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参数资料
| 型号： | MAX1843 |
| 厂商： | Maxim Integrated Products, Inc. |
| 英文描述： | 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package |
| 中文描述： | 2.7A、1MHz、低电压、降压型稳压器，内置同步整流，TQFN封装 |
| 文件页数： | 11/13页 |
| 文件大小： | 366K |
| 代理商： | MAX1843 |
M
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
______________________________________________________________________________________
11
Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit to reduce input surge currents at startup and
at exit from shutdown. A timing capacitor, C
SS
, placed
from SS to GND sets the rate at which the internal cur-
rent limit is changed. Upon power-up, when the device
comes out of undervoltage lockout (2.6V typ) or after
the
SHDN
pin is pulled high, a 4μA constant-current
source charges the soft-start capacitor and the voltage
on SS increases. When the voltage on SS is less than
approximately 0.7V, the current limit is set to zero. As
the voltage increases from 0.7V to approximately 1.8V,
the current limit is adjusted from 0 to the current-limit
threshold (see the
Electrical Characteristics
). The volt-
age across the soft-start capacitor changes with time
according to the equation:
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
where I
LIMIT
is the current threshold from the
Electrical
Characteristics
.
The constant-current source stops charging once the
voltage across the soft-start capacitor reaches 1.8V
(Figure 5).
Frequency Variation with Output Current
The operating frequency of the MAX1843 is determined
primarily by t
OFF
(set by R
TOFF
), V
IN
, and V
OUT
as
shown in the following formula:
f
PWM
= (V
IN
- V
OUT
- V
PMOS
) / [t
OFF
(V
IN
- V
PMOS
+
V
NMOS
)]
However, as the output current increases, the voltage
drop across the NMOS and PMOS switches increases
and the voltage across the inductor decreases. This
causes the frequency to drop. The change in frequency
can be approximated with the following formula:
f
PWM
= -I
OUT
x R
PMOS
/ (V
IN
x t
OFF
)
where R
PMOS
is the resistance of the internal MOSFETs
(90m
typ).
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1843
’
s
intended output power level, high efficiency, and low
noise. Good layout includes the use of ground planes,
careful component placement, and correct routing of
traces using appropriate trace widths. The following
points are in order of decreasing importance:
1) Minimize switched-current and high-current ground
loops. Connect the input capacitor
’
s ground, the out-
put capacitor
’
s ground, and PGND. Connect the
resulting island to GND at only one point.
2) Connect the input filter capacitor less than 5mm
away from IN. The connecting copper trace carries
large currents and must be at least 1mm wide,
preferably 2.5mm.
3) Place the LX node components as close together
and as near to the device as possible. This reduces
resistive and switching losses as well as noise.
4) Ground planes are essential for optimum perfor-
mance. In most applications, the circuit is located on
a multilayer board and full use of the four or more
layers is recommended. For heat dissipation, con-
nect the exposed backside pad to a large analog
ground plane, preferably on a surface of the board
that receives good airflow. If the ground plane is
located on the IC surface, make use of the N.C. pins
adjacent to GND to lower thermal resistance to the
ground plane. If the ground is located elsewhere,
use several vias to lower thermal resistance. Typical
applications use multiple ground planes to minimize
thermal resistance. Avoid large AC currents through
the analog ground plane.
Chip Information
TRANSISTOR COUNT: 3662
SSI
V
V
V
I
LIMIT
SS
LIMIT
=
×
.
1 1
.
0 7
V
A
C
t
SS
SS
×
=
4
μ
0.7V
1.8V
I
LIMIT
t
SHDN
0
0
0
V
SS
(V)
I
LIMIT
(A)
Figure 5. Soft-Start Current-Limit Over Time
相关PDF资料 |
PDF描述 |
|---|---|
| MAX1843EGI | 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package |
| MAX1845 | Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit |
| MAX1845EEI | Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit |
| MAX1845ETX | Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit |
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相关代理商/技术参数 |
参数描述 |
|---|---|
| MAX1843EGI | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| MAX1843EGI+ | 制造商:Maxim Integrated Products 功能描述:2.7A, 1MHZ, INTERNAL SWITCH STEP DOWN REGULATOR - Rail/Tube |
| MAX1843EGI+T | 制造商:Maxim Integrated Products 功能描述:CONV DC-DC SGL-OUT STEP DOWN 28QFN EP - Tape and Reel |
| MAX1843EGI-T | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| MAX1843ETI+ | 功能描述:直流/直流开关调节器 2.7A 1MHz Step-Down RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
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