PM6675 Datasheet, PDF (34/47 Pages) STMicroelectronics – Very fast load transient response using constant on-time control loop

Application information
PM6675
7.1.1 Inductor selection
Once the switching frequency has been defined, the inductance value depends on the
desired inductor ripple current. Low inductance value means great ripple current that brings
poor efficiency and great output noise. On the other hand a great current ripple is desirable
for fast transient response when a load step is applied.
High inductance brings to good efficiency but the transient response is critical, especially if
V_INmin - V_OUT is little. Moreover a minimum output ripple voltage is necessary to assure
system stability and jitter-free operations (see Section 7.1.3: Output capacitor selection on
page 36). The product of the output capacitor's ESR multiplied by the inductor ripple current
must be taken in consideration. A good trade-off between the transient response time, the
efficiency, the cost and the size is choosing the inductance value in order to maintain the
inductor ripple current between 20% and 50% (usually 40%) of the maximum output current.
The maximum inductor ripple current, ∆I_L,MAX , occurs at the maximum input voltage.
Given these considerations, the inductance value can be calculated using the following
expression:
uct(s) ^{Equation 30}
L = ^{VIN − VOUT} ⋅ ^VOUT
fsw ⋅ ∆I_L V_IN
rod ^{where f}SW ^{is the switching frequency, V}IN ^{is the input voltage, V}OUT ^{is the output voltage and}
P ^∆I_L ^{is the inductor ripple current.}
te ^{Once the inductor value is determined, the inductor ripple current is then recalculated:}
Obsole ^{Equation 31}
∆I_L,MAX
=
V_IN,MAX − V_OUT
fsw ⋅ L
⋅
V_OUT
V_IN,MAX
t(s) ^{- The next step is the calculation of the maximum r.m.s. inductor current:}
roduc ^{Equation 32}
I_L,RMS =
(I_LOAD,MAX )²
+
(∆I_L,MAX )²
12
te ^P The inductor must have an r.m.s. current greater than I_L,RMS in order to assure thermal
lestability.
so Then the calculation of the maximum inductor peak current follows:
Ob Equation 33
I_L,PEAK
= I_LOAD,MAX
+
∆I_L,MAX
2
I_L,PEAK is important when choosing the inductor, in term of its saturation current.
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