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参数资料
| 型号： | LTC3612IUDC#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/30页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 3A 20QFN |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 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： | 300kHz ~ 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： | 20-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 20-QFN（3x4） |
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LTC3612
APPLICATIONS INFORMATION
? V OUT ≤ ? I L ? ? ESR +
C OUT ≈
V OUT = 0.6 ? ? 1 +
? V
thecapacitanceisadequateforfiltering.Theoutputripple
? V OUT is determined by:
? 1 ?
?
? 8 ? f SW ?C OUT ?
where f OSC = operating frequency, C OUT = output capaci-
tance 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.
They are attractive for switching regulator use because
of their very low ESR, but great care must be taken when
using only ceramic input and output capacitors.
Ceramic capacitors are prone to temperature effects
which require the designer to check loop stability over
the operating temperature range. To minimize their large
temperature and voltage coefficients, only X5R or X7R
ceramic capacitors 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 sup-
port the load. The time required for the feedback loop to
respond is dependent on the compensation components
and the output capacitor size. Typically, 3 to 4 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 2 to 4 times the linear drop of the first
cycle; however, this behavior can vary depending on the
compensation component values. Thus, a good place to
start is with the output capacitor size of approximately:
3.5 ? ? I OUT
f SW ? V DROOP
This is only an approximation; more capacitance may
be needed 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.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
? R1 ?
? R2 ?
The resistive divider allows pin V FB to sense a fraction of
the output voltage, as shown in Figure 1.
3612fb
For more information www.linear.com/LTC3612
17
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