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| 型号： | LT3740EDHC#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16-DFN |
| 标准包装： | 73 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 330kHz |
| 占空比： | 90% |
| 电源电压： | 2.2 V ~ 22 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： | 16-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 产品目录页面： | 1332 (CN2011-ZH PDF) |
LT3740
APPLICATIONS INFORMATION
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V OUT immediately shifts by an amount
equal to Δ I LOAD *(ESR), where ESR is the effective series
resistance of C OUT . Δ I LOAD also begins to charge or
discharge C OUT generating a feedback error signal used
by the regulator to return V OUT to its steady-state value.
During this recovery time, V OUT can be monitored for
overshoot or ringing that would indicate a stability problem.
The V C pin external components shown in Figure 2 will
provide adequate compensation for most applications. For
a detailed explanation of switching control loop theory see
Application Note 76.
Step-Up Converter Inductor Selection
The step-up converter in the LT3740 provides a BIAS voltage
about 7V higher than the input voltage V IN for the top
MOSFET drive and most of the internal controller circuitry.
The step-up converter has a current limit of 400mA.
An inductor ripple current from 100mA to 200mA is a
reasonable design for the converter. For this consideration,
a 22μH or 47μH inductor is recommended for most of the
LT3740 applications. Small size and high ef?ciency are the
major concerns. Inductors with low core losses and small
DCR at 1MHz are good choices. Some inductors in this
The step-up converter inductor current is the greatest
when the input voltage is the lowest. Larger Cgs of the
MOSFETs results in large inductor current. Connecting
the BGDP pin to the BIAS pin also greatly increases the
inductor current. The saturation current of the inductor
needs to cover the maximum input current.
The step-up inductor current decreases as the input voltage
increases. For high input voltages, the step-up converter
will begin skipping pulses. Although this will result in some
low frequency ripple, the BIAS voltage remains regulated
on an average basis, and the step-down controller opera-
tion is not affected.
For V IN higher than 10V step-up inductor saturation
current should be higher than 400mA. For V IN lower than
10V, a lower current rating inductor could be used. The
inductor RMS current should be higher than 250mA, and
the inductance should not be less than 10μH at 400mA.
Step-Up Converter Capacitor Selection
The small size of ceramic capacitors makes them ideal for the
output of the LT3740 step-up converter. X5R and X7R types
are recommended because they retain their capacitance
over wider voltage and temperature ranges than other types
such as Y5V or Z5U. A 1μF capacitor is recommended for
the output of the LT3740 step-up converter.
Table 3. Recommended Ceramic Capacitor Manufacturers
category with small size are listed in Table 2.
Table 2. Recommended Inductors for Step-Up Converter
CURRENT
PART NUMBER DCR (Ω) RATING (MA) MANUFACTURER
MANUFACTURER
Taiyo Yuden
Murata
Kemet
PHONE
408-573-4150
814-237-1431
408-986-0424
URL
www.t-yuden.com
www.murata.com
www.kemet.com
LQH3C220
ELT5KT-220
CDRH3D16-220
CR32-470
0.71
0.9
0.43
0.97
250
420
400
330
Murata
814-237-1431
www.murata.com
Panasonic
714-373-7334
www.panasonic.com
Sumida
847-956-0666
Design Example
As a design example, take a supply with the following
speci?cations: V IN = 7V to 20V (15V nominal), V OUT = 2.5V
± 5%, I OUT(MAX) = 10A. First, choose the inductor for about
30% ripple current at nominal V IN :
? ? 1–
www.Sumida.com
L =
2.5V ?
(300kHz) ? 0.3 ? 10A ?
2.5V ?
15V ? ?
= 2.3 μ H
3740fc
13
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