LD49150 datasheet(14/20 Pages) STMICROELECTRONICS

14 / 20 page

Application hints

LD49150

14/20

8.6

VIN and VBIAS power sequencing

In common applications where the power on transient of VIN and VBIAS voltages are not

particularly fast (Tr>100µs), no power sequencing is required. Where voltage transient input

(Tr<100µs) is very fast, it is recommended to have the VIN voltage present before or, at least, at

the same time as the VBIAS voltage in order to avoid overvoltage spikes during the power on

transient (refer to the figures in the typical characteristics). Where VIN transient input

(Tr<<100µs) is very fast for the fixed VOUT versions, it is possible to avoid start-up overvoltage

spikes by pulling the VINH pin up to VIN voltage (refer to relative typical characteristics figures at

pages 11 and 12).

8.7

Power dissipation/heatsinking

A heatsink may be required depending on the maximum power dissipation and maximum

ambient temperature of the application. Under all possible conditions, the junction temperature

must be within the range specified under operating conditions. The total power dissipation of

the device is given by:

PD = VIN x IIN + VBIAS x IBIAS - VOUT x IOUT

Where:

●

VIN, Input supply voltage

●

VBIAS, Bias supply voltage

●

VOUT, Output voltage

●

IOUT, Load current

From this data, we can calculate the thermal resistance (

SA) required for the heat sink using

the following formula:

SA = (TJ - TA/PD) - (

JC +

CS)

The maximum allowed temperature rise (TRmax) depends on the maximum ambient

temperature (TAmax) of the application, and the maximum allowable junction temperature

(TJmax):

TRmax = TJmax - TAmax

The maximum allowable value for junction to ambient thermal resistance,

JA, can be

calculated using the formula:

JAmax = TRmax / PD

This part is available for the PPAK package.

The thermal resistance depends on the amount of copper area or heat sink, and on air flow. If

the maximum allowable value of

JA calculated above is ≥100 °C/W for the PPAK package, no

heatsink is needed since the package can dissipate enough heat to satisfy these requirements.

If the value for allowable

JA falls below these limits, a heat sink is required as described below.

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