參數(shù)資料
型號: MAX1856EUB
廠商: MAXIM INTEGRATED PRODUCTS INC
元件分類: 穩(wěn)壓器
英文描述: Replaced by TMS320C6202B : Fixed-Point Digital Signal Processor 352-FCBGA 0 to 0
中文描述: SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO10
封裝: MICRO MAX PACKAGE-10
文件頁數(shù): 14/18頁
文件大?。?/td> 594K
代理商: MAX1856EUB
M
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
14
______________________________________________________________________________________
the feedback resistance that cancels the ESR zero. The
optimum compensation value is:
where R1 and R3 are feedback resistors (Figure 3). If
the calculated value for C
FB
results in a nonstandard
capacitance value, values from 0.5C
FB
to 1.5C
FB
will
also provide sufficient compensation.
Snubber Design
The MAX1856 uses a current-mode controller that
employs a current-sense resistor. Immediately after
turn-on, the MAX1856 uses a 100ns current-sense
blanking period to minimize noise sensitivity. However,
when the MOSFET turns on, the secondary inductance
and the output diode
s parasitic capacitance form a
resonant circuit that causes ringing. Reflected back
through the transformer to the primary side, these oscil-
lations appear across the current-sense resistor and
last well beyond the 100ns blanking period. As shown
in Figure 1, a series RC snubber circuit at the output
diode increases the damping factor, allowing the ring-
ing to settle quickly. Applications with dual output volt-
ages require only one snubber circuit on the higher
voltage output.
The diode
s parasitic capacitance can be estimated
using the diode
s reverse voltage rating (V
RRM
), current
capability (I
O
), and recovery time (t
RR
). A rough
approximation is:
For the CMR1U-02 Central Semiconductor diode used
in Figure 1, the capacitance is roughly 172pF. A value
less than this (100pF) was chosen since the output
snubber only needs to dampen the ringing, so the initial
turn-on spike that occurs during the 100ns blanking
period is still present. Larger capacitance values
require more charge, thereby increasing the power dis-
sipation.
The snubber
s time constant (t
SNUB
) must be smaller
than the 100ns blanking time. A typical RC time con-
stant of 50ns was chosen for Figure 1:
When a MOSFET with a transformer load is turned off,
the drain will fly to a high voltage as a result of the ener-
gy stored in the transformer
s leakage inductance.
During the switch on-time, current is established in the
leakage inductance (L
L
) equal to the peak primary cur-
rent (I
PEAK
). The energy stored in the leakage induc-
tance is:
When the switch turns off, this energy is transferred to
the MOSFET
s parasitic capacitance, causing a voltage
spike at the MOSFET
s drain. For the IRLL2705 MOS-
FET, the capacitance value (C
DS
) is 130pF. If all of the
leakage inductance energy transfers to this capaci-
tance, the drain would fly up to:
The leakage inductance is (worst case) 1% of the pri-
mary inductance value. For a 0.27μH leakage induc-
tance and a 2.5A peak current, the voltage reaches
114V at the MOSFET
s drain, which is much higher than
the MOSFET
s rated breakdown voltage. This causes
the parasitic bipolar transistor to turn on if the dv/dt at
the drain is high enough. Note that the inductive spike
adds on to the sum of the input voltage and the reflect-
ed secondary voltage already present at the drain of
the transistor (see
Power MOSFET Selection
).
A series combination RC snubber (R7 and C6 in Figure
3) across the MOSFET (drain to source) reduces this
spike. The energy stored in the leakage inductance
transfers to the snubber capacitor (C6) as electrostatic
energy. Therefore, C6 must be large enough to guaran-
tee the voltage spike will not exceed the breakdown
voltage, but not so large as to result in excessive power
dissipation:
Typically, a 30% safety margin is chosen so that V
C6
is
at most equal to about 70% of the MOSFET
s V
DS
rat-
ing. For example, the V
DSS
is 55V for the IRLL2705, so
this gives a value of 1000pF for C9. The amount of
energy stored in snubber capacitor C6 has to dis-
charge through series resistor R7 in the snubber net-
work. During turn-off, the drain voltage rises in a time
period (t
f
) characteristic of the MOSFET used, which is
22ns for the IRLL2705. The RC time constant should
therefore equal this time. Hence:
C
L I
V
C
6
2
62
=
V
L I
C
COSS
DS
=
2
E
L I
L
=
2
2
R
t
C
ns
3
C
SNUB
3
4
50
=
=
C
I t
V
RRM
DIODE
=
C
C
ESR
R
×
R
(
R
R
FB
OUT
COUT
)/(
=
+
1
2
1
3
1
3
)
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MAX1856EUB+ 功能描述:電信線路管理 IC Synchronizable PWM SLIC Power Supply RoHS:否 制造商:STMicroelectronics 產(chǎn)品:PHY 接口類型:UART 電源電壓-最大:18 V 電源電壓-最小:8 V 電源電流:30 mA 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VFQFPN-48 封裝:Tray
MAX1856EUB+T 功能描述:電信線路管理 IC Synchronizable PWM SLIC Power Supply RoHS:否 制造商:STMicroelectronics 產(chǎn)品:PHY 接口類型:UART 電源電壓-最大:18 V 電源電壓-最小:8 V 電源電流:30 mA 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VFQFPN-48 封裝:Tray
MAX1856EUB-T 功能描述:電信線路管理 IC RoHS:否 制造商:STMicroelectronics 產(chǎn)品:PHY 接口類型:UART 電源電壓-最大:18 V 電源電壓-最小:8 V 電源電流:30 mA 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VFQFPN-48 封裝:Tray
MAX1856EVKIT 功能描述:電源管理IC開發(fā)工具 Evaluation Kit for the MAX1856 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
MAX1857EUA47 功能描述:低壓差穩(wěn)壓器 - LDO 500mA Ripple-Rejecting RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動電壓(最大值):307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20