LDO5 LDO5 Q R S PWM PWM Comparato Comparato rr Positive Positive Current Current Limit Limit Vr Vr + - Zero Zero--cross. cross. Comp. Comp. bandga bandga p p 1.236V 1.236V SKIP SKIP VREF VREF PHASE PHASE PGND PGND LGATE LGATE HGATE HGATE BOOT BOOT CSENSE CSENSE COMP COMP gm gm OUT OUT Vr Vr Vr Vr + + - Toff Toff--min min Ton Ton + - VIN VIN 0.5V + - 0.5V + - 0.25V LDO5 LDO5 FB FB + - HS HS driver driver Level Level shifte shifte rr LS LS driver driver Q Q R S LDO5 LDO5 Q R S PWM PWM Comparato Comparato rr Positive Positive Current Current Limit Limit Vr Vr + - Zero Zero--cross. cross. Comp. Comp. bandga bandga p p 1.236V 1.236V SKIP SKIP VREF VREF PHASE PHASE PGND PGND LGATE LGATE HGATE HGATE BOOT BOOT CSENSE CSENSE COMP COMP gm gm OUT OUT Vr Vr Vr Vr + + - + - Toff Toff--min min Ton Ton + - VIN VIN 0.5V + - 0.5V + - 0.5V + - 0.25V + - 0.25V LDO5 LDO5 FB FB |
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