AS1310
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
Using TON = 1µs, and IPEAK = 480mA, and VRIPPLE = 50mV, EQ 15 yields:
CIN = 9.6µF
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 = 5mΩ to 10mΩ. For the AS1310,
maximum peak current is 480mA. Ripple due to ESR is 2.4mV to 4.8mV.
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 13) and on-time (Region A in Figure 13) of the power switch.
ILOAD (TON +TWAIT
(1− 0.99)VOUT _ NOM
)
COUT
=
(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: VINmin, VINmax, VOUTmin, VOUTmax, ILOADmin, ILOADmax
Inductor Selection:
Select Max on-time = 0.5µs to 3µs for AS1310. 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 VIN and max VOUT for maximum load conditions.
Input Capacitor Selection: Choose a ripple value and use (EQ 14) to find the value.
Output Capacitor Selection: Determine TWAIT via (EQ 6) or (EQ 13), and use (EQ 16) to find the value.
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