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参数资料
| 型号： | LMZ14201EXTTZE |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, 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： | 3945K |
| 代理商： | LMZ14201EXTTZE |
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 results in 2.2 msec soft-start interval 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 (Approximately 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≥ 1A*0.8V*10μH*24V / (4*3.3V*( 24V — 3.3V)*33mV)
≥ 21.3μF (7)
The LMZ14201EXT demonstration and evaluation boards are
populated with a 100 uF 6.3V X5R output capacitor. Locations
for other output capacitors are provided.
C
IN SELECTION
The LMZ14201EXT 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≥ 1A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V)
≥ 0.9μF
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 LMZ14201EXT 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.
11
www.national.com
LMZ14201EXT
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LMZ14201EXTTZE/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 |
| LMZ14201EXTTZX | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1A SIMPLE SWITCHER? Power Module with 42V Maximum Input Voltage for Military and Rugged Applications |
| LMZ14201EXTTZX/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 |
| LMZ14201H | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1A SIMPLE SWITCHER? Power Module for High Output Voltage |
| LMZ14201H_1106 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1A SIMPLE SWITCHER?? Power Module for High Output Voltage |
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