TPS61045
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SLVS440A–JANUARY 2003–REVISED SEPTEMBER 2003
APPLICATION INFORMATION
INDUCTOR SELECTION, MAXIMUM LOAD CURRENT
Since the PFM peak current control scheme is inherently stable the inductor and capacitor 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 switch on-time of 6 µs (typ). The peak current limit of 375 mA (typ) must be reached
within this 6 µs for proper operation.
The inductor value determines the maximum switching frequency of the converter. Therefore, the inductor value
must be selected for the maximum switching frequency, at maximum load current of the converter and should not
be exceeded. A good inductor value to start with is 4.7 µH. The maximum switching frequency is calculated as:
VI ǒVO * V Ǔ
I
f
+
s
(max)
I
L V
P
O
with:
IP = peak current as described in the previous peak current control section.
V
I
I
+ 375 mA ) 100 ns
P(typ)
L
(2)
L = selected inductor value
If the selected inductor does not exceed the maximum switching frequency of the converter, as a next step, the
switching frequency at the nominal load current is estimated as follows:
ǒVO–V ) VFǓ
2 I
LOAD
I
f
+
s
2
(ILOAD)
I
L
P
with:
IP = peak current as described in the previous chapter peak current control section
V
I
I
+ 375 mA ) 100 ns
P(typ)
L
(3)
L = selected inductor value
I(LOAD) = nominal load current
VF = rectifier diode forward voltage (typically 0.3 V)
The smaller the inductor value, the higher the switching frequency of the converter but the lower the efficiency.
The maximum load current of the converter is determined at the operation point where the converter starts to
enter continuous conduction mode. The converter must always operate in discontinuous conduction mode to
maintain regulation.
Two conditions exist for determining the maximum output current of the converter. One is when the inductor
current fall time is <400 ns, and the other is when the inductor current fall time is >400 ns.
One way to calculate the maximum available load current under certain operation 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 Figure 2 and Figure 3. Then the maximum load current can be estimated:
Inductor fall time:
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