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參數(shù)資料
| 型號： | TPS54320RHLR |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 1320 kHz SWITCHING FREQ-MAX, PQCC14 |
| 封裝： | 3.50 X 3.50 MM, GREEN, PLASTIC, QFN-14 |
| 文件頁數(shù)： | 15/39頁 |
| 文件大?。?/td> | 1358K |
| 代理商： | TPS54320RHLR |
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Vinmax
Vout
L1
Iout KIND
Vinmax
sw
-
=
×
× f
f
-
=
×
Vinmax
Vout
Iripple
L1
Vinmax
sw
(
)
2
1
12
Vout
Vinmax
Vout
ILrms
Iout
Vinmax L1
sw
×
-
=
+
×
÷
÷
×
è
f
2
=
+
Iripple
ILpeak
Iout
2
f
× D
>
× D
Iout
Co
sw
Vout
SLVS982A – AUGUST 2010 – REVISED SEPTEMBER 2010
www.ti.com
Output Inductor Selection
To calculate the value of the output inductor, Equation 18 is used. KIND is a coefficient that represents the
amount of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by
the output capacitor. Therefore, choosing a high inductor ripple current impacts the selection of the output
capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple
current. Normally, the inductor ripple value is at the discretion of the designer; however, KIND is normally from
0.2 to 0.4 for the majority of applications.
(18)
For this design example, using KIND = 0.3 the inductor value is calculated to be 6.2H. The nearest standard
value of 6.8H was chosen. For the output filter inductor, it is important that the RMS current and saturation
current ratings not be exceeded. The inductor ripple current, RMS current, and peak inductor current can be
found from Equation 19, Equation 20, and Equation 21.
(19)
(20)
(21)
For this design, the inductor ripple current is 815 mA, the RMS inductor current is 3.01 A, and the peak inductor
current is 3.41 A. A 6.8H TDK VLP8040 series inductor was chosen for its small size and low DCR. It has a
saturation current rating of 3.6 A and a RMS current rating of 4 A.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the calculated peak inductor current
level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of
the device. For this reason, the most conservative approach is to specify an inductor with a saturation current
rating equal to or greater than the switch current limit rather than the peak inductor current.
Output Capacitor Selection
There are three primary considerations for selecting the value of the output capacitor: minimum capacitance to
meet the load transient requirement, minimum capacitance to meet the output voltage ripple requirement, and
maximum ESR to meet the output voltage ripple requirement. The output capacitor needs to be selected based
on the most stringent of these three criteria.
The desired response to a large change in the load current is the first criterion. The output capacitor needs to
supply the load with current when the regulator cannot. This situation would occur if there are desired hold-up
times for the regulator where the output capacitor must hold the output voltage above a certain level for a
specified amount of time after the input power is removed. The regulator is also temporarily not able to supply
sufficient output current if there is a large, fast increase in the current needs of the load such as transitioning
from no load to a full load. The regulator usually needs two or more clock cycles for the control loop to see the
change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor
must be sized to supply the extra current to the load until the control loop responds to the load change. The
output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing
a tolerable amount of droop in the output voltage. Equation 22 shows the minimum output capacitance necessary
to accomplish this.
(22)
Where
ΔIout is the change in output current, fsw is the regulator's switching frequency and ΔVout is the
allowable change in the output voltage. For this example, the transient load response is specified as a 4%
change in Vout for a load step of 0.75 A. Using these numbers (
ΔIOUT = 0.75 A and ΔVout = 0.04 x 3.3 = 0.132V)
gives a minimum capacitance of 23.7mF. This value does not take the ESR of the output capacitor into account in
the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.
22
Copyright 2010, Texas Instruments Incorporated
Product Folder Link(s) :TPS54320
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| TPS54325 | 制造商:TI 制造商全稱:Texas Instruments 功能描述:4.5-V to 18-V, 3-A OUTPUT SYNCHRONOUS STEP DOWN SWITCHER WITH INTEGRATED FET (SWIFT?) |
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