Datasheet LTC3826-1 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 30µA IQ, Dual, 2-Phase Synchronous Step-Down Controller |
| Pages / Page | 32 / 10 — OPERATION (Refer to Functional Diagram). INTVCC/EXTVCC Power. Light Load … |
| File Format / Size | PDF / 438 Kb |
| Document Language | English |
OPERATION (Refer to Functional Diagram). INTVCC/EXTVCC Power. Light Load Current Operation (Burst Mode Operation,
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Text Version of Document
LTC3826-1
OPERATION (Refer to Functional Diagram) INTVCC/EXTVCC Power
pin. When the voltage on the TRACK/SS pin is less than the 0.8V internal reference, the LTC3826-1 regulates the Power for the top and bottom MOSFET drivers and most V other internal circuitry is derived from the INTV FB voltage to the TRACK/SS pin voltage instead of the CC pin. 0.8V reference. This allows the TRACK/SS pin to be used When the EXTVCC pin is left open or tied to a voltage less to program a soft start by connecting an external capacitor than 4.7V, an internal 5.25V low dropout linear regulator from the TRACK/SS pin to SGND. An internal 1μA pull-up supplies INTVCC power from VIN. If EXTVCC is taken above current charges this capacitor creating a voltage ramp on 4.7V, the 5.25V regulator is turned off and a 7.5V low the TRACK/SS pin. As the TRACK/SS voltage rises linearly dropout linear regulator is enabled that supplies INTVCC from 0V to 0.8V (and beyond), the output voltage V power from EXTV OUT CC. If EXTVCC is less than 7.5V (but rises smoothly from zero to its fi nal value. greater than 4.7V), the 7.5V regulator is in dropout and INTVCC is approximately equal to EXTVCC. When EXTVCC Alternatively the TRACK/SS pin can be used to cause the is greater than 7.5V (up to an absolute maximum rating startup of VOUT to “track” that of another supply. Typically, of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC this requires connecting to the TRACK/SS pin an external pin allows the INTVCC power to be derived from a high resistor divider from the other supply to ground (see effi ciency external source such as one of the LTC3826-1 Applications Information section). switching regulator outputs. When the corresponding RUN pin is pulled low to disable Each top MOSFET driver is biased from the fl oating boot- a controller, or when VIN drops below its undervoltage strap capacitor CB, which normally recharges during each lockout threshold of 3.5V, the TRACK/SS pin is pulled low off cycle through an external diode when the top MOSFET by an internal MOSFET. When in undervoltage lockout, turns off. If the input voltage VIN decreases to a voltage both controllers are disabled and the external MOSFETs close to VOUT, the loop may enter dropout and attempt are held off. to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for
Light Load Current Operation (Burst Mode Operation,
about one twelfth of the clock period every tenth cycle to
Pulse Skipping, or Continuous Conduction)
allow C
(PLLIN/MODE Pin)
B to recharge. The LTC3826-1 can be enabled to enter high effi ciency
Shutdown and Start-Up (RUN1, RUN2 and TRACK/
Burst Mode operation, constant frequency pulse skipping
SS1, TRACK/SS2 Pins)
mode, or forced continuous conduction mode at low load The two channels of the LTC3826-1 can be independently currents. To select Burst Mode operation, tie the PLLIN/ shut down using the RUN1 and RUN2 pins. Pulling either MODE pin to a DC voltage below 0.7V (e.g., SGND). To of these pins below 0.7V shuts down the main control select forced continuous operation, tie the PLLIN/MODE loop for that controller. Pulling both pins low disables pin to INTVCC. To select pulse-skipping mode, tie the both controllers and most internal circuits, including the PLLIN/MODE pin to a DC voltage greater than 0.9V and INTVCC regulator, and the LTC3826-1 draws only 4μA of less than INTVCC – 1.2V. quiescent current. When a controller is enabled for Burst Mode operation, Releasing either RUN pin allows an internal 0.5μA current the peak current in the inductor is set to approximately to pull up the pin and enable that controller. Alternatively, one-tenth of the maximum sense voltage even though the the RUN pin may be externally pulled up or driven directly voltage on the ITH pin indicates a lower value. If the aver- by logic. Be careful not to exceed the Absolute Maximum age inductor current is lower than the load current, the rating of 7V on this pin. error amplifi er EA will decrease the voltage on the ITH pin. When the I The start-up of each controller’s output voltage V TH voltage drops below 0.4V, the internal sleep OUT is con- signal goes high (enabling “sleep” mode) and both external trolled by the voltage on the TRACK/SS1 and TRACK/SS2 38261fb 10
OPERATION (Refer to Functional Diagram) INTVCC/EXTVCC Power
pin. When the voltage on the TRACK/SS pin is less than the 0.8V internal reference, the LTC3826-1 regulates the Power for the top and bottom MOSFET drivers and most V other internal circuitry is derived from the INTV FB voltage to the TRACK/SS pin voltage instead of the CC pin. 0.8V reference. This allows the TRACK/SS pin to be used When the EXTVCC pin is left open or tied to a voltage less to program a soft start by connecting an external capacitor than 4.7V, an internal 5.25V low dropout linear regulator from the TRACK/SS pin to SGND. An internal 1μA pull-up supplies INTVCC power from VIN. If EXTVCC is taken above current charges this capacitor creating a voltage ramp on 4.7V, the 5.25V regulator is turned off and a 7.5V low the TRACK/SS pin. As the TRACK/SS voltage rises linearly dropout linear regulator is enabled that supplies INTVCC from 0V to 0.8V (and beyond), the output voltage V power from EXTV OUT CC. If EXTVCC is less than 7.5V (but rises smoothly from zero to its fi nal value. greater than 4.7V), the 7.5V regulator is in dropout and INTVCC is approximately equal to EXTVCC. When EXTVCC Alternatively the TRACK/SS pin can be used to cause the is greater than 7.5V (up to an absolute maximum rating startup of VOUT to “track” that of another supply. Typically, of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC this requires connecting to the TRACK/SS pin an external pin allows the INTVCC power to be derived from a high resistor divider from the other supply to ground (see effi ciency external source such as one of the LTC3826-1 Applications Information section). switching regulator outputs. When the corresponding RUN pin is pulled low to disable Each top MOSFET driver is biased from the fl oating boot- a controller, or when VIN drops below its undervoltage strap capacitor CB, which normally recharges during each lockout threshold of 3.5V, the TRACK/SS pin is pulled low off cycle through an external diode when the top MOSFET by an internal MOSFET. When in undervoltage lockout, turns off. If the input voltage VIN decreases to a voltage both controllers are disabled and the external MOSFETs close to VOUT, the loop may enter dropout and attempt are held off. to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for
Light Load Current Operation (Burst Mode Operation,
about one twelfth of the clock period every tenth cycle to
Pulse Skipping, or Continuous Conduction)
allow C
(PLLIN/MODE Pin)
B to recharge. The LTC3826-1 can be enabled to enter high effi ciency
Shutdown and Start-Up (RUN1, RUN2 and TRACK/
Burst Mode operation, constant frequency pulse skipping
SS1, TRACK/SS2 Pins)
mode, or forced continuous conduction mode at low load The two channels of the LTC3826-1 can be independently currents. To select Burst Mode operation, tie the PLLIN/ shut down using the RUN1 and RUN2 pins. Pulling either MODE pin to a DC voltage below 0.7V (e.g., SGND). To of these pins below 0.7V shuts down the main control select forced continuous operation, tie the PLLIN/MODE loop for that controller. Pulling both pins low disables pin to INTVCC. To select pulse-skipping mode, tie the both controllers and most internal circuits, including the PLLIN/MODE pin to a DC voltage greater than 0.9V and INTVCC regulator, and the LTC3826-1 draws only 4μA of less than INTVCC – 1.2V. quiescent current. When a controller is enabled for Burst Mode operation, Releasing either RUN pin allows an internal 0.5μA current the peak current in the inductor is set to approximately to pull up the pin and enable that controller. Alternatively, one-tenth of the maximum sense voltage even though the the RUN pin may be externally pulled up or driven directly voltage on the ITH pin indicates a lower value. If the aver- by logic. Be careful not to exceed the Absolute Maximum age inductor current is lower than the load current, the rating of 7V on this pin. error amplifi er EA will decrease the voltage on the ITH pin. When the I The start-up of each controller’s output voltage V TH voltage drops below 0.4V, the internal sleep OUT is con- signal goes high (enabling “sleep” mode) and both external trolled by the voltage on the TRACK/SS1 and TRACK/SS2 38261fb 10
