TPS61045
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SLVS440A–JANUARY 2003–REVISED SEPTEMBER 2003
APPLICATION INFORMATION (continued)
I
L
P
t
+
fall
V –V
O
I
For tf≥ 400 ns
I
V
P
I
I
+ h
load max
2 V
O
(4)
(5)
tf≤ 400 ns
2
P
I
L V
I
I
+ h
load max
ǒ2 I L ) 2 400 ns V Ǔ
ǒVO–VIǓ
P
I
with:
L = selected inductor value
η = expected converter efficiency (typically between 70% to 85%)
IP = peak current as described in the previous peak current control section.
V
I
I
+ 300 mA ) 100 ns
P
2
(6)
The above formula contains the expected converter efficiency that allows calculating the expected maximum load
current the converter can support. The efficiency can be taken out of the efficiency graphs shown in Figures 2
and 3 or 80% can be used as a good estimation.
The selected inductor must have a saturation current which meets the maximum peak current of the converter as
calculated in the peak current control section. Use the maximum value for ILim (450mA) for this calculation.
Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency
of the converter. Refer to the Table 1 and the inductor selection section under typical applications.
Table 2. Possible Inductor Selection
INDUCTOR VALUE
COMPONENT SUPPLIER
Sumida CR32-100
COMMENTS
10 µH
10 µH
10 µH
High efficiency
High efficiency
Sumida CDRH3D16-100
Murata LQH43CN100K01
Sumida CDRH3D16-4R7
muRata LQH32CN4R7M51
4.7 µH
4.7 µH
Small solution size
Small solution size
SETTING THE OUTPUT VOLTAGE
When the converter is programmed to the minimum output voltage, the DAC output (DO) equals the reference
voltage of 1.233 V (typ). Therefore, only the feedback resistor network (R1) and (R2) determines the output
voltage under these conditions. This gives the minimum output voltage possible and can be calculated as:
R1
R2
ǒ Ǔ
V
+ V
) 1
O(min)
(FB)
The maximum output voltage is determined as the DAC output (DO) is set to 0 V:
R1
R3
R1
R2
ǒ Ǔ
V
+ V
) V
) 1
O(max)
(FB)
(FB)
12
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