LED6000PHR datasheet(31/49 Pages) STMICROELECTRONICS

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LED6000

Application notes - step-down conversion

Equation 24

where TRISE and TFALL are the overlap times of the voltage across the power switch (VDS)

and the current flowing into it during turn ON and turn OFF phases. TTR is the equivalent

switching time. For this device the typical value for the equivalent switching time is 40 ns.



Quiescent current losses, calculated as:

Equation 25

where IQOPVIN and IQOPVBIAS are the LED6000 quiescent current in case of the separate

bias supply. If the switchover feature is not used, the IC quiescent current is the only one

from VIN, IQUIESC, as summarized inTable 5 on page 8.

The junction temperature TJ can be calculated as:

Equation 26

where TA is the ambient temperature and PTOT is the sum of the power losses just seen.

RthJA is the equivalent thermal resistance junction to ambient of the device; it can be

calculated as the parallel of many paths of heat conduction from the junction to the ambient.

For this device the path through the exposed pad is the one conducting the largest amount

of heat. The RthJA, measured on the demonstration board described in the following

paragraph, is about 40 °C/W for the HTSSOP16 package.

5.5

Layout considerations

The PCB layout of the switching DC/DC regulators is very important to minimize the noise

injected in high impedance nodes and interference generated by the high switching current

loops. Two separated ground areas must be considered: the signal ground and the power

ground.

In a step-down converter the input loop (including the input capacitor, the power MOSFET

and the freewheeling diode) is the most critical one. This is due to the fact that high value

pulsed currents are flowing through it. In order to minimize the EMI, this loop must be as

short as possible. The input loop, including also the output capacitor, must be referred to the

power ground. All the other components are referred to the signal ground.

The feedback pin (FB) connection to the external current sensing resistor is a high

impedance node, so the interference can be minimized by placing the routing of the

feedback node as far as possible from the high current paths. To reduce the pick-up noise,

the compensation network involving FB and COMP pins should be placed very close to the

device.

To filter the high frequency noise, a small bypass capacitor (1

F or higher) must be added

as close as possible to the input voltage pin of the device, for both VIN and VCC pins.

Thanks to the exposed pad of the device, the ground plane helps to reduce the junction to

ambient thermal resistance; so a wide ground plane enhances the thermal performance of

the converter, allowing high power conversion.



OUT

FALL

RISE

OUT













QOPVBIAS

BIAS

QOPVIN









TOT







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