Datasheet LTC3824 (Analog Devices) - 9

LTC3824
applicaTions inForMaTion Burst Mode Operation
If shutdown mode will be invoked after startup, it is The LTC3824 can be configured for Burst Mode operation recommended to connect a 1MΩ to 10MΩ resistor from to enhance light load efficiency (only 40µA quiescent cur- SS to ground to reset the SS capacitor during shutdown. rent) and extend battery run time by leaving the SYNC/ This ensures proper soft-start operation when exiting MODE pin open or pulling it higher than 2V. In this mode, shutdown mode. when output load drops the loop control voltage VC also
Overvoltage Protection
drops and when VC reaches approximately 0.9V at low duty cycle the LTC3824 goes into sleep mode with the To achieve good output regulation in Burst Mode operation, switch turned off. During sleep mode the output voltage an overvoltage comparator, OVP, with a threshold adap- drops and VC rises up. When VC goes up to around 70mV tive to the VC voltage is used to monitor the FB voltage. the LTC3824 will turn on the switch and the burst cycle In Burst Mode operation with low VC voltage, the OVP repeats. If the SYNC/MODE pin is grounded the Burst threshold is approximately 2% above VREF and the VREF Mode operation will be disabled and the LTC3824 skips is also shifted lower by 2% to contain the output ripple cycles at light load. and to keep output regulation constant. As output load increases, OVP threshold increases with VC voltage to up
Oscillation Frequency Setting and Synchronization
to 8% above VREF. The switching frequency of the LTC3824 can be set up to
Shutdown Mode Quiescent Current
600kHz by a resistor, RFREQ, from the RSET pin to ground. When the V For 200kHz, R C pin is pulled down below 25mV the LTC3824 FREQ = 392k. See the Switching Frequency goes into micropower shutdown mode and only draws 7µA. vs RFREQ graph in the Typical Performance Characteris- tics section. With a 100ns one-shot timer on-chip, the For proper operation, shutdown mode should not be LTC3824 provides flexibility on the sync pulse width. The engaged again too soon after exiting shutdown mode. sync pulse threshold voltage level is about 1.2V. This minimum time is a function of CSS and is calculated by tMIN = 5.5e5 • CSS. For example, if CSS = 0.1μF, then
Short-Circuit Protection
a minimum of 55ms must elapse after exiting shutdown before engaging it again. In normal operation when the output voltage is in regulation, VFB is regulated to 0.8V. If the output is shorted to ground
Output Voltage Programming
and VFB drops below 0.5V the switching frequency will be reduced to 50kHz to allow the inductor current to discharge With a 0.8V feedback reference voltage, VREF, the output and prevent current runaway. Note that synchronization voltage, VOUT, is programmed by a resistor divider as is enabled only when V shown in the Block Diagram. FB is above 0.5V. V
Soft-Start
OUT = 0.8V (1+RF1/RF2) During soft-start, the voltage on the SS pin (V
Current Sense Resistor R
SS) is the
S and Current Limit
reference voltage that controls the output voltage and the The maximum current the LTC3824 can deliver is deter- output ramps up following VSS. The effective range of VSS mined by: is from 0V to 0.8V. The typical time for the output to reach the programmed level is: IOUT(MAX) = 100mV/RS – IRIPPLE/2 C where 100mV is the internal 100mV threshold across VCC t SS • 0.8V SS = and VSENSE, and IRIPPLE is the inductor peak-to-peak ripple 5μA current. RS should be placed very close to the power switch where C with very short traces. Good kelvin sensing is required for SS is the capacitor connected from the SS pin to GND. accurate current limit. 3824fh For more information www.linear.com/LTC3824 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Applications Information Typical Application Related Parts