參數(shù)資料
型號(hào)： MAX1887
廠商： Maxim Integrated Products, Inc.
英文描述： Quick-PWM Slave Controllers for Multiphase, Step-Down
中文描述： Quick-PWM從控制器，用于多相、降壓型電源
文件頁(yè)數(shù)： 19/33頁(yè)
文件大?。?/td> 1018K
代理商： MAX1887
M
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
______________________________________________________________________________________
19
in MOSFET technology that are making higher frequen-
cies more practical.
Setting Switch On Time:
The constant on-time control
algorithm in the master results in a nearly constant
switching frequency despite the lack of a fixed-frequen-
cy clock generator. In the slave, the high-side switch on
time is inversely proportional to V+ and directly propor-
tional to the compensation voltage (V
COMP
):
where K is internally preset to 3.3μs for the MAX1887 or
externally set by the TON pin-strap connection for the
MAX1897 (Table 3)
Set the nominal on time in the slave to match the on
time in the master. An exact match is not necessary
because the MAX1887/MAX1897 have wide t
ON
adjust-
ment ranges (±40%). For example, if t
ON
in the master
is set to 250kHz, the slave can be set to either 200kHz
or 300kHz and still achieve good performance. Care
should be taken to ensure that the COMP voltage
remains within its output voltage range (0.42V to
2.80V).
Inductor Operating Point:
This choice provides trade-
offs between size vs. efficiency and transient response
vs. output noise. Low inductor values provide better
transient response and smaller physical size, but also
result in lower efficiency and higher output noise due to
increased ripple current. The minimum practical induc-
tor value is one that causes the circuit to operate at the
edge of critical conduction (where the inductor current
just touches zero with every cycle at maximum load).
Inductor values lower than this grant no further size-
reduction benefit. The optimum operating point is usu-
ally found between 20% and 50% ripple current.
Inductor Selection
The switching frequency and operating point (% ripple
or LIR) determine the inductor value as follows:
where
η
is the number of phases. Example:
η
= 2,
I
LOAD
= 40A, V
IN
= 12V, V
OUT
= 1.3V, f
SW
= 300kHz,
30% ripple current or LIR = 0.3:
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
PEAK
):
where
η
is the number of phases.
Transient Response
The inductor ripple current affects transient-response
performance, especially at low V
IN
- V
OUT
differentials.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output fil-
ter capacitors by a sudden load step. The amount of
output sag also is a function of the maximum duty fac-
tor, which can be calculated from the on time and mini-
mum off time:
where t
OFF(MIN)
is the minimum off time (see the
Electrical Characteristics
section),
η
is the number of
phases, and K is from Table 3.
The amount of overshoot due to stored inductor energy
can be calculated as:
≈
(
2
η
Setting the Current Limits
The master and slave current-limit thresholds must be
great enough to support the maximum load current,
even under worst-case operating conditions. Since the
master
’
s current limit determines the maximum load
(see the
Current-Limit Circuitry
section), the procedure
for setting the current limit is sequential. First, the mas-
ter
’
s current limit is set based on the operating condi-
tions and the characteristics of the low-side MOSFETs.
Then the slave controller is configured to adjust the
master
’
s current-limit threshold based on the precise
current-sense resistor value and variation in the MOS-
FET characteristics. Finally, the resulting valley current
limit for the slave
’
s inductor occurs above the master
’
s
V
I
L
C
SOAR
LOAD MAX
OUT OUT
)
(
)
2
V
L
I
V
K
V
t
C
V
V
V
K
)
t
SAG
LOAD MAX
OUT
IN
OFF MIN
(
OUT OUT
IN
OUT
IN
OFF MIN
(
=
(
)
(
+
(
)
)
)
2
2
η
I
I
LIR
2
η
PEAK
LOAD MAX
=
+
(
)
2
L
Vx
300
V
V x
Ax
Vx
kHzx
H
=
(
=
1 3
.
12
1 3
40
2
0 3
.
12
0 64
.
.
μ
L
V
x V
xI
V
x
xLIR
V
xf
OUT
IN
OUT
IN
SW
LOAD MAX
=
)
)
η
(
t
K
V
V
ON
COMP
IN
=
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
MAX1887EEE 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(shí)間: 下降時(shí)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
MAX1887EEE+ 功能描述:電流型 PWM 控制器 CDMA Cellular/PCS System Power Supplies RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)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
MAX1887EEE+T 功能描述:電流型 PWM 控制器 CDMA Cellular/PCS System Power Supplies RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)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
MAX1887EEE-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(shí)間: 下降時(shí)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
MAX1888EUA 功能描述:其他電源管理 RoHS:否 制造商:Texas Instruments 輸出電壓范圍: 輸出電流:4 mA 輸入電壓范圍:3 V to 3.6 V 輸入電流: 功率耗散: 工作溫度范圍:- 40 C to + 110 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-48 封裝:Reel
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