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| 型号： | MAX1856EUB |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 14/18页 |
| 文件大小： | 0K |
| 描述： | IC PS PWM SLIC SYNCH 10-UMAX |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 50 |
| 应用： | 控制器，卫星接收机顶盒，xDSL 和线缆调制解调器 |
| 输入电压： | 3 V ~ 28 V |
| 输出数： | 1 |
| 输出电压： | -24V，-72V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 供应商设备封装： | 10-uMAX |
| 包装： | 管件 |
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
C FB =
( R 1 × R 3 ) /( R 1 + R 3 ) ?
C OUT ? ?
?
L I
E L = L PEAK
the feedback resistance that cancels the ESR zero. The
optimum compensation value is:
1 ? ESR COUT ?
2
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
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:
2
2
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:
blanking period to minimize noise sensitivity. However,
when the MOSFET turns on, the secondary inductance
and the output diode ’ s parasitic capacitance form a
V COSS =
L L I PEAK 2
C DS
C DIODE = O RR
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:
I t
V RRM
For the CMR1U-02 Central Semiconductor diode used
in Figure 1, the capacitance is roughly 172pF. A value
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:
C 6 = L PEAK
V C 6
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
L I
2
2
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:
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-
R 4 = =
t SNUB
C 3
50 ns
C 3
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
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.
therefore equal this time. Hence:
14
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| MAX1856EUB+ | 功能描述:电信线路管理 IC Synchronizable PWM SLIC Power Supply RoHS:否 制造商:STMicroelectronics 产品:PHY 接口类型:UART 电源电压-最大:18 V 电源电压-最小:8 V 电源电流:30 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 安装风格:SMD/SMT 封装 / 箱体:VFQFPN-48 封装:Tray |
| MAX1856EUB+T | 功能描述:电信线路管理 IC Synchronizable PWM SLIC Power Supply RoHS:否 制造商:STMicroelectronics 产品:PHY 接口类型:UART 电源电压-最大:18 V 电源电压-最小:8 V 电源电流:30 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 安装风格:SMD/SMT 封装 / 箱体:VFQFPN-48 封装:Tray |
| MAX1856EUB-T | 功能描述:电信线路管理 IC RoHS:否 制造商:STMicroelectronics 产品:PHY 接口类型:UART 电源电压-最大:18 V 电源电压-最小:8 V 电源电流:30 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 安装风格:SMD/SMT 封装 / 箱体:VFQFPN-48 封装:Tray |
| MAX1856EVKIT | 功能描述:电源管理IC开发工具 Evaluation Kit for the MAX1856 RoHS:否 制造商:Maxim Integrated 产品:Evaluation Kits 类型:Battery Management 工具用于评估:MAX17710GB 输入电压: 输出电压:1.8 V |
| MAX1857EUA47 | 功能描述:低压差稳压器 - LDO 500mA Ripple-Rejecting RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20 |
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