Datasheet LTC3772 (Analog Devices) - 8
| Fabricante | Analog Devices |
| Descripción | Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller |
| Páginas / Página | 20 / 8 — OPERATIO. (Refer to the Functional Diagram). Main Control Loop (Normal … |
| Formato / tamaño de archivo | PDF / 259 Kb |
| Idioma del documento | Inglés |
OPERATIO. (Refer to the Functional Diagram). Main Control Loop (Normal Operation). Dropout Operation
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LTC3772
U OPERATIO (Refer to the Functional Diagram) Main Control Loop (Normal Operation)
operation. With the switch held off, average inductor cur- rent will decay to zero and the load will eventually cause the The LTC3772 is a constant frequency current mode step- error amplifier output to start drifting higher. When the error down switching regulator controller. During normal op- amplifier output rises to 0.87V, the sleep comparator will eration, the external P-channel MOSFET is turned on each untrip and normal operation will resume. The next oscilla- cycle when the oscillator sets the RS latch and turned off when the current comparator resets the latch. The peak tor cycle will turn the external MOSFET on and the switch- inductor current at which the current comparator trips is ing cycle will repeat. controlled by the voltage on the ITH/RUN pin, which is the
Dropout Operation
output of the error amplifier. The negative input to the error amplifier is the output feedback voltage VFB, which is When the input supply voltage decreases towards the generated by an external resistor divider connected be- output voltage, the rate of change of inductor current tween VOUT and ground. When the load current increases, during the on cycle decreases. This reduction means that it causes a slight decrease in VFB relative to the 0.8V at some input-output differential, the external P-channel reference, which in turn causes the ITH/RUN voltage to MOSFET will remain on for more than one oscillator cycle increase until the average inductor current matches the (start dropping off-cycles) since the inductor current has new load current. not ramped up to the threshold set by the error amplifier. The main control loop is shut down by pulling the I Further reduction in input supply voltage will eventually TH/RUN pin to ground. Releasing the I cause the external P-channel MOSFET to be turned on TH/RUN pin allows an internal 1µA current source to charge up the external 100%; i.e., DC. The output voltage will then be determined compensation network. When the I by the input voltage minus the voltage drop across the TH/RUN pin voltage reaches approximately 0.6V, the main control loop is sense resistor, the MOSFET and the inductor. enabled and the ITH/RUN voltage is pulled up by a clamp
Undervoltage Lockout Protection
to its zero current level of approximately one diode voltage drop (0.7V). As the external compensation net- To prevent operation of the external P-channel MOSFET work continues to charge up, the corresponding peak with insufficient gate drive, an undervoltage lockout cir- inductor current level follows, allowing normal operation. cuit is incorporated into the LTC3772. When the input The maximum peak inductor current attainable is set by a supply voltage drops below approximately 2V, the clamp on the ITH/RUN pin at 1.2V above the zero current P-channel MOSFET and all internal circuitry other than the level (approximately 1.9V). undervoltage block itself are turned off. Input supply current in undervoltage is approximately 1µA.
Burst Mode Operation
The LTC3772 incorporates Burst Mode operation at low
Short-Circuit Protection
load currents (<10% of IMAX). In this mode, an internal If the output is shorted to ground, the frequency of the clamp sets the peak current of the inductor at a level cor- oscillator is folded back from 550kHz to approximately responding to an ITH/RUN voltage 0.925V, even though the 200kHz while maintaining the same minimum on time. actual ITH/RUN voltage is lower. When the inductor’s av- This lower frequency allows the inductor current to safely erage current is greater than the load requirement, the discharge, thereby preventing current runaway. After the voltage at the ITH/RUN pin will drop. When the ITH/RUN short is removed, the oscillator frequency will gradually voltage falls to 0.85V, the sleep comparator will trip, turn- increase back to 550kHz as VFB rises through 0.3V on its ing off the external MOSFET. In sleep, the input DC supply way back to 0.8V. current to the IC is reduced to 40µA from 250µA in normal 3772f 8
U OPERATIO (Refer to the Functional Diagram) Main Control Loop (Normal Operation)
operation. With the switch held off, average inductor cur- rent will decay to zero and the load will eventually cause the The LTC3772 is a constant frequency current mode step- error amplifier output to start drifting higher. When the error down switching regulator controller. During normal op- amplifier output rises to 0.87V, the sleep comparator will eration, the external P-channel MOSFET is turned on each untrip and normal operation will resume. The next oscilla- cycle when the oscillator sets the RS latch and turned off when the current comparator resets the latch. The peak tor cycle will turn the external MOSFET on and the switch- inductor current at which the current comparator trips is ing cycle will repeat. controlled by the voltage on the ITH/RUN pin, which is the
Dropout Operation
output of the error amplifier. The negative input to the error amplifier is the output feedback voltage VFB, which is When the input supply voltage decreases towards the generated by an external resistor divider connected be- output voltage, the rate of change of inductor current tween VOUT and ground. When the load current increases, during the on cycle decreases. This reduction means that it causes a slight decrease in VFB relative to the 0.8V at some input-output differential, the external P-channel reference, which in turn causes the ITH/RUN voltage to MOSFET will remain on for more than one oscillator cycle increase until the average inductor current matches the (start dropping off-cycles) since the inductor current has new load current. not ramped up to the threshold set by the error amplifier. The main control loop is shut down by pulling the I Further reduction in input supply voltage will eventually TH/RUN pin to ground. Releasing the I cause the external P-channel MOSFET to be turned on TH/RUN pin allows an internal 1µA current source to charge up the external 100%; i.e., DC. The output voltage will then be determined compensation network. When the I by the input voltage minus the voltage drop across the TH/RUN pin voltage reaches approximately 0.6V, the main control loop is sense resistor, the MOSFET and the inductor. enabled and the ITH/RUN voltage is pulled up by a clamp
Undervoltage Lockout Protection
to its zero current level of approximately one diode voltage drop (0.7V). As the external compensation net- To prevent operation of the external P-channel MOSFET work continues to charge up, the corresponding peak with insufficient gate drive, an undervoltage lockout cir- inductor current level follows, allowing normal operation. cuit is incorporated into the LTC3772. When the input The maximum peak inductor current attainable is set by a supply voltage drops below approximately 2V, the clamp on the ITH/RUN pin at 1.2V above the zero current P-channel MOSFET and all internal circuitry other than the level (approximately 1.9V). undervoltage block itself are turned off. Input supply current in undervoltage is approximately 1µA.
Burst Mode Operation
The LTC3772 incorporates Burst Mode operation at low
Short-Circuit Protection
load currents (<10% of IMAX). In this mode, an internal If the output is shorted to ground, the frequency of the clamp sets the peak current of the inductor at a level cor- oscillator is folded back from 550kHz to approximately responding to an ITH/RUN voltage 0.925V, even though the 200kHz while maintaining the same minimum on time. actual ITH/RUN voltage is lower. When the inductor’s av- This lower frequency allows the inductor current to safely erage current is greater than the load requirement, the discharge, thereby preventing current runaway. After the voltage at the ITH/RUN pin will drop. When the ITH/RUN short is removed, the oscillator frequency will gradually voltage falls to 0.85V, the sleep comparator will trip, turn- increase back to 550kHz as VFB rises through 0.3V on its ing off the external MOSFET. In sleep, the input DC supply way back to 0.8V. current to the IC is reduced to 40µA from 250µA in normal 3772f 8