Datasheet LTC3900 (Analog Devices) - 9

LTC3900
applicaTions inForMaTion
time. The current sinking capability of the circuit is around 10.5mV to prevent tripping under light load conditions. 1mA. The timeout function can be disabled by connecting If the product of the inductor negative peak current and the timer pin to GND. MOSFET RDS(ON) is higher than 10.5mV, the LTC3900 will operate in discontinuous current mode. Figure 6 shows
Current Sense
the LTC3900 operating in discontinuous current mode; The differential input current sense comparator is used the CG output goes low before the next negative SYNC for sensing the voltage across the drain-to-source termi- pulse, as soon as the inductor current becomes negative. nals of Q4 through the CS+ and CS– pins. If the inductor Discontinuous current mode is sometimes undesirable. current reverses into the Q4 causing CS+ to rise above To disable discontinuous current mode operation, add a CS– by more than 10.5mV, the LTC3900 pulls CG low. This resistor divider, RCS1 and RCS2 at the CS+ pin to increase comparator is used to prevent inductor reverse current the 10.5mV threshold so that the LTC3900 operates in buildup during power down or Burst Mode operation, which continuous mode at no load. may cause damage to the MOSFET. The 10.5mV input The LTC3900 CS+ pin has an internal current sinking threshold has a positive temperature coefficient, which clamp circuit (ZCS in the Block Diagram) that clamps the closely matches the TC of the external MOSFET RDS(ON). pin to 11V. The clamp circuit is to be used together with The current sense comparator is only active 250ns after the external series resistor, RCS1 to protect the CS+ pin CG goes high; this is to avoid any ringing immediately from high Q4 drain voltage in the power transfer cycle. after Q4 is switched on. During the power transfer cycle, Q4 is off, the drain volt- Under light load conditions, if the inductor average cur- age of Q4 is determined by the primary input voltage and rent is less than half of its peak-to-peak ripple current, the transformer turns ratio. This voltage can be high and the inductor current will reverse into Q4 during a portion may damage the LTC3900 if CS+ is connected directly to of the switching cycle, forcing CS+ to rise above CS–. the drain of Q4. The current sinking capability of the clamp The current sense comparator input threshold is set at circuit is 5mA minimum. TIMER DO NOT RESET SG MISSING/LOW AT SECOND NEGATIVE TIMER RESET AFTER POSITIVE SYNC PULSE RECEIVING POSITIVE SYNC PULSE SYNC PULSE SYNC SYNC FG FG CG CG TIMER RESET (INTERNAL) TIMEOUT INDUCTOR 0A CURRENT CURRENT SENSE COMPARATOR TRIP TIMEOUT 3900 F06a THRESHOLD TIMER
Figure 6a. Discontinuous Current Mode Operation at No Load
3900 F05
Figure 5. Timer Waveforms with Incorrect SYNC Pulses
3900fb 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts