AST1S31HF Datasheet, PDF (20/33 Pages) STMicroelectronics – Internal soft-start and enable

Application information
AST1S31HF
The placement of the input capacitor is very important to avoid noise injection and voltage
spikes on the input voltage pin. So the C_IN must be placed as close as possible to the
VIN_SW pin. In Table 6 some multilayer ceramic capacitors suitable for this device are
given.
Manufacturer
Murata
TDK
TAIYO YUDEN
Table 6. Input MLCC capacitors
Series
Cap value (µF)
GCM
47
CGA6
47
LMK325
47
Rated voltage (V)
6.3
6.3
10
A ceramic bypass capacitor, as close as possible to the VINA pin so that additional parasitic
ESR and ESL are minimized, is suggested in order to prevent instability on the output
voltage due to noise. The value of the bypass capacitor can go from 330 nF to 1 µF.
7.2
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value to have the expected current ripple must be selected. The rule to
fix the current ripple value is to have a ripple at 20% - 40% of the output current.
In the continuous current mode (CCM), the inductance value can be calculated by Equation
28:
Equation 28
I_L = -^V----^I-^N-----–---_L--^V----^O----^U----^T-  T_ON = ^V-----^O-_L---^U---^T--  T_OFF
where T_ON is the conduction time of the high-side switch and T_OFF is the conduction time of
the low-side switch [in CCM, F_SW = 1 / (T_ON + T_OFF)]. The maximum current ripple, given
the V_OUT, is obtained at maximum T_OFF, that is, at a minimum duty cycle (see previous
section to calculate minimum duty). So by fixing I_L = 20% to 30% of the maximum output
current, the minimum inductance value can be calculated:
Equation 29
L_MIN = _-^V----_I-^OM----^U-_A--^T-_X--  ¹-_F----_S^–----_W-^D----_MM----_I-^IN-^N--
where F_SWMIN is the minimum switching frequency, according to Table 4 on page 7. The
slope compensation, to prevent the sub-harmonic instability in the peak current control loop,
is internally managed and so fixed. This implies a further lower limit for the inductor value.
To assure sub-harmonic stability:
Equation 30
L  V_out  2  V_pp  f_sw
where V_PP is the peak-to-peak value of the slope compensation ramp. The inductor value
selected based on Equation 29 must satisfy Equation 30.
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