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参数资料
| 型号： | LM3151MHE-2.5 |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO14 |
| 封装： | TSSOP-14 |
| 文件页数： | 7/20页 |
| 文件大小： | 479K |
| 代理商： | LM3151MHE-2.5 |
ESR
min ≥ [ET / (VIN - VOUT)] / CO
ESR
min ≥ (15 mV x 1.65 H) / 5.7 V s = 4.3 m
ESR
min ≥ [5.7 V s / (12 - 3.3)] / 169 F = 3.9 m
Based on the above criteria two 150 F polymer aluminum
capacitors with a ESR = 12 m
each for a effective ESR in
parallel of 6 m
was chosen from Panasonic. The part num-
ber is EEF-UE0J151P.
5. MOSFET Selection
The LM3151/2/3 are designed to drive N-channel MOSFETs.
For a maximum input voltage of 24V we should choose N-
channel MOSFETs with a maximum drain-source voltage,
V
DS, greater than 1.2 x 24V = 28.8V. FETs with maximum
V
DS of 30V will be the first option. The combined total gate
charge Q
gtotal of the high-side and low-side FET should satisfy
the following:
Q
gtotal ≤ IVCCL / fs
Q
gtotal ≤ 65 mA / 500 kHz
Q
gtotal ≤ 130 nC
Where I
VCCL is the minimum current limit of VCC, over the
temperature range, specified in the electrical characteristics
table. The MOSFET gate charge Q
g is gathered from reading
the V
GS vs Qg curve of the MOSFET datasheet at the VGS =
5V for the high-side, M1, MOSFET and V
GS = 6V for the low-
side, M2, MOSFET.
The Renesas MOSFET RJK0305DPB has a gate charge of
10 nC at V
GS = 5V, and 12 nC at VGS = 6V. This combined
gate charge for a high-side, M1, and low-side, M2, MOSFET
12 nC + 10 nC = 22 nC is less than 130 nC calculated
Q
gtotal.
The calculated MOSFET power dissipation must be less than
the max allowed power dissipation, Pdmax, as specified in the
MOSFET datasheet. An approximate calculation of the FET
power dissipated Pd, of the high-side and low-side FET is
given by:
High-Side MOSFET
The max power dissipation of the RJK0305DPB is rated as
45W for a junction temperature that is 125°C higher than the
case temperature and a thermal resistance from the FET
junction to case,
θ
JC, of 2.78°C/W. When the FET is mounted
onto the PCB, the PCB will have some additional thermal re-
sistance such that the total system thermal resistance of the
FET package and the PCB,
θ
JA, is typically in the range of 30°
C/W for this type of FET package. The max power dissipation,
Pdmax, with the FET mounted onto a PCB with a 125°C junc-
tion temperature rise above ambient temperature and
θ
JA =
30°C/W, can be estimated by:
Pdmax = 125°C / 30°C/W = 4.1W
The system calculated Pdh of 0.674W is much less than the
FET Pdmax of 4.1W and therefore the RJK0305DPB max al-
lowable power dissipation criteria is met.
Low-Side MOSFET
Primary loss is conduction loss given by:
Pdl = Iout2 x R
DS(ON) x (1-D) = 122 x 0.01 x (1-0.275) = 1W
Pdl is also less than the Pdmax specified on the RJK0305DPB
MOSFET datasheet.
However, it is not always necessary to use the same MOS-
FET for both the high-side and low-side. For most applications
it is necessary to choose the high-side MOSFET with the low-
est gate charge and the low-side MOSFET is chosen for the
lowest allowed R
DS(ON). The plateau voltage of the FET VGS
vs Q
g curve must be less than VCC - 750 mV.
The current limit, I
OCL, is calculated by estimating the RDS
(ON) of the low-side FET at the maximum junction temperature
of 100°C. Then the following calculation of I
OCL is:
I
OCL = ICL + ΔIL / 2
I
CL = 200 mV / 0.014 = 14.2A
I
OCL = 14.2A + 3.6 / 2 = 16A
6. Calculate Input Capacitance
The input capacitor should be chosen so that the voltage rat-
ing is greater than the maximum input voltage which for this
example is 24V. Similar to the output capacitor, the voltage
rating needed will depend on the type of capacitor chosen.
The input capacitor should also be able to handle the input
rms current which is approximately 0.5 x I
OUT. For this exam-
ple the rms input current is approximately 0.5 x 12A = 6A.
The minimum capacitance with a maximum 5% input ripple
ΔV
IN-MAX = (0.05 x 12) = 0.6V:
C
IN = [12 x 0.275 x (1-0.275)] / [500 kHz x 0.6] = 8 F
To handle the large input rms current 2 ceramic capacitors
are chosen at 10 F each with a voltage rating of 50V and
case size of 1210, that can handle 3A of rms current each. A
100 F aluminum electrolytic is chosen to help dampen input
ringing.
C
BYP = 0.1 F ceramic with a voltage rating greater than max-
imum V
IN
7. Calculate Soft-Start Capacitor
The soft start-time should be greater than the input voltage
rise time and also satisfy the following equality to maintain a
smooth transition of the output voltage to the programmed
regulation voltage during startup.
t
SS ≥ (VOUT x COUT) / (IOCL - IOUT)
5 ms > (3.3V x 300 F) / (1.2 x 12A - 12A)
5 ms > 0.412 ms
The desired soft-start time, t
SS, of 5 ms satisfies the equality
as shown above. Therefore, the soft-start capacitor, C
SS, is
calculated as:
C
SS = (7.7 A x 5 ms) / 0.6V = 0.064 F
15
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LM3151/LM3152/LM3153
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM3151MHE-3.3 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Texas Instruments 功能描述:SIMPLE SWITCHER 6~42V 14TSSOP 制造商:Texas Instruments 功能描述:SIMPLE SWITCHER, 6~42V, 14TSSOP 制造商:Texas Instruments 功能描述:SIMPLE SWITCHER, 6~42V, 14TSSOP, Primary Input Voltage:18V, No. of Outputs:1, Output Voltage:3.3V, Output Current:12A, Voltage Regulator Case Style:TSSOP, No. of Pins:14, Operating Temperature Min:-40C, Operating Temperature , RoHS Compliant: Yes |
| LM3151MHE-3.3/NOPB | 功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
| LM3151MHX-3.3 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:SIMPLE SWITCHER? CONTROLLER, High Input Voltage Synchronous Step-Down |
| LM3151MHX-3.3/NOPB | 功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
| LM3152 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:SIMPLE SWITCHER? CONTROLLER, High Input Voltage Synchronous Step-Down |
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