Datasheet LTC3602 (Analog Devices) - 7

LTC3602
OPERATION Main Control Loop Burst Mode Operation
The LTC3602 is a monolithic, constant-frequency, current- Connecting the SYNC/MODE pin to a voltage in the range mode step-down DC/DC converter. During normal opera- of 0.42V to 1V enables Burst Mode operation. In Burst tion, the internal top power switch (N-channel MOSFET) is Mode operation, the internal power MOSFETs operate turned on at the beginning of each clock cycle. Current in intermittently at light loads. This increases effi ciency by the inductor increases until the current comparator trips minimizing switching losses. During Burst Mode opera- and turns off the top power MOSFET. The peak inductor tion, the minimum peak inductor current is externally set current at which the current comparator shuts off the top by the voltage on the SYNC/MODE pin and the voltage power switch is controlled by the voltage on the ITH pin. on the ITH pin is monitored by the burst comparator to The error amplifi er adjusts the voltage on the ITH pin by determine when sleep mode is enabled and disabled. comparing the feedback signal from a resistor divider on When the average inductor current is greater than the the VFB pin with an internal 0.6V reference. When the load load current, the voltage on the ITH pin drops. As the ITH current increases, it causes a reduction in the feedback voltage falls below 330mV, the burst comparator trips and voltage relative to the reference. The error amplifi er raises enables sleep mode. During sleep mode, the top power the ITH voltage until the average inductor current matches MOSFET is held off and the ITH pin is disconnected from the new load current. When the top power MOSFET shuts the output of the error amplifi er. The majority of the internal off, the synchronous power switch (N-channel MOSFET) circuitry is also turned off to reduce the quiescent current turns on until either the bottom current limit is reached or to 75μA while the load current is solely supplied by the the beginning of the next clock cycle. The bottom current output capacitor. When the output voltage drops, the ITH limit is set at –2.5A for forced continuous mode and 0A pin is reconnected to the output of the error amplifi er and for Burst Mode operation. the top power MOSFET along with all the internal circuitry is switched back on. This process repeats at a rate that The operating frequency is externally set by an external is dependent on the load demand. Pulse-skipping opera- resistor connected between the RT pin and ground. The tion is implemented by connecting the SYNC/MODE pin practical switching frequency can range from 300kHz to to ground. This forces the burst clamp level to be at 0V. 3MHz. As the load current decreases, the peak inductor current Overvoltage and undervoltage comparators will pull the will be determined by the voltage on the ITH pin until the PGOOD output low if the output voltage comes out of ITH voltage drops below 330mV. At this point, the peak regulation by ±7.5%. In an overvoltage condition, the top inductor current is determined by the minimum on-time power MOSFET is turned off and the bottom power MOSFET of the current comparator. If the load demand is less than is switched on until either the overvoltage condition clears the average of the minimum on-time inductor current, or the bottom MOSFET’s current limit is reached. switching cycles will be skipped to keep the output volt- age in regulation.
Forced Continuous Mode
Connecting the SYNC/MODE pin to INTV
Frequency Synchronization
CC will disable Burst Mode operation and force continuous current operation. The internal oscillator of the LTC3602 can be synchronized At light loads, forced continuous mode operation is less to an external clock connected to the SYNC/MODE pin. effi cient than Burst Mode operation, but may be desirable in The frequency of the external clock can be in the range of some applications where it is necessary to keep switching 300kHz to 3MHz. For this application, the oscillator timing harmonics out of a signal band. The output voltage ripple resistor should be chosen to correspond to a frequency is minimized in this mode. that is 25% lower than the synchronization frequency. When synchronized, the LTC3602 will operate in pulse- skipping mode. 3602fb 7