Datasheet LT3798 (Analog Devices) - 8

LT3798
OPERATION
The LT3798 is a current mode switching controller IC part begins to switch. The VIN hysteresis is set by the EN/ designed specifically for generating a constant current/ UVLO resistor divider. The third winding provides power constant voltage supply in an isolated flyback topology. to VIN when its voltage is higher than the VIN voltage. A The special problem normally encountered in such circuits voltage shunt is provided for fault protection and can sink is that information relating to the output voltage and cur- 8mA of current when VIN is over 40V. rent on the isolated secondary side of the transformer During a typical cycle, the gate driver turns the external must be communicated to the primary side in order to MOSFET on and a current flows through the primary wind- maintain regulation. Historically, this has been done with ing. This current increases at a rate proportional to the an opto-isolator. The LT3798 uses a novel method of using input voltage and inversely proportional to the magnetizing the external MOSFETs peak current information from the inductance of the transformer. The control loop determines sense resistor to calculate the output current of a flyback the maximum current and the current comparator turns converter without the need of an opto-coupler. the switch off when the current level is reached. When the Active power factor correction is becoming a requirement switch turns off, the energy in the core of the transformer for offline power supplies and the power levels are de- flows out the secondary winding through the output diode, creasing. A power factor of one is achieved if the current D1. This current decreases at a rate proportional to the drawn is proportional to the input voltage. The LT3798 output voltage. When the current decreases to zero, the modulates the peak current limit with a scaled version output diode turns off and voltage across the secondary of the input voltage. This technique can provide power winding starts to oscillate from the parasitic capacitance factors of 0.97 or greater. and the magnetizing inductance of the transformer. Since The Block Diagram shows an overall view of the system. The all windings have the same voltage across them, the third external components are in a flyback topology configura- winding rings too. The capacitor connected to the DCM tion. The third winding senses the output voltage and also pin, C1, trips the comparator A2, which serves as a dv/dt supplies power to the part in steady-state operation. The detector, when the ringing occurs. This timing information V is used to calculate the output current and will be described IN pin supplies power to an internal LDO that generates 10V at the INTV below. The dv/dt detector waits for the ringing waveform CC pin. The novel control circuitry consists of two error amplifiers, a minimum circuit, a multiplier, to reach its minimum value and then the switch turns back a transmission gate, a current comparator, a low output on. This switching behavior is similar to zero volt switching current oscillator and a master latch, which will be ex- and minimizes the amount of energy lost when the switch plained in the following sections. The part also features a is turned back on and improves efficiency as much as sample-and-hold to sample the output voltage from the 5%. Since this part operates on the edge of continuous third winding. A comparator is used to detect discontinu- conduction mode and discontinuous conduction mode, ous conduction mode (DCM) with a cap connected to the the operating mode is called critical conduction mode (or third winding. The part features a 1.9A gate driver. boundary conduction mode). The LT3798 is designed for both off-line and DC applica-
Primary Side Control Loops
tions. The EN/UVLO and a resistor divider can be configured The LT3798 achieves constant current/constant voltage for a micropower hysteretic start-up. In the Block Diagram, operation by using two separate error amplifiers. These R3 is used to stand off the high voltage supply voltage. two amplifiers are then fed to a circuit that outputs the The internal LDO starts to supply current to the INTVCC lower voltage of the two, shown as the "minimum" block in when VIN is above 2.5V. The VIN and INTVCC capacitors are the Block Diagram. This voltage is converted to a current charged by the current from R3. When VIN exceeds the before being fed into the multiplier. turn-on threshold and INTVCC is in regulation at 10V, the 3798fa 8 Document Outline Description Features Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts