Datasheet

Application hints LD49150XX08, LD49150XX10, LD49150XX12
14/22 Doc ID 13446 Rev 3
8.6 Power sequencing recommendations
In order to ensure the correct biasing and settling of the regulator internal circuitry during the
startup phase, as well as to avoid overvoltage spikes at the output, it is recommended to
provide for the correct power sequencing.
As a general rule the V
IN
and V
INH
signals timings at startup should be chosen properly, so
that they are applied to the device after the V
BIAS
voltage is already settled at its minimum
operative value (see paragraph 8.2: Bias supply voltage (VBIAS)). This can be achieved, for
instance, by avoiding too slow V
BIAS
rising edges (T
r
>10 ms).
Provided that the above condition is satisfied, when fast V
IN
transient input (T
r
< 100 µs) is
present, a smooth startup, with limited overvoltage on the output, can be obtained by
applying V
IN
voltage at the same time as the V
BIAS
voltage (refer to Figure 20, Figure 21
and Figure 22 on page 11).
In the fixed voltage versions it is possible to reduce overvoltage spikes during very fast
startup (T
r
<< 100 µs) by pulling the V
INH
pin up to V
IN
voltage (see Figure 23 on page 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:
P
D
= V
IN
x I
IN
+ V
BIAS
x I
BIAS
- V
OUT
x I
OUT
Where:
● V
IN
, input supply voltage
● V
BIAS
, bias supply voltage
● V
OUT
, output voltage
● I
OUT
, load current
From this data, we can calculate the thermal resistance (
θ
SA
) required for the heat sink
using the following formula:
θ
SA
= (T
J
- T
A
/P
D
) - (θ
JC
+ θ
CS
)
The maximum allowed temperature rise (T
Rmax
) depends on the maximum ambient
temperature (T
Amax
) of the application, and the maximum allowable junction temperature
(T
Jmax
):
T
Rmax
= T
Jmax
- T
Amax
The maximum allowable value for junction to ambient thermal resistance, θ
JA
, can be
calculated using the formula:
θ
JAmax
= T
Rmax
/ P
D
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
θ
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
θ
JA
falls below these limits, a heat sink is
required as described below.