Datasheet LTC3806 (Analog Devices) - 8

LTC3806
U OPERATIO Main Control Loop
the RUN pin below 1.139V, the chip is off and the input supply current is typically only 50μA. If the RUN pin is The LTC3806 is a constant frequency, current mode above 1.230V, most internal circuitry remains off until V flyback converter controller. A secondary-side gate driver IN exceeds the undervoltage comparator UV2 threshold. This capable of driving several MOSFET synchronous rectifiers reduces start-up current to approximately 80μA allowing is provided. To insure best cross regulation, DC/DC con- smaller values for C1 and larger values for R1 to be used. verters using this controller operate in forced continuous conduction (current is always flowing in either the primary The undervoltage comparator UV1 keeps G1 and G2 low or secondary winding(s) of the transformer.) until INTVCC voltage is >4.7V to insure that gate drivers will switch the external power MOSFETs properly. For circuit operation, please refer to the Block Diagram of the IC and Figure 1. In normal operation, the primary-side Prior to normal operation, soft-start pin SS is low clamp- power MOSFET is turned on when the oscillator sets the ing the output of the V-to-I converter to a low value causing PWM latch and is turned off when the current comparator current comparator C1 to trip at a low threshold. Once C1 resets the latch. VOUT1 is divided down and compared operation begins, the SS pin ramps up causing the clamp to an internal 1.230V reference by error amplifier EA, voltage to rise as well. This allows progressively higher which outputs an error signal at the ITH pin. The voltage of trip points on comparator C1 and progressively higher the ITH pin sets the current comparator C1 input threshold. peak currents to be supplied to the primary of the trans- When the load current on either output increases, a fall in former. Soft-start is completed when the voltage on the SS the FB voltage relative to the reference voltage causes the pin exceeds the voltage on the ITH pin. ITH pin to rise increasing the primary-side peak current The nominal operating frequency of the LTC3806 is 250kHz. thereby maintaining regulation. Regulation of VOUT2 is Since forced continuous operation is used, the noise indirect, occurring via transformer action. spectrum over all operating conditions is well controlled The RUN pin and undervoltage comparators control with virtually all noise occurring at the operating frequency whether the IC is enabled or is in a low current state. With and its harmonics. 3806fb 8