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参数资料
| 型号： | LT1941EFE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/24页 |
| 文件大小： | 0K |
| 描述： | IC REG MULTI CONFIG TRPL 28TSSOP |
| 标准包装： | 50 |
| 类型： | 降压（降压），升压（升压），反相，Sepic |
| 输出类型： | 可调式 |
| 输出数： | 3 |
| 输出电压： | 1.25 V ~ 40 V |
| 输入电压： | 3.5 V ~ 25 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 1.1MHz |
| 电流 - 输出： | 3A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SOIC（0.173"，4.40mm 宽）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 28-TSSOP 裸露焊盘 |
LT1941
APPLICATIONS INFORMATION
and is largest when V IN = 2 V OUT (50% duty cycle). As
the second, lower power channel draws input current, the
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. The ripple current contribution
from the third channel will be minimal. Considering that
the maximum load current from a single channel is ~2.8A,
RMS ripple current will always be less than 1.4A.
The high frequency of the LT1941 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is often less than 10μF. The combi-
nation 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 along with a larger electrolytic capacitor, for example
a 1μF ceramic capacitor in parallel with a low ESR tantalum
capacitor. For the electrolytic capacitor, a value larger than
10μF will be required to meet the ESR and ripple current
requirements. Because the input capacitor is likely to see
high surge currents when the input source is applied, tan-
talum capacitors should be surge rated. The manufacturer
may also recommend operation below the rated voltage
of the capacitor. Be sure to place the 1μF ceramic as close
as possible to the V IN and GND pins on the IC for optimal
noise immunity.
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 LT1941. 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.
Frequency Compensation
The LT1941 uses current mode control to regulate the
output. This simpli?es loop compensation. In particular, the
LT1941 does not depend on the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size.
The components tied to the V C pin provide frequency
compensation. Generally, a capacitor and a resistor in
series to ground determine loop gain. In addition, there
is a lower value capacitor in parallel. This capacitor ?lters
noise at the switching frequency and is not part of the
loop compensation.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and the type of output capacitor. A practical approach is to
start with one of the circuits in this data sheet that is similar
to your application and tune the compensation network
to optimize the performance. Check stability across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Application
Note 76 is an excellent source as well.
Figure 5 shows an equivalent circuit for the LT1941 control
loop. The error amp is a transconductance ampli?er with
?nite output impedance. The power section, consisting of
the modulator, power switch and inductor is modeled as
a transconductance ampli?er generating an output cur-
rent proportional to the voltage at the V C pin. Note that
the output capacitor integrates this current and that the
capacitor on the V C pin (C C ) integrates the error ampli-
?er output current, resulting in two poles in the loop. In
most cases, a zero is required and comes either from the
output capacitor ESR or from a resistor in series with C C .
This model works well as long as the inductor current
ripple is not too low (ΔI RIPPLE > 5% I OUT ) and the loop
crossover frequency is less than ? SW /5. A phase lead
capacitor (C PL ) across the feedback divider may improve
the transient response.
The equivalent circuit for the LT1941 inverter control loop
is slightly different than is shown in Figure 5. The feedback
resistors are connected as shown for negative outputs in
Figure 2. The operational ampli?er is fast enough to have
minimal effect on the loop dynamics.
1941fb
16
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