ALED6000 datasheet(24/45 Pages) STMICROELECTRONICS

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PQ=VIN⋅IQOPVIN+VBIAS⋅IQOPVBIAS

(25)

where IQOPVIN and IQOPVBIAS are the ALED6000 quiescent current in case of separate bias supply. If the

switch-over feature is not used, the IC quiescent current is the only one from VIN, IQUIESC, as summarized in

Table 5. Electrical characteristics .

The junction temperature TJ can be calculated as

TJ=TA+Rtℎ,JA⋅PTOT

(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-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 HTTSOP16

package.

6.5

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 μF 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.

The exposed pad must be connected to the signal GND pin. The connection to the ground plane must be

achieved by taking care of the above mentioned input loop, in order to avoid high current flowing through the

signal GND.

Refer to Section 7 Demonstration board for the ALED6000 layout example.

ALED6000

Layout considerations

DS13018 - Rev 4

page 24/45

ALED6000 Datasheet(PDF) 24 Page - STMicroelectronics