ams Datasheet Page 15
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AS1310 − Detailed Description
Inductor Choice Example
For the AS1310 V
IN_MIN
= 0.9V, V
OUT_MAX
= 3.3V, EQ 5 gives
T
ON
=2.66T
OFF
.
Let the maximum operating on-time = 1μs.
Note that this is shorter than the minimum limit ON-time of
3.6μs. Therefore from EQ 5, T
OFF
= 0.376μs. Using EQ 3, L
MAX
is
obtained:
L
MAX
= 1.875μH. The nearest preferred value is 2.2μH.
This value provides the maximum energy storage for the chosen
fixed ON-time limit at the minimum V
IN
.
Energy stored during the ON time is given by:
Joules (Region A in Figure 17)
If the overall time period (T
ON
+ T
OFF
) is T, the power taken from
the input is:
Watts
Assume output power is 0.8 P
IN
to establish an initial value of
operating period T.
T
WAIT
is determined by the time taken for the output voltage to
fall to 0.99xV
OUT
. The longer the wait time, the lower will be the
supply current of the converter. Longer wait times require
increased output capacitance. Choose T
WAIT
= 10% T as a
minimum starting point for maximum energy transfer. For very
low power load applications, choose T
WAIT
≥ 50% T.
Output Loop Timing
The output loop consists of the main inductor, P-channel
synchronous switch (or diode if fitted), output capacitor and
load. When the input loop is interrupted, the voltage on the LX
pin rises (Lenz’s Law). At the same time a comparator enables
the synchronous switch, and energy stored in the inductor is
transferred to the output capacitor and load. Inductor peak
current supports the load and replenishes the charge lost from
the output capacitor. The magnitude of the current from the
inductor is monitored, and as it approaches zero, the
synchronous switch is turned ON. No switching action
continues until the output voltage falls below the output
reference point (0.99 x V
OUT
).
Output power is composed of the DC component (Region C in
Figure 17):
P
REGION_C
=
Output power is also composed of the inductor component
(Region B in Figure 17), neglecting efficiency loss:
P
IN
0.5L I
PK
()
2
T
---------------------------
=
V
IN
I
PK
2
--------
T
OFF
T
------------ -
