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| 型号： | LTC3823IGN#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/26页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-SSOP |
| 标准包装： | 49 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 90% |
| 电源电压： | 4.5 V ~ 36 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 28-SSOP（0.154"，3.90mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1336 (CN2011-ZH PDF) |
LTC3823
APPLICATIONS INFORMATION
V OUT ?
? ? ?
?
?
? ? 1 ?
L = ?
does not exceed a speci?ed maximum, the inductance
should be chosen according to:
V OUT
? ? ?
? f Δ I L ( MAX ) ? ? V IN ( MAX ) ?
Once the value for L is known, the type of inductor must
be selected. High ef?ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ ? cores. A variety of inductors designed for
high current, low voltage applications are available from
manufacturers such as Sumida, Panasonic, Coiltronics,
Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 12 conducts dur-
ing the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
of the bottom MOSFET from turning on and storing charge
during the dead time, which can cause a modest (about
1%) ef?ciency loss. The diode can be rated for about one
half to one ?fth of the full load current since it is on for
only a fraction of the duty cycle. In order for the diode
to be effective, the inductance between it and the bottom
MOSFET must be as small as possible, mandating that
these components be placed adjacently. The diode can
be omitted if the ef?ciency loss is tolerable.
C IN and C OUT Selection
The input capacitance C IN is required to ?lter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
The selection of C OUT is primarily determined by the
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔV OUT is approximately
bounded by:
? 1 ?
Δ V OUT ≤ Δ I L ? ESR + ?
? 8 fC OUT ?
Since ΔI L increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satis?ed, the capacitance is adequate
for ?ltering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount pack-
ages. Special polymer capacitors offer very low ESR but
have lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have signi?cantly higher ESR, but can be used
in cost-sensitive applications providing that consideration
is given to ripple current ratings and long term reliability.
Ceramic capacitors have excellent low ESR characteris-
tics but can have a high voltage coef?cient and audible
piezoelectric effects. The high Q of ceramic capacitors with
trace inductance can also lead to signi?cant ringing. When
used as input capacitors, care must be taken to ensure that
ringing from inrush currents and switching does not pose
an overvoltage hazard to the power switches and control-
ler. To dampen input voltage transients, add a small 5μF
to 50μF aluminum electrolytic capacitor with an ESR in
the range of 0.5Ω to 2Ω. High performance through-hole
I RMS ? I OUT ( MAX )
V OUT
V IN
V IN
V OUT
–1
capacitors may also be used, but an additional ceramic
capacitor in parallel is recommended to reduce the effect
of their lead inductance.
This formula has a maximum at V IN = 2V OUT , where
I RMS = I OUT(MAX) /2. This simple worst-case condition is
commonly used for design because even signi?cant de-
viations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
Top MOSFET Driver Supply (C B , D B )
An external bootstrap capacitor C B connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D B from INTV CC
when the switch node is low. When the top MOSFET turns
the capacitor.
3823fd
14
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