SLVS413C – OCTOBER 2002 – REVISED AUGUST 2007
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APPLICATION INFORMATION
INDUCTOR SELECTION, MAXIMUM LOAD CURRENT
Because the PFM peak current control scheme is inherently stable, the inductor value does not affect the stability
of the regulator. The selection of the inductor together with the nominal load current, input and output voltage of
the application determines the switching frequency of the converter. Depending on the application, inductor
values between 2.2
μH
and 47
μH
are recommended. The maximum inductor value is determined by the
maximum on time of the switch, typically 6
μs.
The peak current limit of 400 mA/250 mA (typically) should be
reached within this 6-μs period for proper operation.
The inductor value determines the maximum switching frequency of the converter. Therefore, select the inductor
value that ensures the maximum switching frequency at the converter maximum load current is not exceeded.
The maximum switching frequency is calculated by the following formula:
V
(V
*
V
IN(min)
OUT
IN)
fS max
+
I
L V
P
OUT
(2)
Where:
I
P
= Peak current as described in the
section
L = Selected inductor value
V
IN(min)
= The highest switching frequency occurs at the minimum input voltage
If the selected inductor value does not exceed the maximum switching frequency of the converter, the next step
is to calculate the switching frequency at the nominal load current using the following formula:
2 I
(V
*
V
)
Vd)
load
OUT
IN
fS I
+
2
load
I
L
P
(3)
Where:
I
P
= Peak current as described in the
section
L = Selected inductor value
I
load
= Nominal load current
Vd = Rectifier diode forward voltage (typically 0.3V)
A smaller inductor value gives a higher converter switching frequency, but lowers the efficiency.
The inductor value has less effect on the maximum available load current and is only of secondary order. The
best way to calculate the maximum available load current under certain operating conditions is to estimate the
expected converter efficiency at the maximum load current. This number can be taken out of the efficiency
graphs shown in
through
The maximum load current can then be estimated as follows:
I
2
L fS max
I
+
h
P
load max
2 (V
*
V
OUT
IN)
(4)
Where:
I
P
= Peak current as described in the
section
L = Selected inductor value
fS
max
= Maximum switching frequency as calculated previously
η
= Expected converter efficiency. Typically 70% to 85%
10
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