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)： 23/33頁(yè)
文件大?。?/td> 1018K
代理商： MAX1887
M
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
______________________________________________________________________________________
23
In non-CPU applications, the output capacitor selection
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor
’
s ESR. When operating multiphase systems out-of-
phase, the peak inductor currents of each phase are
staggered, resulting in lower output ripple voltage by
reducing the total inductor ripple current. For out-of-
phase operation, the maximum ESR to meet ripple
requirements is:
This equation may be rewritten as the single phase rip-
ple current minus a correction due to the additional
phases:
where t
TRIG
is the MAX1887/MAX1897
’
s trigger propa-
gation delay,
η
is the number of phases, and K is from
Table 3. When operating the MAX1897 in-phase (POL
= GND), the high-side MOSFETs turn on together, so
the output capacitors must simultaneously support the
combined inductor ripple currents of each phase. For
in-phase operation, the maximum ESR to meet ripple
requirements is:
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, OS-CONs, and other electrolytics).
When using low-capacity filter capacitors such as
ceramic or polymer types, capacitor size is usually
determined by the capacity needed to prevent V
SAG
and V
SOAR
from causing problems during load tran-
sients. Generally, once enough capacitance is added to
meet the overshoot requirement, undershoot at the ris-
ing load edge is no longer a problem (see the V
SAG
and
V
SOAR
equations in the
Transient Response
section).
Output Capacitor Stability Considerations
For Quick-PWM controllers, stability is determined by
the value of the ESR zero relative to the switching fre-
quency. The boundary of instability is given by the fol-
lowing equation:
f
ESR
≤
π
where:
For a standard 300kHz application, the ESR zero fre-
quency must be well below 95kHz, preferably below
50kHz. Tantalum, Sanyo POSCAP, and Panasonic SP
capacitors in wide-spread use at the time of publication
have typical ESR zero frequencies below 30kHz. In the
standard application used for inductor selection, the
ESR needed to support a 30mV
P-P
ripple is 30mV/(40A
x 0.3) = 2.5m
. Eight 270μF/2V Panasonic SP capaci-
tors in parallel provide 1.9m
(max) ESR. Their typical
combined ESR results in a zero at 39kHz.
Don
’
t put high-value ceramic capacitors directly across
the output without taking precautions to ensure stability.
Ceramic capacitors have a high ESR zero frequency
and may cause erratic, unstable operation. However,
it
’
s easy to add enough series resistance by placing
the capacitors a couple of centimeters downstream
from the junction of the inductor and FB pin.
Unstable operation manifests itself in two related but
distinctly different ways: double-pulsing and feedback
loop instability. Double-pulsing occurs due to noise on
the output or because the ESR is so low that there isn
’
t
enough voltage ramp in the output voltage signal. This
“
fools
”
the error comparator into triggering a new cycle
immediately after the minimum off-time period has
expired. Double-pulsing is more annoying than harmful,
resulting in nothing worse than increased output ripple.
However, it can indicate the possible presence of loop
instability due to insufficient ESR. Loop instability can
result in oscillations at the output after line or load
steps. Such perturbations are usually damped, but can
cause the output voltage to rise above or fall below the
tolerance limits.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output voltage ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the switching waveforms (V
LX
and/or I
INDUCTOR
). Don
’
t
allow more than one cycle of ringing after the initial
step-response under/overshoot.
f
f
R
C
SW
ESR
ESR
OUT
=
π
1
2
R
V
I
LIR
)
V
f
L
V
V
V
V
ESR
RIPPLE
(
LOAD MAX
RIPPLE
SW
OUT
IN
IN
OUT
≤
=
(
)
η
R
V
I
LIR
)
V
L
t
t
ESR
RIPPLE
)
LOAD MAX
OUT
ON
TRIG
≤
+
(
)
(
η η
1
R
V
L
V
V
f
V
V
V
ESR
RIPPLE
IN
OUT
SW
OUT
IN
OUT TRIG
≤
(
)
η
η
η
1
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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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