Datasheet

Table Of Contents
ST1S10 Application information
Doc ID 13844 Rev 5 13/29
recommended to select a V
OUT
level which maintains the junction temperature below the
thermal shut-down protection threshold (150°C typ.) at the rated output current. The
following equation can be used to calculate the junction temperature (T
J
):
Equation 8
T
J
= {[V
OUT
x I
OUT
x R
thJA
x (1-η)] / η} +T
AMB
where R
thJA
is the junction-to-ambient thermal resistance, η is the efficiency at the rated I
OUT
current and T
AMB
is the ambient temperature.
To ensure safe operating conditions the application should be designed to keep T
J
< 140°C.
5.6 Inductor (V
OUT
> 2.5 V)
The inductor value fixes the ripple current flowing through output capacitor and switching
peak current. The ripple current should be kept in the range of 20-40% of I
OUT_MAX
(for
example it is 0.6 - 1.2 A at I
OUT
= 3 A). The approximate inductor value can be obtained with
the following equation:
Equation 9
L = [(V
IN
- V
OUT
) / I
SW
] x T
ON
where T
ON
is the ON time of the internal switch, given by:
T
ON
= D/F
S
The inductor should be selected with saturation current (I
SAT
) equal to or higher than the
inductor peak current, which can be calculated with the following equation:
Equation 10
I
PK
= I
O
+ (I
SW
/2), I
SAT
I
PK
The inductor peak current must be designed so that it does not exceed the switching current
limit.
5.7 Inductor (0.8 V < V
OUT
< 2.5 V)
For applications with lower output voltage levels (V
out
< 2.5 V) the description in the previous
section is still valid but it is recommended to keep the inductor values in a range from 1µH to
2.2 µH in order to improve the DC-DC control loop behavior, and increase the output
capacitance depending on the V
IN
level as shown in the Figure 4 and Figure 5. In most
application conditions a 2.2 µH inductor is the best compromise between DC-DC control
loop behavior and output voltage ripple.
5.8 Function operation
5.8.1 Sync operation
The ST1S10 operates at a fixed frequency or can be synchronized to an external frequency
with the SYNC pin. The ST1S10 switches at a frequency of 900 kHz when the SYNC pin is
connected to ground, and can synchronize the switching frequency between 400 kHz to 1.2