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参数资料
| 型号： | LMZ14202TZE-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 PIN |
| 文件页数： | 3/18页 |
| 文件大小： | 1157K |
| 代理商： | LMZ14202TZE-ADJ |
abled. This implements the feature of programmable under
voltage lockout. This is often used in battery powered systems
to prevent deep discharge of the system battery. It is also
useful in system designs for sequencing of output rails or to
prevent early turn-on of the supply as the main input voltage
rail rises at power-up. Applying the enable divider to the main
input rail is often done in the case of higher input voltage sys-
tems such as 24V AC/DC systems where a lower boundary
of operation should be established. In the case of sequencing
supplies, the divider is connected to a rail that becomes active
earlier in the power-up cycle than the LMZ14202 output rail.
The two resistors should be chosen based on the following
ratio:
R
ENT / RENB = (VIN UVLO/ 1.18V) – 1 (1)
The LMZ14202 demonstration and evaluation boards use
11.8k
for R
ENB and 68.1k for RENT resulting in a rising UV-
LO of 8V. This divider presents 6.25V to the EN input when
the divider input is raised to 42V.
The EN pin is internally pulled up to VIN and can be left float-
ing for always-on operation.
OUTPUT VOLTAGE SELECTION
Output voltage is determined by a divider of two resistors
connected between V
O and ground. The midpoint of the di-
vider is connected to the FB input. The voltage at FB is
compared to a 0.8V internal reference. In normal operation
an on-time cycle is initiated when the voltage on the FB pin
falls below 0.8V. The main MOSFET on-time cycle causes the
output voltage to rise and the voltage at the FB to exceed
0.8V. As long as the voltage at FB is above 0.8V, on-time
cycles will not occur.
The regulated output voltage determined by the external di-
vider resistors R
FBT and RFBB is:
V
O = 0.8V * (1 + RFBT / RFBB) (2)
Rearranging terms; the ratio of the feedback resistors for a
desired output voltage is:
R
FBT / RFBB = (VO / 0.8V) - 1 (3)
These resistors should be chosen from values in the range of
1.0 kohm to 10.0 kohm.
For V
O = 0.8V the FB pin can be connected to the output di-
rectly so long as an output preload resistor remains that draws
more than 20uA. Converter operation requires this minimum
load to create a small inductor ripple current and maintain
proper regulation when no load is present.
A feed-forward capacitor is placed in parallel with R
FBT to im-
prove load step transient response. Its value is usually deter-
mined experimentally by load stepping between DCM and
CCM conduction modes and adjusting for best transient re-
sponse and minimum output ripple.
A table of values for R
FBT , RFBB , CFF and RON is included in
the applications schematic.
SOFT-START CAPACITOR SELECTION
Programmable soft-start permits the regulator to slowly ramp
to its steady state operating point after being enabled, thereby
reducing current inrush from the input supply and slowing the
output voltage rise-time to prevent overshoot.
Upon turn-on, after all UVLO conditions have been passed,
an internal 8uA current source begins charging the external
soft-start capacitor. The soft-start time duration to reach
steady state operation is given by the formula:
t
SS = VREF * CSS / Iss = 0.8V * CSS / 8uA (4)
This equation can be rearranged as follows:
C
SS = tSS * 8 μA / 0.8V (5)
Use of a 0.022
μF capacitor results in 2.2msec soft-start du-
ration which is recommended as a minimum value.
As the soft-start input exceeds 0.8V the output of the power
stage will be in regulation. The soft-start capacitor continues
charging until it reaches approximately 3.8V on the SS pin.
Voltage levels between 0.8V and 3.8V have no effect on other
circuit operation. Note that the following conditions will reset
the soft-start capacitor by discharging the SS input to ground
with an internal 200
μA current sink.
The enable input being “pulled low”
Thermal shutdown condition
Over-current fault
Internal Vcc UVLO (Approx 4V input to V
IN)
C
O SELECTION
None of the required C
O output capacitance is contained with-
in the module. At a minimum, the output capacitor must meet
the worst case minimum ripple current rating of 0.5 * I
LR P-P,
as calculated in equation (19) below. Beyond that, additional
capacitance will reduce output ripple so long as the ESR is
low enough to permit it. A minimum value of 10
μF is generally
required. Experimentation will be required if attempting to op-
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 LMZ14202 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 LMZ14202 module contains an internal 0.47 F input ce-
ramic capacitor. Additional input capacitance is required ex-
ternal 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)
11
www.national.com
LMZ14202
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LMZ14202TZE-ADJ/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 |
| LMZ14202TZX-ADJ | 制造商:NSC 制造商全称:National Semiconductor 功能描述:2A SIMPLE SWITCHER? Power Module with 42V Maximum Input Voltage |
| LMZ14202TZX-ADJ/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 |
| LMZ14202TZXADJNOPB | 制造商:Texas Instruments 功能描述:Module DC-DC 1-OUT 0.8V to 6V 2A 12W 7-Pin TO-PMOD EP |
| LMZ14203 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:3A SIMPLE SWITCHER? Power Module with 42V Maximum |
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