Datasheet

h
P IN
load max
O
I V
I
2 V
´
=
´
( ) ( )
2
L
2 2 400
h
P IN
load max
O I P IN
I V
I
V V I L ns V
´ ´
=
´ ´ ´ ´ + ´ ´
300 100
2
IN
P
V
I mA ns= + ´
V
O(min)
+ V
(FB)
ǒ
R1
R2
) 1
Ǔ
V
O(max)
+ V
(FB)
R1
R3
) V
(FB)
ǒ
R1
R2
) 1
Ǔ
V
O(step)
+
19.6 mV R1
R3
TPS61045
www.ti.com SLVS440B –JANUARY 2003–REVISED MARCH 2009
t
f
≤ 400 ns
with:
L = selected inductor value
h = expected converter efficiency (typically between 70% to 85%)
I
P
= peak current as described in the previous peak current control section.
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 I
Lim
(450mA) for this calculation.
Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency
of the converter. See Table 3 and the inductor selection section under typical applications.
Table 3. Possible Inductor Selection
INDUCTOR VALUE COMPONENT SUPPLIER COMMENTS
10 mH Sumida CR32-100 High efficiency
10 mH Sumida CDRH3D16-100 High efficiency
10 mH Murata LQH43CN100K01
4.7 mH Sumida CDRH3D16-4R7 Small solution size
4.7 mH muRata LQH32CN4R7M51 Small solution size
SETTING THE OUTPUT VOLTAGE (see Figure 1)
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:
The maximum output voltage is determined as the DAC output (DO) is set to 0 V:
The output voltage can be digitally programmed by pulling the CTRL pin low for a certain period of time as
described in the DIGITAL INTERFACE (CTRL) section. Pulling the signal applied to the CTRL pin low and again
high (for related timing see the ELECTRICAL CHARACTERISTICS) increases or decreases the DAC output DO
(pin 3) one-step where one step is typically 19.6 mV. A voltage step on DO of 19.6mV (typ) changes the output
voltage by one step and is calculated as:
The possible output voltage range is determined by selecting R1, R2 and R3. A possible larger output voltage
range gives a larger output voltage step size. The smaller the possible output voltage range, the smaller the
output voltage step size.
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