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参数资料
| 型号： | LMZ14202EXTTZX-ADJ |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, PSSO7 |
| 封装： | 10.16 X 13.77 MM, 4.57 MM HEIGHT, ROHS COMPLIANT, TO-PMOD-7 |
| 文件页数： | 4/18页 |
| 文件大小： | 3941K |
| 代理商： | LMZ14202EXTTZX-ADJ |
erate with a minimum value. Ceramic capacitors or other low
ESR types are recommended. See AN-2024 for more detail.
The following equation provides a good first pass approxima-
tion of C
O for load transient requirements:
C
O≥ISTEP*VFB*L*VIN/ (4*VO*(VIN—VO)*VOUT-TRAN)(6)
Solving:
C
O≥ 2A*0.8V*10μH*24V / (4*3.3V*( 24V — 3.3V)*33mV)
≥ 43μF (7)
The LMZ14202EXT demonstration and evaluation boards are
populated with a 100 uF 6.3V X5R output capacitor. Locations
for extra output capacitors are provided. See AN-2024 for lo-
cations.
C
IN SELECTION
The LMZ14202EXT module contains an internal 0.47 F input
ceramic capacitor. Additional input capacitance is required
external to the module to handle the input ripple current of the
application. This input capacitance should be located in very
close proximity to the module. Input capacitor selection is
generally directed to satisfy the input ripple current require-
ments rather than by capacitance value. Worst case input
ripple current rating is dictated by the equation:
I(C
IN(RMS)) 1 /2 * IO * √ (D / 1-D) (8)
where D
V
O / VIN
(As a point of reference, the worst case ripple current will oc-
cur when the module is presented with full load current and
when V
IN = 2 * VO).
Recommended minimum input capacitance is 10uF X7R ce-
ramic with a voltage rating at least 25% higher than the
maximum applied input voltage for the application. It is also
recommended that attention be paid to the voltage and tem-
perature deratings of the capacitor selected. It should be
noted that ripple current rating of ceramic capacitors may be
missing from the capacitor data sheet and you may have to
contact the capacitor manufacturer for this rating.
If the system design requires a certain minimum value of input
ripple voltage
ΔV
IN be maintained then the following equation
may be used.
C
IN ≥ IO * D * (1–D) / fSW-CCM * ΔVIN(9)
If
ΔV
IN is 1% of VIN for a 24V input to 3.3V output application
this equals 240 mV and f
SW = 400 kHz.
C
IN≥ 2A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V)
≥ 2.5μF
Additional bulk capacitance with higher ESR may be required
to damp any resonant effects of the input capacitance and
parasitic inductance of the incoming supply lines.
R
ON RESISTOR SELECTION
Many designs will begin with a desired switching frequency in
mind. For that purpose the following equation can be used.
f
SW(CCM) VO / (1.3 * 10
-10
* R
ON) (10)
This can be rearranged as
R
ON VO / (1.3 * 10
-10
* f
SW(CCM) )(11)
The selection of RON and f
SW(CCM) must be confined by limi-
tations in the on-time and off-time for the COT control section.
The on-time of the LMZ14202EXT timer is determined by the
resistor R
ON and the input voltage VIN. It is calculated as fol-
lows:
t
ON = (1.3 * 10
-10
* R
ON) / VIN (12)
The inverse relationship of t
ON and VIN gives a nearly constant
switching frequency as V
IN is varied. RON should be selected
such that the on-time at maximum V
IN is greater than 150 ns.
The on-timer has a limiter to ensure a minimum of 150 ns for
t
ON. This limits the maximum operating frequency, which is
governed by the following equation:
f
SW(MAX) = VO / (VIN(MAX) * 150 nsec) (13)
This equation can be used to select R
ON if a certain operating
frequency is desired so long as the minimum on-time of 150
ns is observed. The limit for R
ON can be calculated as follows:
R
ON ≥ VIN(MAX) * 150 nsec / (1.3 * 10
-10
) (14)
If R
ON calculated in (11) is less than the minimum value de-
termined in (14) a lower frequency should be selected. Alter-
natively, V
IN(MAX) can also be limited in order to keep the
frequency unchanged.
Additionally note, the minimum off-time of 260 ns limits the
maximum duty ratio. Larger R
ON (lower FSW) should be se-
lected in any application requiring large duty ratio.
Discontinuous Conduction and Continuous Conduction
Modes
At light load the regulator will operate in discontinuous con-
duction mode (DCM). With load currents above the critical
conduction point, it will operate in continuous conduction
mode (CCM). When operating in DCM the switching cycle
begins at zero amps inductor current; increases up to a peak
value, and then recedes back to zero before the end of the
off-time. Note that during the period of time that inductor cur-
rent is zero, all load current is supplied by the output capacitor.
The next on-time period starts when the voltage on the at the
FB pin falls below the internal reference. The switching fre-
quency is lower in DCM and varies more with load current as
compared to CCM. Conversion efficiency in DCM is main-
tained since conduction and switching losses are reduced
with the smaller load and lower switching frequency. Operat-
ing frequency in DCM can be calculated as follows:
f
SW(DCM)VO*(VIN-1)*10μH*1.18*10
20
*I
O/(VIN–VO)*RON
2 (15)
In CCM, current flows through the inductor through the entire
switching cycle and never falls to zero during the off-time. The
switching frequency remains relatively constant with load cur-
rent and line voltage variations. The CCM operating frequen-
cy can be calculated using equation 7 above.
Following is a comparison pair of waveforms of the showing
both CCM (upper) and DCM operating modes.
CCM and DCM Operating Modes
V
IN = 24V, VO = 3.3V, IO = 2A/0.32A 2 μsec/div
30117712
The approximate formula for determining the DCM/CCM
boundary is as follows:
www.national.com
12
LMZ14202EXT
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LMZ14202H | 制造商:NSC 制造商全称:National Semiconductor 功能描述:2A SIMPLE SWITCHER Power Module for High Output Voltage |
| LMZ14202HEVAL/NOPB | 功能描述:电源管理IC开发工具 LMZ14202H EVAL BOARD 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 |
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| LMZ14202HTZ/NOPB | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LMZ14202HTZE | 制造商:NSC 制造商全称:National Semiconductor 功能描述:2A SIMPLE SWITCHER Power Module for High Output Voltage |
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