AST1S31HF Datasheet, PDF (21/33 Pages) STMicroelectronics – Internal soft-start and enable

AST1S31HF
Application information
The peak current through the inductor is given by Equation 31:
Equation 31
I_{L PK} = I_O + ^---₂--^I-^L-
So if the inductor value decreases, the peak current (which must be lower than the current
limit of the device) increases. The higher the inductor value, the higher the average output
current that can be delivered, without reaching the current limit.
In Table 7 some inductor part numbers are listed.
Manufacturer
COILTRONICS
COILCRAFT
Table 7. Inductors
Series
Inductor value (µH)
DRA73
XAL40XX
0.6 to 2.2
0.6 to 2.2
Saturation current (A)
5.5 to 7.9
5.4 to 8.35
7.3
Output capacitor selection
The current in the output capacitor has a triangular waveform which generates a voltage
ripple across it. This ripple is due to the capacitive component (charge or discharge of the
output capacitor) and the resistive component (due to the voltage drop across its ESR). So
the output capacitor must be selected in order to have a voltage ripple compliant with the
application requirements.
The amount of the voltage ripple can be calculated starting from the current ripple obtained
by the inductor selection.
Equation 32
V_OUT = ESR  I_MAX + ₈-----_---_C---^--_O--^I--^MU----_TA---^X-_---_f--_S---_W---
For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the
resistive one. While for an electrolytic capacitor the opposite is true.
As the compensation network is internal, the output capacitor should be selected in order to
have a proper phase margin and then a stable control loop.
The equations of Section 6.3 on page 11 help to check loop stability given the application
conditions, the value of the inductor and of the output capacitor.
In Table 8 some capacitor series are listed.
Manufacturer
MURATA
TDK
Table 8. Output capacitors
Series
Cap value (µF) Rated voltage (V)
GCM
22 to 470
10
CGA6
22 to 470
16
ESR (m)
5
10
DocID026968 Rev 2
21/33
33