Datasheet LTC3850-2 (Analog Devices) - 10

LTC3850-2
OPERATION Main Control Loop Shutdown and Start-Up (RUN1, RUN2 and TK/SS1, TK/SS2 Pins)
The LTC3850-2 is a constant-frequency, current mode step-down controller with two channels operating 180 The two channels of the LTC3850-2 can be independently degrees out-of-phase. During normal operation, each shut down using the RUN1 and RUN2 pins. Pulling either top MOSFET is turned on when the clock for that channel of these pins below 1.2V shuts down the main control sets the RS latch, and turned off when the main current loop for that controller. Pulling both pins low disables both comparator, ICMP, resets the RS latch. The peak inductor controllers and most internal circuits, including the INTVCC current at which ICMP resets the RS latch is controlled by regulator. Releasing either RUN pin allows an internal the voltage on the ITH pin, which is the output of each error 0.5μA current to pull up the pin and enable that control- amplifi er EA. The VFB pin receives the voltage feedback ler. Alternatively, the RUN pin may be externally pulled up signal, which is compared to the internal reference voltage or driven directly by logic. Be careful not to exceed the by the EA. When the load current increases, it causes a Absolute Maximum Rating of 6V on this pin. slight decrease in VFB relative to the 0.8V reference, which The start-up of each controller’s output voltage V in turn causes the I OUT is TH voltage to increase until the average controlled by the voltage on the TK/SS1 and TK/SS2 pins. inductor current matches the new load current. After the When the voltage on the TK/SS pin is less than the 0.8V top MOSFET has turned off, the bottom MOSFET is turned internal reference, the LTC3850-2 regulates the V on until either the inductor current starts to reverse, as FB voltage to the TK/SS pin voltage instead of the 0.8V reference. This indicated by the reverse current comparator IREV, or the allows the TK/SS pin to be used to program a soft-start beginning of the next cycle. by connecting an external capacitor from the TK/SS pin to SGND. An internal 1.3μA pull-up current charges this
INTVCC/EXTVCC Power
capacitor, creating a voltage ramp on the TK/SS pin. As the Power for the top and bottom MOSFET drivers and most TK/SS voltage rises linearly from 0V to 0.8V (and beyond), other internal circuitry is derived from the INTVCC pin. When the output voltage VOUT rises smoothly from zero to its fi nal the EXTVCC pin is left open or tied to a voltage less than value. Alternatively the TK/SS pin can be used to cause the 4.7V, an internal 5V linear regulator supplies INTVCC power start-up of VOUT to “track” that of another supply. Typically, from VIN. If EXTVCC is taken above 4.7V, the 5V regulator is this requires connecting to the TK/SS pin an external resistor turned off and an internal switch is turned on connecting divider from the other supply to ground (see the Applica- EXTVCC. Using the EXTVCC pin allows the INTVCC power tions Information section). When the corresponding RUN to be derived from a high effi ciency external source such pin is pulled low to disable a controller, or when INTVCC as one of the LTC3850-2 switching regulator outputs. drops below its undervoltage lockout threshold of 3V, the Each top MOSFET driver is biased from the fl oating boot- TK/SS pin is pulled low by an internal MOSFET. When in strap capacitor C undervoltage lockout, both controllers are disabled and B, which normally recharges during each off cycle through an external diode when the top MOSFET the external MOSFETs are held off. turns off. If the input voltage VIN decreases to a voltage
Light Load Current Operation (Burst Mode Operation,
close to VOUT, the loop may enter dropout and attempt
Pulse-Skipping, or Continuous Conduction)
to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for The LTC3850-2 can be enabled to enter high effi ciency Burst about one-twelfth of the clock period every third cycle to Mode operation, constant-frequency pulse-skipping mode, allow CB to recharge. However, it is recommended that a or forced continuous conduction mode. To select forced load be present during the drop-out transition to ensure continuous operation, tie the MODE/PLLIN pin to a DC CB is recharged. voltage below 0.8V (e.g., SGND). To select pulse-skipping 38502f 10