Texas Instruments TPS61080/81-147 Evaluation Board TPS61080/81EVM-147 TPS61080EVM-147 TPS61080EVM-147 데이터 시트
제품 코드
TPS61080EVM-147
K
ref
+
K
2
Vclp
Vclp
)
K
2
Iss
Iss
)
K
2
R
R
Ǹ
I
in_ss
+
I
in_cout
)
V
out
V
in
h
I
load
I
in_cout
+
C
out
I
ss
V
out
C
ss
V
in
h
t
ss
+
C
ss
1.229
I
ss
SLVS644D – FEBRUARY 2006 – REVISED APRIL 2013
The 100
Ω
resistor is added to reduce noise coupling from the OUT to the FB pin through the feed forward
capacitor. Without the resistor, the regulator may oscillate at high output current.
SOFT START CAPACITOR
The voltage at the SS pin clamps the internal reference voltage, which allows the output voltage to ramp up
slowly. The soft start time is calculated as
slowly. The soft start time is calculated as
(8)
where
C
ss
= soft start capacitor
I
ss
= soft start bias current (TYP 5
μ
A)
1.229V is the typical value of the reference voltage.
During start up, input current has to be supplied to charge the output capacitor. This current is proportional to
rising slope of the output voltage, and peaks when output reaches regulation.
rising slope of the output voltage, and peaks when output reaches regulation.
(9)
Where
I
in_cout
= additional input current for charging the output capacitor
The maximum input during soft start is
(10)
Output overshoot can occur if the input current at startup exceeds the inductor saturation current and/or reaches
current limit because the error amplifier loses control of the voltage feedback loop. The in-rush current can also
pull down input sources, potentially causing system reset. Therefore, select C
current limit because the error amplifier loses control of the voltage feedback loop. The in-rush current can also
pull down input sources, potentially causing system reset. Therefore, select C
ss
to make I
in_ss
stay below the
inductor saturation current, the IC over current limit and the input's maximum supply current.
TPS61080/1 can also be configured for constant current output, as shown in the typical applications. In this
configuration, a current sense resistor is connected to FB pin for output current regulation. In order to reduce
power loss on the sense resistor, FB pin reference voltage can be lowered by connecting a resistor to the SS pin
The new reference voltage is simply the resistor value times the SS pin bias current. However, keep in mind that
this reference has higher tolerance due to the tolerance of the bias current and sense resistor, and the offset of
the clamp circuit. Refer to the specification V
configuration, a current sense resistor is connected to FB pin for output current regulation. In order to reduce
power loss on the sense resistor, FB pin reference voltage can be lowered by connecting a resistor to the SS pin
The new reference voltage is simply the resistor value times the SS pin bias current. However, keep in mind that
this reference has higher tolerance due to the tolerance of the bias current and sense resistor, and the offset of
the clamp circuit. Refer to the specification V
CLP
and I
SS
to calculate the tolerance as following.
(11)
Where
K
ref
= percentage tolerance of the FB reference voltage.
K
Vclp
= percentage tolerance of the clamp circuit.
K
lss
= percentage tolerance of the SS pin bias current.
K
R
= percentage tolerance of the SS pin resistor.
Without considering the SS pin resistor tolerance, the FB reference voltage has ±5.6% under the room
temperature.
temperature.
INDUCTOR SELECTION
Because the selection of the inductor affects steady state operation, transient behavior and loop stability, the
inductor is the most important component in power regulator design. There are three important inductor
specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not
enough.
inductor is the most important component in power regulator design. There are three important inductor
specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not
enough.
14
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