參數(shù)資料
型號： MAX1845ETX
廠商： MAXIM INTEGRATED PRODUCTS INC
元件分類： 穩(wěn)壓器
英文描述： Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit
中文描述： DUAL SWITCHING CONTROLLER, 620 kHz SWITCHING FREQ-MAX, QCC36
封裝： 6 X 6 MM, 0.80 MM, MO220WJJD-1, TQFN-36
文件頁數(shù)： 21/27頁
文件大?。?/td> 543K
代理商： MAX1845ETX
M
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
______________________________________________________________________________________
21
Applications Information
Dropout Performance
The output voltage adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 500ns
(max) minimum off-time one-shot. For best dropout per-
formance, use the slower on-time settings. When work-
ing with low input voltages, the duty-cycle limit must be
calculated using the worst-case values for on- and off-
times. Manufacturing tolerances and internal propaga-
tion delays introduce an error to the TON K-factor. This
error is greater at higher frequencies (Table 4). Also,
keep in mind that transient response performance of
buck regulators operating close to dropout is poor, and
bulk output capacitance must often be added (see the
V
SAG
equation in the
Design Procedure
section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (
I
DOWN
)
as much as it ramps up during the on-time (
I
UP
). The
ratio h =
I
UP
/
I
DOWN
is an indicator of ability to slew
the inductor current higher in response to increased
load and must always be greater than 1. As h ap-
proaches 1, the absolute minimum dropout point, the
inductor current will be less able to increase during each
switching cycle, and V
SAG
will greatly increase unless
additional output capacitance is used.
A reasonable minimum value for h is 1.5, but this may
be adjusted up or down to allow trade-offs between
V
SAG
, output capacitance, and minimum operating
voltage. For a given value of h, calculate the minimum
operating voltage as follows:
V
IN(MIN)
= [(V
OUT
+ V
DROP1
) / {1 - (t
OFF(MIN)
h / K)}]
+ V
DROP2
- V
DROP1
where V
DROP1
and V
DROP2
are the parasitic voltage
drops in the discharge and charge paths (see the
On-
Time One-Shot (TON
) section), t
OFF(MIN)
is from the
Electrical Characteristics
, and K is taken from Table 4.
The absolute minimum input voltage is calculated with h
= 1.
If the calculated V
IN(MIN)
is greater than the required
minimum input voltage, then reduce the operating fre-
quency or add output capacitance to obtain an accept-
able V
SAG
. If operation near dropout is anticipated,
calculate V
SAG
to ensure adequate transient response.
Dropout Design Example:
V
OUT
= 1.8V
fsw = 600kHz
K = 1.63μs, worst-case K = 1.4175μs
t
OFF(MIN)
= 500ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
V
IN(MIN)
= (1.8V + 0.1V) / [1 - (0.5μs
1.5) / 1.4175μs]
+ 0.1V - 0.1V = 3.8V
Calculating again with h = 1 gives an absolute limit of
dropout:
V
IN(MIN)
= (1.8V + 0.1V) / [1 - (0.5μs
1) / 1.4175μs]
+ 0.1V - 0.1V = 2.8V
Therefore, V
IN
must be greater than 2.8V, even with
very large output capacitance, and a practical input
voltage with reasonable output capacitance would be
3.8V.
Fixed Output Voltages
The MAX1845
’
s dual-mode operation allows the selec-
tion of common voltages without requiring external
components (Figure 8). Connect FB1 to GND for a fixed
1.8V output or to V
CC
for a 1.5V output, or connect FB1
directly to OUT1 for a fixed 1V output.
Connect FB2 to GND for a fixed 2.5V output or to OUT2
for a fixed 1V output.
Setting V
OUT
_ with a Resistor-Divider
The output voltage can be adjusted from 1V to 5.5V
with a resistor-divider network (Figure 9). The equation
for adjusting the output voltage is:
where V
FB
_ is 1.0V and R2 is about 10k
.
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. This is
especially true for dual converters, where one channel
can affect the other. The switching power stages
require particular attention (Figure 10). Refer to the
MAX1845 evaluation kit data sheet for a specific layout
example.
Use a four-layer board. Use the top side for power
components and the bottom side for the IC and the
sensitive ground components. Use the two middle lay-
ers as ground planes, with interconnections between
the top and bottom layers as needed. If possible,
V
V
1
R1
R2
OUT_
FB_
=
+
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
MAX1845ETX+ 功能描述:電流型 PWM 控制器 Step-Down Controller RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14
MAX1845ETX+T 功能描述:電流型 PWM 控制器 Step-Down Controller RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14
MAX1845ETX-T 功能描述:電流型 PWM 控制器 RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14
MAX1845ETX-TG068 制造商:Rochester Electronics LLC 功能描述: 制造商:Maxim Integrated Products 功能描述:
MAX1845EVKIT 功能描述:電源管理IC開發(fā)工具 Evaluation Kit for the MAX1845 RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V
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