TPS61040
TPS61041
SLVS413 – FEBRUARY 2002
APPLICATION INFORMATION
inductor selection, maximum load current
Since 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 up to 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/250mA (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:
Vin
(Vout–Vin)
min
L Vout
fS
+
max
I
P
Where:
I = Peak current as described in the previous peak current control section
P
L = Selected inductor value
Vin
= The highest switching frequency occurs at the minimum input voltage
min
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
(Vout–Vin ) Vd)
load
ǒ Ǔ +
fS I
load
2
I
L
P
Where:
I = Peak current as described in the previous peak current control section
P
L = Selected inductor value
I
= Nominal load current
load
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 Figures 1, 2, 3, and 4. The maximum load current can then be estimated as follows:
2
P
I
L fS
max
2 Vout
I
+ h
load max
Where:
I = Peak current as described in the previous peak current control section
P
L = Selected inductor value
fS
= Maximum switching frequency as calculated previously
max
η = Expected converter efficiency. Typically 70% to 85%
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