Datasheet LTC3836 (Analog Devices) - 10

LTC3836
OPERATION (Refer to Functional Diagram) Main Control Loop
source), the EAMP regulates the VFB1 proportionally from 0V to 0.6V. The LTC3836 uses a constant-frequency, current mode architecture with the two controllers operating 180 degrees The start-up of VOUT2 is controlled by the voltage on the out-of-phase. During normal operation, the top external TRACK/SS2 pin. When the voltage on the TRACK/SS2 power MOSFET is turned on when the clock for its chan- pin is less than the 0.6V internal reference, the LTC3836 nel sets the RS latch, and turned off when the current regulates the VFB2 voltage to the TRACK/SS2 pin voltage comparator (ICMP) resets the latch. The peak inductor instead of the 0.6V reference. This allows the TRACK/SS2 current at which ICMP resets the RS latch is determined pin to be used to program a soft-start by connecting an by the voltage on the ITH pin, which is driven by the output external capacitor from the TRACK/SS2 pin to SGND. of the error amplifi er (EAMP). The VFB pin receives the An internal 1μA pull-up current charges this capacitor, output voltage feedback signal from an external resistor creating a voltage ramp on the TRACK/SS2 pin. As the divider. This feedback signal is compared to the internal TRACK/SS2 voltage rises linearly from 0V to 0.6V (and 0.6V reference voltage by the EAMP. When the load current beyond), the output voltage VOUT2 rises smoothly from increases, it causes a slight decrease in VFB relative to the zero to its fi nal value. 0.6V reference, which in turn causes the ITH voltage to Alternatively, the TRACK/SS2 pin can be used to cause the increase until the average inductor current matches the start-up of V new load current. While the top N-channel MOSFET is off, OUT2 to “track” that of another supply. Typi- cally, this requires connecting to the TRACK/SS2 pin an the bottom N-channel MOSFET is turned on until either external resistor divider from the other supply to ground the inductor current starts to reverse, as indicated by the (see Applications Information section). current reversal comparator, IRCMP, or the beginning of the next cycle. When the RUN/SS pin is pulled low to disable the LTC3836, or when VIN drops below its undervoltage
Shutdown, Soft-Start and Tracking Start-Up
lockout threshold, the TRACK/SS2 pin is pulled low by
(RUN/SS and TRACK/SS2 Pins)
an internal MOSFET. When in undervoltage lockout, both controllers are disabled and the external MOSFETs are The LTC3836 is shut down by pulling the RUN/SS pin held off. low. In shutdown, all controller functions are disabled and the chip draws only 6.5μA. The TG and BG outputs are
Light Load Operation (Pulse-Skipping or Continuous
held low (off) in shutdown. Releasing RUN/SS allows an
Conduction) (SYNC/FCB Pin)
internal 0.65μA current source to charge up the RUN/SS The LTC3836 can be enabled to enter high effi ciency pulse- pin. When the RUN/SS pin reaches 0.65V, the LTC3836’s skipping operation or forced continuous conduction mode two controllers are enabled. at low load currents. To select pulse-skipping operation, tie The start-up of VOUT1 is controlled by the LTC3836’s the SYNC/FCB pin to a DC voltage above 0.6V (e.g., VIN). internal soft-start. During soft-start, the error amplifi er To select forced continuous operation, tie the SYNC/FCB EAMP compares the feedback signal VFB1 to the internal to a DC voltage below 0.6V (e.g., SGND). soft-start ramp (instead of the 0.6V reference), which rises In forced continuous operation, the inductor current is linearly from 0V to 0.6V in about 1ms. This allows the allowed to reverse at light loads or under large transient output voltage to rise smoothly from 0V to its fi nal value, conditions. The peak inductor current is determined by while maintaining control of the inductor current. the voltage on the ITH pin. The main N-channel MOSFET The 1ms soft-start time can be increased by con- is turned on every cycle (constant-frequency) regardless necting the optional external soft-start capacitor CSS of the ITH pin voltage. In this mode, the effi ciency at light between the RUN/SS and SGND pins. As the RUN/SS loads is lower than in pulse-skipping operation. However, pin continues to rise linearly from approximately 0.65V continuous mode has the advantages of lower output ripple to 1.3V (being charged by the internal 0.65μA current and less interference with audio circuitry. 3836fb 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS FUNCTIONAL DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY RELATED PARTS