1.5A, 280kHz, Boost Regulator
LM5171
Finally, the total on−chip power losses are
PD = PBIAS + PDRIVER + PSAT
Power dissipation in a semiconductor device results in the generation of heat in the junctions at the surface of
the chip. This heat is transferred to the surface of the IC package, but a thermal gradient exists due to the
resistive properties of the package molding compound. The magnitude of the thermal gradient is expressed in
manufacturers’ data sheets as θJA, or junction−to−ambient thermal resistance.
The on−chip junction
temperature can be calculated if θJA, the air temperature near the surface of the IC, and the on−chip power
dissipation are known.
TJ = TA + (PD • θJA)
where:
TJ = IC or FET junction temperature (°C);
TA = ambient temperature (°C);
PD = power dissipated by part in question (W);
θJA = junction–to–ambient thermal resistance (°C/W). For the LM5171 θJA=165°C/W.
Once the designer has calculated TJ, the question of whether the LM5171 can be used in an application is
settled. If TJ exceeds 150°C, the absolute maximum allowable junction temperature, the LM5171 is not suitable
for that application.
If TJ approaches 150°C, the designer should consider possible means of reducing the junction temperature.
Perhaps another converter topology could be selected to reduce the switch current. Increasing the airflow
across the surface of the chip might be considered to reduce TA.
Circuit Layout Guidelines
In any switching power supply, circuit layout is very important for proper operation. Rapidly switching currents
combined with trace inductance generates voltage transitions that can cause problems. Therefore the following
guidelines should be followed in the layout.
1. In boost circuits, high AC current circulates within the loop composed of the diode, output capacitor, and
on−chip power transistor. The length of associated traces and leads should be kept as short as possible.
In the flyback circuit, high AC current loops exist on both sides of the transformer. On the primary side, the
loop consists of the input capacitor, transformer, and on−chip power transistor, while the transformer, rectifier
diodes, and output capacitors form another loop on the secondary side. Just as in the boost circuit, all
traces and leads containing large AC currents should be kept short.
2. Separate the low current signal grounds from the power grounds. Use single point grounding or ground
plane construction for the best results.
3. Locate the voltage feedback resistors as near the IC as possible to keep the sensitive feedback wiring short.
Connect feedback resistors to the low current analog ground.
Dec. 2010 - Rev. 1.2.1
- 19 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
