L7987L datasheet(30/41 Pages) STMICROELECTRONICS

30 / 41 page

Application information

L7987L

30/41

DocID026362 Rev 3

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

switching time is 20 ns.



Quiescent current losses, calculated as

Equation 31

PQ = VIN • IQOPVIN + VBIAS • IQOPVBIAS

where IQOPVIN and IQOPVBIAS are the L7987L quiescent current in case of separate bias

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

VIN, IQUIESC, as summarized in Table 5 on page 8.

The junction temperature TJ can be calculated as:

Equation 32

TJ = TA + Rth,JA • PTOT

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 Section 5.6, is about

40 °C/W for the HTSSOP16 package.

5.6

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 resistor divider 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 resistor divider must

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.

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 6 for the L7987L layout

example.

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