Datasheet

V
O
+ 1.233 V
ǒ
1 )
R1
R2
Ǔ
C
FF
[
1
2 p
f
s
20
R1
1
D
O
P
O P
O s(ILOAD) O F IN
I
I L
V I ESR
C f V V V
æ ö
´
÷
ç
÷
ç
= ´ - + ´
÷
ç
÷
ç
+ - ÷
ç
è ø
IN
P
V
I = 375 mA + x 100 ns
L
TPS61045
SLVS440B –JANUARY 2003–REVISED MARCH 2009 www.ti.com
To reduce the overall operating quiescent current in battery powered applications a high impedance voltage
divider must be used with a typical value for R2 of ≤ 200 kΩ and a maximum value for R1 of 2.2 MΩ.
Some applications may not need the digital interface to program the output voltage. In this case the output DO
can be left open as shown in Figure 18 and the output voltage is calculated as for any standard boost converter:
In such a configuration a high impedance voltage divider must also be used to minimize ground current and a
typical value for R2 of ≤ 200 kΩ and a maximum value for R1 of 2.2 MΩ are recommended.
A feed-forward capacitor (C
(FF)
), across the upper feedback resistor (R1), is required to provide sufficient
overdrive for the error comparator. Without a feed-forward capacitor or a too small feed-forward capacitor value,
the device shows double pulses or a pulse burst instead of single pulses at the switch node (SW). This can
cause higher output voltage ripple. If a higher output voltage ripple is acceptable, the feedforward capacitor can
be left out too.
The lower the switching frequency of the converter, the larger the feed-forward capacitor value needs to be. A
good starting point is the use of a 10 pF feed-forward capacitor. As a first estimation, the required value for the
feed-forward capacitor can be calculated at the operation point:
with:
R1 = upper resistor of voltage divider
f
S
= switching frequency of the converter at the nominal load current. (For the calculation of the switching
frequency see previous section)
For C
(FF)
choose a value which comes closest to the calculation result.
The larger the feed-forward capacitor, the worse the line regulation of the device. Therefore, the feed-forward
capacitor must be selected as small as possible if good line regulation is of concern.
OUTPUT CAPACITOR SELECTION
For better output voltage filtering a low ESR output capacitor is recommended. Ceramic capacitors have low
ESR values but depending on the application, tantalum capacitors can also be used. See Table 4 and the
TYPICAL APPLICATIONS for the selection of the output capacitor.
Assuming the converter does not show double pulses or pulse bursts on the switch node (SW) the output voltage
ripple is calculated as:
with:
I
P
= peak current as described in the previous section peak current control
L = selected inductor value
I
O(LOAD)
= Nominal load current
f
S(ILoad)
= switching frequency at the nominal load current as calculated previously.
V
F
= rectifier diode forward voltage (typically 0.3 V)
C
O
= selected output capacitor
ESR = output capacitor ESR value
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