Datasheet LTM4668 (Analog Devices) - 9

LTM4668
APPLICATIONS INFORMATION Burst Mode Operation Force Continuous Current Mode (CCM)
The LTM4668 is capable of Burst Mode operation in which In applications where fixed frequency operation is more the power MOSFETs operate intermittently based on load critical than low current efficiency, and where the low- demand, thus saving quiescent current. For applications est output ripple is desired, forced continuous opera- where maximizing the efficiency at very light loads is a tion should be used. Forced continuous operation can high priority, Burst Mode operation should be applied. To be enabled by tying the MODE pin to INTVCC/2. In this enable Burst Mode operation, simply tie the MODE/SYNC mode, inductor current is allowed to reverse during low pin to INTVCC. output loads, the output of the error amplifier is in control During Burst Mode operation, the peak current of the of the current comparator threshold throughout, and the inductor is set to approximately 400mA in normal opera- top MOSFET always turns on with each oscillator pulse. tion even though the output of the error amplifier (COMP) During start-up, the module operates in pulse-skipped indicates a lower value. The COMP voltage drops when mode regardless of the mode programmed on the MODE/ the inductor’s average current is greater than the load SYNC pin to prevent inductor current from reversing until requirement. As the COMP voltage drops below 0.2V, the the LTM4668’s output voltage is in regulation. burst comparator trips, causing the internal sleep line to go high and turn off both power MOSFETs.
Operating Frequency
In sleep mode, the internal circuitry is partially turned The operating frequency of the LTM4668 is optimized off, reducing the quiescent current. The load current is to achieve the compact package size and the minimum now being supplied from the output capacitors. When the output ripple voltage while keeping high efficiency. The output voltage drops, causing COMP voltage to rise, the default operating frequency is internally set to 1MHz. In internal sleep line goes low, and the LTM4668 resumes most applications, no additional frequency adjusting is normal operation. The next oscillator cycle will turn on required. the top power MOSFET and the switching cycle repeats. If any operating frequency other than 1MHz is required by When all channels are in sleep mode, the LTM4668 mod- application, the µModule can be externally synchronized ule draws only 8µA of quiescent current from VIN. to a clock from 500kHz to 1.5MHz.
Pulse-Skipping Mode Operation Frequency Synchronization and Clock In
In applications where low output ripple and high efficiency The power module has a phase-locked loop comprised at intermediate currents are desired, pulse-skipping mode of an internal voltage controlled oscillator and a phase should be used by grounding the MODE/SYNC pin. In detector. This allows all internal top MOSFET turn-on to LTM4668, pulse-skipping mode is implemented similarly be locked to the rising edge of the same external clock. to Burst Mode operation with the peak inductor current The external clock frequency range must be within ±50% set to be at least 66mA. This results in lower ripple than in around the 1MHz set frequency. A pulse detection circuit Burst Mode operation with the trade-off of slightly lower is used to detect a clock on the MODE/SYNC pin to turn efficiency. on the phase-locked loop. The pulse width of the clock has to be at least 400ns. The clock high level must be above Both modes, Burst Mode operation and pulse-skipping 2V and clock low level below 0.3V. During the start-up of mode, automatically switch from continuous operation to the regulator, the phase-locked loop function is disabled. the selected mode when the load current is low. And once engaged in frequency sync, the LTM4668 runs in forced continuous mode at the external clock frequency. Rev. A For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Package Description Revision History Package Photo Design Resources Related Parts