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参数资料
| 型号： | LTC3615EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 18/32页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 3A DL 24QFN |
| 标准包装： | 91 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 0.6 V ~ 5.5 V |
| 输入电压： | 2.25 V ~ 5.5 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 400kHz ~ 4MHz |
| 电流 - 输出： | 3A |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 24-QFN 裸露焊盘（4x4） |
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LTC3615/LTC3615-1
APPLICATIONS INFORMATION
8 ? f SW ? C OUT ?
C OUT ≈
OutputCapacitorC OUT Selection
The selection of C OUT is typically driven by the required
ESR to minimize voltage ripple and load step transients
(low-ESR ceramic capacitors are discussed in the next
section). Typically, once the ESR requirement is satisfied,
the capacitance is adequate for filtering. The output ripple
?V OUT is determined by:
? 1 ?
Δ V OUT ≤ Δ I L ? ? ESR + ?
?
where f SW = operating frequency, C OUT = output capacitance
and ?I L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ?I L increases
with input voltage.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
Tantalum capacitors have the highest capacitance density,
but can have higher ESR and must be surge tested for
use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can often
be used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long term reliability.
Ceramic Input and Output Capacitors
Ceramic capacitors have the lowest ESR and can be cost
effective, but also have the lowest capacitance density,
high voltage and temperature coefficients, and exhibit
audible piezoelectric effects. In addition, the high-Q of
ceramic capacitors along with trace inductance can lead
to significant ringing.
Capacitors are tempting for switching regulator use
because of their very low ESR. Great care must be taken
when using only ceramic input and output capacitors.
Ceramic caps are prone to temperature effects which re-
quire the designer to check loop stability over the operating
temperature range. To minimize their large temperature
and voltage coefficients, only X5R or X7R ceramic capaci-
tors should be used.
When a ceramic capacitor is used at the input, and the
power is being supplied through long wires, such as from
a wall adapter, a load step at the output can induce ringing
at the V IN pin. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, the ringing
at the input can be large enough to damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation com-
ponents and the output capacitor size. Typically, three to
four cycles are required to respond to a load step, but
only in the first cycle does the output drop linearly. The
output droop, V DROOP , is usually about two to three times
the linear drop of the first cycle. Thus, a good place to
start is with the output capacitor size of approximately:
2.5 ? Δ I OUT
f SW ? V DROOP
More capacitance may be required depending on the duty
cycle and load step requirements. In most applications, the
input capacitor is merely required to supply high frequency
bypassing, since the impedance to the supply is very low.
18
For more information www.linear.com/LTC3615
3615fb
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