Datasheet LT3439 (Analog Devices) - 8

LT3439
U U W U APPLICATIO S I FOR ATIO
mean greater switching losses in the internal output
Table 1
switches. However, efficiency is only modestly reduced
NOMINAL NOMINAL
for a large improvement in EMI.
INPUT OUTPUT OUTPUT COILTRONICS VOLTAGE VOLTAGE POWER PART NUMBER
Care should be taken to ensure that the worst-case input 5V 12V 1.5W CTX02-13716-X1 voltage and load current conditions do not cause an 5V 12V 3.0W CTX02-13665-X1 excessive die temperature. The total power dissipation of 5V ±15V 1.5W CTX02-13713-X1 5V ±15V 3.0W CTX02-13664-X1 the IC is dominated by three loss terms, regulator losses, 5V 12V 1.5W CTX02-13834-X3 saturation losses and switching losses. The following 5V 12V 10W CTX02-13949-X1 formulas may be used to approximate these losses: 12V –12V 6W CTX02-16076 1. Regulator Dissipation: These transformers will yield slightly high output voltages  I  so that they can accommodate an LDO regulator on the P = V m 12 A VIN IN +  60 output. where I is the average switch current. If your application is not listed, the LTC Applications group is available to assist in the choice and/or the design of the 2. Switch Saturation Dissipation: transformer. PVSAT = (VSAT)(I) In the design/selection of the transformer the following 3. Switch Switching Dissipation: characteristics are critical and should be considered. 
Turns Ratio
I P = 10 6 – • V •I • f SW IN OSC  The turns ratio of the transformer determines the output –2.3 • 10 4 – • R  ( 10 8 . ) + SL + voltage. The following equation can be used as a first pass  to calculate the turns ratio: V  –1.7 • 10 3 – • R ( 65.8) SL +  N V V S OUT F = + N V – V P IN SW Total IC power dissipation (PD) is the sum of these three where VF is the forward voltage of the output diode and terms. Die junction temperature can be computed as VSW is the voltage drop across the internal switches (see follows: Typical Performance curves). TJ = TAMB + (PD)(θJA) Sufficient margin should be added to the turns ratio to where T account for voltage drops due to transformer winding AMB is the ambient temperature, TJ is the junction temperature and θ resistances. Also, if using an LDO for regulating the output JA is the thermal resistance from junc- tion to ambient. voltage, don’t forget to take into account the voltage drop that should be added to V The LT3439 comes in the 16-pin TSSOP with exposed OUT. backside package that has a very low junction-to-ambient
Magnetizing Current
thermal resistance (θJA) of approximately 40°C/W. The primary inductance of the transformer causes a ripple
Transformer Design
current that is independent of load current. The ripple current manifests itself in the output voltage through the Table 1 lists recommended center tapped transformers for parasitic resistances of the supply. Increasing the trans- a variety of input voltage, output voltage and power former magnetizing inductance can reduce the ripple combinations. sn3439 3439fs 8