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| 型号： | MAX1845EEI+ |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 20/27页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-QSOP |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 540kHz |
| 电源电压： | 4.5 V ~ 5.5 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 |
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Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
(
)
I RMS LOAD ?
C RSS × V IN(MAX)2 × f × I LOAD
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I RMS ) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resistance to power-up
surge currents:
? V V -V ?
= I OUT IN OUT ?
? V IN ?
? ?
Power MOSFET Selection
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
(>5A) when using high-voltage (>20V) AC adapters.
Low-current applications usually require less attention.
For maximum efficiency, choose a high-side MOSFET
(Q1) that has conduction losses equal to the switching
losses at the optimum battery voltage (15V). Check to
ensure that the conduction losses at the minimum
input voltage do not exceed the package thermal limits
or violate the overall thermal budget. Check to ensure
that conduction losses plus switching losses at the
maximum input voltage do not exceed the package
ratings or violate the overall thermal budget.
Choose a low-side MOSFET (Q2) that has the lowest
possible R DS(ON) , comes in a moderate to small pack-
age (i.e., SO-8), and is reasonably priced. Ensure that
become an issue until the input is greater than approxi-
mately 15V.
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied, due to the squared term in the
CV 2 f switching loss equation. If the high-side MOSFET
chosen for adequate R DS(ON) at low battery voltages
becomes extraordinarily hot when subjected to
V IN(MAX) , reconsider the choice of MOSFET.
Calculating the power dissipation in Q1 due to switch-
ing losses is difficult since it must allow for difficult
quantifying factors that influence the turn-on and turn-
off times. These factors include the internal gate resis-
tance, gate charge, threshold voltage, source
inductance, and PC board layout characteristics. The
following switching-loss calculation provides only a
very rough estimate and is no substitute for bench eval-
uation, preferably including a verification using a ther-
mocouple mounted on Q1:
PD(Q1 switching) =
I GATE
where C RSS is the reverse transfer capacitance of Q1,
and I GATE is the peak gate-drive source/sink current
(1A typ).
For the low-side MOSFET, Q2, the worst-case power
dissipation always occurs at maximum battery voltage:
?
the MAX1845 DL gate driver can drive Q2; in other
words, check that the gate is not pulled up by the high-
side switch turning on due to parasitic drain-to-gate
capacitance, causing cross-conduction problems.
?
PD(Q2) = ? 1 -
?
V OUT ? ?
?
V IN ( MAX ) ?
I LOAD2 × R DS ( ON )
Switching losses are not an issue for the low-side MOS-
FET since it is a zero-voltage switched device when
used in the buck topology.
MOSFET Power Dissipation
Worst-case conduction losses occur at the duty cycle
extremes. For the high-side MOSFET, the worst-case-
power dissipation (PD) due to resistance occurs at min-
imum battery voltage:
The absolute worst case for MOSFET power dissipation
occurs under heavy overloads that are greater than
I LOAD(MAX) but are not quite high enough to exceed
the current limit. To protect against this possibility,
“overdesign” the circuit to tolerate:
I LOAD = I LIMIT(HIGH) + (LIR / 2) ? I LOAD(MAX)
where I LIMIT(HIGH) is the maximum valley current
allowed by the current-limit circuit, including threshold
? V IN ( MIN ) ? LOAD
× R DS ( )
? V OUT ?
PD(Q1 resistance) = ? ? I
? ?
2
ON
tolerance and on-resistance variation. If short-circuit
protection without overload protection is adequate,
enable overvoltage protection, and use I LOAD(MAX) to
calculate component stresses.
Generally, a small high-side MOSFET is desired in
order to reduce switching losses at high input voltages.
However, the R DS(ON) required to stay within package
power-dissipation limits often limits how small the MOS-
FET can be. Again, the optimum occurs when the
switching (AC) losses equal the conduction (R DS(ON) )
losses. High-side switching losses do not usually
Choose a Schottky diode (D1) having a forward voltage
low enough to prevent the Q2 MOSFET body diode
from turning on during the dead time. As a general rule,
a diode having a DC current rating equal to 1/3 of the
load current is sufficient. This diode is optional and can
be removed if efficiency is not critical.
20
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相关代理商/技术参数 |
参数描述 |
|---|---|
| MAX1845EEI+ | 功能描述:电流型 PWM 控制器 Step-Down Controller RoHS:否 制造商:Texas Instruments 开关频率:27 KHz 上升时间: 下降时间: 工作电源电压:6 V to 15 V 工作电源电流:1.5 mA 输出端数量:1 最大工作温度:+ 105 C 安装风格:SMD/SMT 封装 / 箱体:TSSOP-14 |
| MAX1845EEI+T | 功能描述:电流型 PWM 控制器 Step-Down Controller RoHS:否 制造商:Texas Instruments 开关频率:27 KHz 上升时间: 下降时间: 工作电源电压:6 V to 15 V 工作电源电流:1.5 mA 输出端数量:1 最大工作温度:+ 105 C 安装风格:SMD/SMT 封装 / 箱体:TSSOP-14 |
| MAX1845EEI-T | 功能描述:电流型 PWM 控制器 RoHS:否 制造商:Texas Instruments 开关频率:27 KHz 上升时间: 下降时间: 工作电源电压:6 V to 15 V 工作电源电流:1.5 mA 输出端数量:1 最大工作温度:+ 105 C 安装风格:SMD/SMT 封装 / 箱体:TSSOP-14 |
| MAX1845EEI-TG05 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Maxim Integrated Products 功能描述: |
| MAX1845EEI-TG068 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Maxim Integrated Products 功能描述: |
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