AS1312
Datasheet - Detailed Description
Energy stored during the on time is given by:
2
E = 0,5L(IPK
)
Joules (Region A in Figure 10)
(EQ 6)
(EQ 7)
If the overall time period (T + T ) is T, the power taken from the input is:
ON
OFF
2
0,5L(IPK
--------------------------
Watts
)
PIN
=
T
Assume output power is 0.8 P to establish an initial value of operating period T.
IN
T
is determined by the time taken for the output voltage to fall to 0.99xV . The longer the wait time, the lower will be the supply current of
OUT
WAIT
the converter. Longer wait times require increased output capacitance. Choose T
= 10% T as a minimum starting point for maximum energy
WAIT
transfer. For very low power load applications, choose T
≥ 50% T.
WAIT
8.1.3 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 off.
No switching action continues until the output voltage falls below the output reference point (0.99 x VOUT).
Output power is composed of the dc component (Region C in Figure 10):
IPKTOFF
IN-------------------
PREGION_C = V
(EQ 8)
(EQ 9)
2
T
Output power is also composed of the inductor component (Region B in Figure 10), neglecting efficiency loss:
2
0,5L(IPK
)
--------------------------
=
PREGION_B
T
Total power delivered to the load is the sum of (EQ 8) and (EQ 9):
2
IPKTOFF 0,5L(IPK
)
------------------- --------------------------
PTOTAL = VIN
+
(EQ 10)
(EQ 11)
2
T
T
From (EQ 3) (using nominal values) peak current is given by:
TONVIN
-------------------
L
IPK
=
Substituting (EQ 11) into (EQ 10) and re-arranging:
V2INT
ON
---------------------
PTOTAL
=
(0,9T)
(EQ 12)
2TL
0.9T incorporates a wait time T
= 10% T
WAIT
Output power in terms of regulated output voltage and load resistance is:
V2
RLOAD
OUT
----------------
POUT
=
(EQ 13)
(EQ 14)
Combining (EQ 12) and (EQ 13):
V2INT
V2
OUT
ON
----------------
---------------------
(0,9T)η
=
RLOAD
2TL
Symbol
η reflects total energy loss between input and output and is approximately 0.8 for these calculations. Use (EQ 14) to plot duty cycle
(T /T) changes for various output loadings and changes to VIN
ON
.
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