參數(shù)資料
型號： MAX1854EEG
廠商： MAXIM INTEGRATED PRODUCTS INC
元件分類： 穩(wěn)壓器
英文描述： High-Speed, Adjustable, Synchronous Step-Down Controllers with Integrated Voltage Positioning
中文描述： SWITCHING CONTROLLER, 550 kHz SWITCHING FREQ-MAX, PDSO24
封裝： 0.150 INCH, 0.025 INCH PITCH, QSOP-24
文件頁數(shù)： 27/33頁
文件大?。?/td> 840K
代理商： MAX1854EEG
M
High-Speed, Adjustable, Synchronous Step-Down
Controllers with Integrated Voltage Positioning
______________________________________________________________________________________
27
R
LOAD
= V
OUT
/ I
OUT
3) Calculate the output current that would exist for
each R
LOAD
data point in a nonpositioned
application:
I
NP
= V
NP
/ R
LOAD
where V
NP
= 1.6V (in this example).
4) Calculate effective efficiency as:
Effective efficiency = (V
NP
×
I
NP
) / (V
IN
×
I
IN
) =
calculated nonpositioned power output divided by
the measured voltage-positioned power input.
5) Plot the efficiency data point at the nonpositioned
current, I
NP
.
The effective efficiency of voltage-positioned circuits is
shown in the
Typical Operating Characteristics
.
Dropout Performance
The output-voltage adjustable range for continuous-
conduction operation is restricted by the nonadjustable
500ns (max) minimum off-time one-shot. For best
dropout performance, use the slower (200kHz) on-time
settings. When working with low input voltages, the
duty-factor limit must be calculated using worst-case
values for on- and off-times. Manufacturing tolerances
and internal propagation delays introduce an error to
the TON K-factor. This error is greater at higher fre-
quencies (Table 3). Also, keep in mind that transient
response performance of buck regulators operated
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 >1. As h approaches 1, the
absolute minimum dropout point, the inductor current
cannot increase as much during each switching cycle,
and V
SAG
greatly increases unless additional output
capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows trade-offs between V
SAG
, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
where V
DROP1
and V
DROP2
are the parasitic voltage
drops in the discharge and charge paths (see
On-Time
One-Sho
t), t
OFF(MIN)
is from the
Electrical
Characteristics
table, and K is taken from Table 3. 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 be sure of adequate transient re-
sponse.
Dropout Design Example:
V
OUT
= 1.6V
SW
= 550kHz
K = 1.8μs, worst-case K = 1.58μs
t
OFF(MIN)
= 500ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
V
IN(MIN)
= [(1.6V + 0.1V) / (1 - (0.5μs
×
1.5 / 1.58μs))] +
0.1V - 0.1V = 3.2V
Calculating again with h = 1 gives the absolute limit of
dropout:
V
IN(MIN)
= [(1.6V + 0.1V) / (1 - (0.5μs
×
1.0 / 1.58μs))] +
0.1V - 0.1V = 2.5V
Therefore, V
IN
must be greater than 2.5V, even with
very large output capacitance, and a practical input
voltage with reasonable output capacitance would be
3.2V.
Adjusting V
OUT
with a Resistive Divider
The output voltage can be adjusted with a resistive-
divider rather than the DAC if desired (Figure 9). The
drawback is that the on-time doesn
’
t automatically
receive correct compensation for changing output volt-
age levels. This can result in variable switching fre-
quency as the resistor ratio is changed, and/or
excessive switching frequency. The equation for adjust-
ing the output voltage is:
V
OUT
= V
FB
(1 + R1 / (R2 || R
INT
))
where V
FB
is the currently selected DAC value, and
R
INT
is the FB input resistance. In resistor-adjusted cir-
cuits, the DAC code should be set as close as possible
to the actual output voltage in order to minimize the
shift in switching frequency.
Adjusting V
OUT
Above 2V
The feed-forward circuit that makes the on-time depen-
dent on the input voltage maintains a nearly constant
switching frequency as V+, I
LOAD
, and the DAC code
are changed. This works extremely well as long as FB
is connected directly to the output. When the output is
V
IN(MIN) =
1
–
K
V
OUT +
V
DROP1
t
OFF(MIN)
h
+ V
DROP2
–
V
DROP1
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
MAX1854EEG/GG8 功能描述:DC/DC 開關(guān)控制器 High-Speed Adjustable Synchronous Step-Down Controllers with Integrated Voltage Positioning RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
MAX1854EEG+ 制造商:Maxim Integrated Products 功能描述:HIGH-SPEED,DIGITALLY ADJUSTABLE SYNCHRONOUS STEPDOWN CONTROL - Rail/Tube
MAX1854EEG+T 制造商: 功能描述: 制造商:undefined 功能描述:
MAX1854EEGT 制造商: 功能描述:
MAX1854EEG-T 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
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