參數(shù)資料
型號(hào)： LT3508IUF#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24
封裝： 4 X 4 MM, PLASTIC, MO-220WGGD-X, QFN-24
文件頁(yè)數(shù)： 4/24頁(yè)
文件大?。?/td> 365K
代理商： LT3508IUF#TR
LT3508
12
3508f
The high frequency of the LT3508 reduces the energy stor-
age requirements of the input capacitor. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
An alternative to a high value ceramic capacitor is a lower
value ceramic along with a larger electrolytic capacitor. The
electrolytic capacitor likely needs to be greater than 10F
in order to meet the ESR and ripple current requirements.
The input capacitor is likely to see high surge currents
when the input source is applied. Tantalum capacitors
can fail due to an oversurge of current. Only use tantalum
capacitors with the appropriate surge current rating. The
manufacturer may also recommend operation below the
rated voltage of the capacitor.
A nal caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3508. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details see Application Note 88.
Output Capacitor Selection
The output capacitor has two essential functions. Along
with the inductor, it lters the square wave generated by the
LT3508 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3508’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is:
C
V
MHz
f
OUT
=
50
1
where COUT is in F. Use X5R or X7R types. This choice
will provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor if the compensation network is also adjusted to
maintain the loop bandwidth. A lower value of output capaci-
tor can be used, but transient performance will suffer. With
an external compensation network, the loop gain can be
lowered to compensate for the lower capacitor value. Look
carefully at the capacitor’s data sheet to nd out what the
actual capacitance is under operating conditions (applied
voltage and temperature). A physically larger capacitor, or
one with a higher voltage rating, may be required. High
performance electrolytic capacitors can be used for the
output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specied by the supplier, and should be 0.05
Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR
PART SERIES
COMMENTS
Panasonic
Ceramic
Polymer
Tantalum
EEF Series
Kemet
Ceramic
Tantalum
T494, T495
Sanyo
Ceramic
Polymer
Tantalum
POSCAP
Murata
Ceramic
AVX
Ceramic
Tantalum
TPS Series
Taiyo Yuden
Ceramic
TDK
Ceramic
Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
I
IV
V
D AVG
OUT
IN
OUT
IN
()
–
=
()
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current.
APPLICATIONS INFORMATION
相關(guān)PDF資料
PDF描述
LT3508IUF 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24
LT3508IFE#TR 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PDSO16
LT3508EUF 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24
LT3509HMSE#TRPBF 1.2 A DUAL SWITCHING CONTROLLER, 2500 kHz SWITCHING FREQ-MAX, PDSO16
LT3509IMSE#PBF 1.2 A DUAL SWITCHING CONTROLLER, 2500 kHz SWITCHING FREQ-MAX, PDSO16
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