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參數(shù)資料
| 型號： | LT3724MPFE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 225 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 6/26頁 |
| 文件大?。?/td> | 309K |
| 代理商： | LT3724MPFE#TR |
LT3724
14
3724fd
APPLICATIONS INFORMATION
Step-Down Converter: Input Capacitor Selection
Alocalinputbypasscapacitorisrequiredforbuckconvert-
ers because the input current is pulsed with fast rise and
fall times. The input capacitor selection criteria are based
on the bulk capacitance and RMS current capability. The
bulk capacitance will determine the supply input ripple
voltage. The RMS current capability is used to keep from
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple, VIN:
CIN(BULK) =
IOUT(MAX) VOUT
VIN fSW VIN(MIN)
VIN is typically chosen at a level acceptable to the user.
100mV-200mV is a good starting point. Aluminum elec-
trolytic capacitors are a good choice for high voltage, bulk
capacitance due to their high capacitance per unit area.
The capacitor’s RMS current is:
ICIN(RMS) = IOUT
VOUT(VIN – VOUT)
(VIN)
2
If applicable, calculate it at the worst case condition,
VIN = 2VOUT. The RMS current rating of the capacitor
is specified by the manufacturer and should exceed the
calculated ICIN(RMS). Due to their low ESR (Equivalent
Series Resistance), ceramic capacitors are a good choice
for high voltage, high RMS current handling. Note that the
ripplecurrentratingsfromaluminumelectrolyticcapacitor
manufacturersarebasedon2000hoursoflife.Thismakes
it advisable to further derate the capacitor or to choose a
capacitor rated at a higher temperature than required.
The combination of aluminum electrolytic capacitors and
ceramic capacitors is an economical approach to meet-
ing the input capacitor requirements. The capacitor volt-
age rating must be rated greater than VIN(MAX). Multiple
capacitors may also be paralleled to meet size or height
requirementsinthedesign.Locatethecapacitorveryclose
to the MOSFET switch and use short, wide PCB traces to
minimize parasitic inductance.
Step-Down Converter: Output Capacitor Selection
The output capacitance, COUT, selection is based on the
design’s output voltage ripple, VOUT, and transient load
requirements. VOUT is a function of IL and the COUT
ESR. It is calculated by:
VOUT = IL ESR+
1
(8 fSW COUT)
The maximum ESR required to meet a VOUT design
requirement can be calculated by:
ESR(MAX)
=
(
VOUT)(L)(fSW)
VOUT 1–
VOUT
VIN(MAX)
Worst-case VOUT occurs at highest input voltage. Use
paralleled multiple capacitors to meet the ESR require-
ments. Increasing the inductance is an option to lower the
ESR requirements. For extremely low VOUT, an additional
LC filter stage can be added to the output of the supply.
Application Note 44 has some good tips on sizing an ad-
ditional output filter.
Output Voltage Programming
A resistive divider sets the DC output voltage according
to the following formula:
R2
=R1
VOUT
1.231V
– 1
The external resistor divider is connected to the output
of the converter as shown in Figure 2. Tolerance of the
feedback resistors will add additional error to the output
voltage.
Example: VOUT = 12V; R1 = 10kΩ
R2
= 10k
12V
1.231V
1
= 87.48k use 86.6k 1%
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