Datasheet LTC1627 (Analog Devices) - 10

LTC1627
U U W U APPLICATIO S I FOR ATIO Auxiliary Winding Control Using SYNC/FCB Pin
Efficiency = 100% – (L1 + L2 + L3 + ...) The SYNC/FCB pin can be used as a secondary feedback where L1, L2, etc. are the individual losses as a percentage input to provide a means of regulating a flyback winding of input power. output. When this pin drops below its ground referenced Although all dissipative elements in the circuit produce 0.8V threshold, continuous mode operation is forced. In losses, two main sources usually account for most of the continuous mode, the main and synchronous MOSFETs losses in LTC1627 circuits: V are switched continuously regardless of the load on the IN quiescent current and I2R losses. main output. 1. The V Synchronous switching removes the normal limitation IN quiescent current is due to two components: the DC bias current as given in the electrical character- that power must be drawn from the inductor primary istics and the internal main switch and synchronous winding in order to extract power from auxiliary windings. switch gate charge currents. The gate charge current With continuous synchronous operation power can be results from switching the gate capacitance of the drawn from the auxiliary windings without regard to the internal power MOSFET switches. Each time the gate is primary output load. switched from high to low to high again, a packet of The secondary output voltage is set by the turns ratio of the charge dQ moves from VIN to ground. The resulting transformer in conjunction with a pair of external resistors dQ/dt is the current out of VIN that is typically larger returned to the SYNC/FCB pin as shown in Figure 6. The than the DC bias current. In continuous mode, IGATECHG secondary regulated voltage VSEC in Figure 6 is given by: = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias  R4 and gate charge losses are proportional to V V ≅ N ( +1 V −V >0.8V )( ) IN and thus SEC OUT DIODE 1+  R3 their effects will be more pronounced at higher supply voltages. where N is the turns ratio of the transformer, VOUT is the main output voltage sensed by V 2. I2R losses are calculated from the resistances of the FB and VDIODE is the voltage drop across the Schottky diode. internal switches RSW and external inductor RL. In continuous mode the average output current flowing through inductor L is “chopped” between the main R4 VSEC switch and the synchronous switch. Thus, the series SYNC/FCB + resistance looking into SW pin from L is a function of R3 L1 1µF LTC1627 1:N both top and bottom MOSFET RDS(ON) and the duty VOUT cycle (DC) as follows: SW + C R OUT SW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) 1627 F06 The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteris-
Figure 6. Secondary Output Loop Connection
tics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply by the square of the average output
Efficiency Considerations
current. The efficiency of a switching regulator is equal to the Other losses including CIN and COUT ESR dissipative losses, output power divided by the input power times 100%. It is MOSFET switching losses and inductor core losses generally often useful to analyze individual losses to determine what account for less than 2% total additional loss. is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: 10