Texas Instruments TPS61160(1)-243 Evaluation Module TPS61160EVM-243 TPS61160EVM-243 Datenbogen
Produktcode
TPS61160EVM-243
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û
ù
ê
ë
é
+
-
+
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V
1
V
V
V
1
(
F
L
1
I
in
in
f
out
s
P
I
out_max
+
Vin
ǒ
I
lim
*
I
P
ń
2
Ǔ
h
Vout
I
in_DC
+
Vout
Iout
Vin
h
SLVS791C – NOVEMBER 2007 – REVISED APRIL 2012
APPLICATION INFORMATION
MAXIMUM OUTPUT CURRENT
The overcurrent limit in a boost converter limits the maximum input current and thus maximum input power for a
given input voltage. Maximum output power is less than maximum input power due to power conversion losses.
Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current
output. The current limit clamps the peak inductor current; therefore, the ripple has to be subtracted to derive
maximum DC current. The ripple current is a function of switching frequency, inductor value and duty cycle. The
following equations take into account of all the above factors for maximum output current calculation.
given input voltage. Maximum output power is less than maximum input power due to power conversion losses.
Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current
output. The current limit clamps the peak inductor current; therefore, the ripple has to be subtracted to derive
maximum DC current. The ripple current is a function of switching frequency, inductor value and duty cycle. The
following equations take into account of all the above factors for maximum output current calculation.
(3)
Where:
I
p
= inductor peak to peak ripple
L = inductor value
V
V
f
= Schottky diode forward voltage
Fs = switching frequency
V
V
out
= output voltage of the boost converter. It is equal to the sum of VFB and the voltage drop across LEDs.
(4)
Where:
I
out_max
= maximum output current of the boost converter
I
lim
= over current limit
η
= efficiency
For instance, when VIN is 3.0V, 8 LEDs output equivalent to VOUT of 26V, the inductor is 22
μ
H, the Schottky
forward voltage is 0.2V; and then the maximum output current is 65mA in typical condition. When VIN is 5V, 10
LEDs output equivalent to VOUT of 32V, the inductor is 22
LEDs output equivalent to VOUT of 32V, the inductor is 22
μ
H, the Schottky forward voltage is 0.2V; and then the
maximum output current is 85mA in typical condition.
INDUCTOR SELECTION
The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These
factors make it 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.
factors make it 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.
The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary
peak current without saturating, according to half of the peak-to-peak ripple current given by
peak current without saturating, according to half of the peak-to-peak ripple current given by
, pause
the inductor DC current given by:
(5)
Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation
level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines
saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM when the
inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter’s
maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value
provides much more output current and higher conversion efficiency. For these reasons, a 10
level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines
saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM when the
inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter’s
maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value
provides much more output current and higher conversion efficiency. For these reasons, a 10
μ
H to 22
μ
H
inductor value range is recommended. A 22
μ
H inductor optimized the efficiency for most application while
maintaining low inductor peak to peak ripple.
lists the recommended inductor for the TPS61160/1. When
recommending inductor value, the factory has considered –40% and +20% tolerance from its nominal value.
14
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