AS1312
Datasheet - Detailed Description
8.1.4 Input Capacitor Selection
The input capacitor supports the triangular current during the on-time of the power switch, and maintains a broadly constant input voltage during
this time. The capacitance value is obtained from choosing a ripple voltage during the on-time of the power switch. Additionally, ripple voltage is
generated by the equivalent series resistance (ESR) of the capacitor. For worst case, use maximum peak current values from the datasheet.
IPEAKTON
-------------------------
CIN
=
(EQ 15)
VRIPPLE
= 50mV, EQ 15 yields:
Using T = 1µs, and I
= 400mA (typ), and V
RIPPLE
ON
PEAK
CIN = 8.0µF
Nearest preferred would be 10µF.
VPK _ RIPPLE _ ESR = IPK RESR
(EQ 16)
Typically, the ripple due to ESR is not dominant. ESR for the recommended capacitors (Murata GMR), ESR = 5m
Ω
to 10mΩ
. For the AS1312,
maximum peak current is 400mA. Ripple due to ESR is 2.0mV to 4.0mV.
Ripple at the input propagates through the common supply connections, and if too high in value can cause problems elsewhere in the system.
The input capacitance is an important component to get right.
8.1.5 Output Capacitor Selection
The output capacitor supports the triangular current during the off-time of the power switch (inductor discharge period), and also the load current
during the wait time (Region D in Figure 10) and on-time (Region A in Figure 10) of the power switch.
ILOAD (TON +TWAIT
)
COUT
=
(1− 0.99)VOUT _ NOM
(EQ 17)
Note: There is also a ripple component due to the equivalent series resistance (ESR) of the capacitor.
8.2 Summary
User Application Defines:
V
INmin, VINmax, VOUTmin, VOUTmax, I
min, I
max
LOAD
LOAD
Inductor Selection:
Select Max on-time = 0.5µs to 3µs for AS1312. Use (EQ 3) to calculate inductor value.
Use (EQ 5) to determine off-time.
Use (EQ 6) to check that power delivery matches load requirements assume 70% conversion efficiency.
Use (EQ 13) to find overall timing period value of T at min V and max V for maximum load conditions.
IN
OUT
Input Capacitor Selection: Choose a ripple value and use (EQ 14) to find the value.
Output Capacitor Selection: Determine T
via (EQ 6) or (EQ 13), and use (EQ 16) to find the value.
WAIT
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