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| 型号： | LMZ12003EXTTZE-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： | 16/16页 |
| 文件大小： | 3896K |
| 代理商： | LMZ12003EXTTZE-ADJ |
General Description
The LMZ12003EXT SIMPLE SWITCHER power module is an
easy-to-use step-down DC-DC solution capable of driving up
to 3A load with exceptional power conversion efficiency, line
and
load
regulation,
and
output
accuracy.
The
LMZ12003EXT is available in an innovative package that en-
hances thermal performance and allows for hand or machine
soldering.
The LMZ12003EXT can accept an input voltage rail between
4.5V and 20V and deliver an adjustable and highly accurate
output voltage as low as 0.8V. The LMZ12003EXT only re-
quires three external resistors and four external capacitors to
complete the power solution. The LMZ12003EXT is a reliable
and robust design with the following protection features: ther-
mal shutdown, input under-voltage lockout, output over-volt-
age protection, short-circuit protection, output current limit,
and allows startup into a pre-biased output. A single resistor
adjusts the switching frequency up to 1 MHz.
COT Control Circuit Overview
Constant On Time control is based on a comparator and an
on-time one shot, with the output voltage feedback compared
with an internal 0.8V reference. If the feedback voltage is be-
low the reference, the main MOSFET is turned on for a fixed
on-time determined by a programming resistor R
ON. RON is
connected to V
IN such that on-time is reduced with increasing
input supply voltage. Following this on-time, the main MOS-
FET remains off for a minimum of 260 ns. If the voltage on the
feedback pin falls below the reference level again the on-time
cycle is repeated. Regulation is achieved in this manner.
Design Steps for the LMZ12003EXT
Application
The LMZ12003EXT is fully supported by Webench and of-
fers the following: Component selection, electrical and ther-
mal simulations as well as the build-it board for a reduction in
design time. The following list of steps can be used to manu-
ally design the LMZ12003EXT application.
Select minimum operating V
IN with enable divider resistors
Program V
O with divider resistor selection
Program turn-on time with soft-start capacitor selection
Select C
O
Select C
IN
Set operating frequency with R
ON
Determine module dissipation
Layout PCB for required thermal performance
ENABLE DIVIDER, R
ENT AND RENB SELECTION
The enable input provides a precise 1.18V band-gap rising
threshold to allow direct logic drive or connection to a voltage
divider from a higher enable voltage such as Vin. The enable
input also incorporates 90 mV (typ) of hysteresis resulting in
a falling threshold of 1.09V. The maximum recommended
voltage into the EN pin is 6.5V. For applications where the
midpoint of the enable divider exceeds 6.5V, a small zener
can be added to limit this voltage.
The function of this resistive divider is to allow the designer to
choose an input voltage below which the circuit will be dis-
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 where a lower boundary of operation should be estab-
lished. In the case of sequencing supplies, the divider is
connected to a rail that becomes active earlier in the power-
up cycle than the LMZ12003EXT output rail. The two resistors
should be chosen based on the following ratio:
R
ENT / RENB = (VIN UVLO / 1.18V) – 1 (1)
The LMZ12003EXT demonstration and evaluation boards
use 11.8k
for R
ENB and 32.4k for RENT resulting in a rising
UVLO of 4.5V. This divider presents 5.34V to the EN input
when the divider input is raised to 20V.
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 RFBT 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 2.2 msec soft-start du-
ration. This 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.
9
www.national.com
LMZ12003EXT
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LMZ12003EXTTZX | 制造商:TI 制造商全称:Texas Instruments 功能描述:3A SIMPLE SWITCHER?? Power Module with 20V Maximum Input Voltage for Military and Rugged Applications |
| LMZ12003EXTTZX/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 |
| LMZ12003TZ-ADJ | 制造商:NSC 制造商全称:National Semiconductor 功能描述:3A SIMPLE SWITCHER? Power Module with 20V Maximum |
| LMZ12003TZ-ADJ/NOPB | 功能描述:直流/直流开关转换器 20V,3A PWR MODULE 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 |
| LMZ12003TZ-ADJ-NOPB | 制造商:TI 制造商全称:Texas Instruments 功能描述:3A SIMPLE SWITCHER?? Power Module with 20V Maximum Input Voltage |
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