Datasheet

PM8903A Application information
Doc ID 024147 Rev 1 21/33
6.3 Inductor design
The inductance value is defined by a compromise between the dynamic response time, the
efficiency, the cost, and the size. The inductor must be calculated to maintain the ripple
current (ΔI
L
) between 20% and 30% of the maximum output current (typ.). The inductance
value can be calculated with the following relationship:
where F
SW
is the switching frequency, V
IN
is the input voltage, and V
OUT
is the output
voltage.
Increasing the value of the inductance reduces the current ripple but, at the same time,
increases the converter response time to a dynamic load change. The response time is the
time required by the inductor to change its current from the initial to the final value. Until the
inductor finishes its charging time, the output current is supplied by the output capacitors.
Minimizing the response time can minimize the output capacitance required. If the
compensation network is well designed, during a load variation the device is able to set a
duty cycle value very different (0% or 100%) from the steady-state one. When this condition
is reached, the response time is limited by the time required to change the inductor current.
6.4 Output capacitors
The output capacitors are basic components to define the ripple voltage across the output
and for the fast transient response of the power supply. They depend on the output voltage
ripple requirements, as well as any output voltage deviation requirement during a load
transient.
During steady-state conditions, the output voltage ripple is influenced by both the ESR and
the capacitive value of the output capacitors as follows:
where ΔI
L
is the inductor current ripple. In particular, the expression that defines ΔV
OUT_C
takes into consideration the output capacitor charge and discharge as a consequence of the
inductor current ripple.
During a load variation, the output capacitor supplies the current to the load or absorbs the
current stored in the inductor until the converter reacts. In fact, even if the controller
immediately recognizes the load transient and sets the duty cycle at 100% or 0%, the
current slope is limited by the inductor value. The output voltage has a drop that, also in this
case, depends on the ESR and capacitive charge/discharge as follows:
where ΔV
L
is the voltage applied to the inductor during the transient response
( for the load appliance or V
OUT
for the load removal).
MLCC capacitors have typically low ESR to minimize the ripple but also have low
capacitance that does not minimize the voltage deviation during dynamic load variations.
L
V
IN
V
OUT
–
F
SW
ΔI
L
⋅
------------------------------
V
OUT
V
IN
--------------
⋅=
ΔV
OUT_ESR
ΔI
L
ESR⋅=
ΔV
OUT_C
ΔI
L
1
8C
OUT
F
SW
⋅⋅
---------------------------------------
⋅=
ΔV
OUT_ESR
ΔI
OUT
ESR⋅=
ΔV
OUT_C
ΔI
OUT
L ΔI
OUT
⋅
2C
OUT
ΔV
L
⋅⋅
--------------------------------------
⋅=
D
MAX
V
IN
V
OUT
–⋅