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PDF LMZ12002 Data sheet ( Hoja de datos )
| Número de pieza | LMZ12002 | |
| Descripción | 2A SIMPLE SWITCHER Power Module | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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DataSheet.in
LMZ12002
April 12, 2010
2A SIMPLE SWITCHER® Power Module with 20V Maximum
Input Voltage
Easy to use 7 pin package
Performance Benefits
■ Operates at high ambient temperature with no thermal
derating
■ High efficiency reduces system heat generation
■ Low radiated emissions (EMI) complies with EN55022
class B standard
■ Low external component count
TO-PMOD 7 Pin Package
30114786
10.16 x 13.77 x 4.57 mm (0.4 x 0.542 x 0.18 in)
θJA = 20°C/W, θJC = 1.9°C/W
RoHS Compliant
System Performance
Efficiency VIN = 12V VOUT = 5.0V
Electrical Specifications
■ 12W maximum total power output
■ Up to 2A output current
■ Input voltage range 4.5V to 20V
■ Output voltage range 0.8V to 6V
■ Efficiency up to 92%
Key Features
■ Integrated shielded inductor
■ Simple PCB layout
■ Flexible startup sequencing using external soft-start
capacitor and precision enable
■ Protection against inrush currents and faults such as input
UVLO and output short circuit
■ -40°C to 125°C junction temperature range
■ Single exposed pad and standard pinout for easy
mounting and manufacturing
■ Fast transient response for FPGAs and ASICs
■ Low output voltage ripple
■ Pin-to-pin compatible family:
LMZ14203/2/1 (42V max 3A, 2A, 1A)
LMZ12003/2/1 (20V max 3A, 2A, 1A)
■ Fully Webench® Power Designer enabled
Applications
■ Point of load conversions from 5V and 12V input rail
■ Time critical projects
■ Space constrained high thermal requirement applications
■ Negative output voltage applications (See AN-2027)
30114718
Thermal derating curve
VIN = 12V VOUT = 5.0V
30114719
Radiated Emissions (EN 55022 Class B)
from Evaluation Board
© 2010 National Semiconductor Corporation 301147
30114750
www.national.com
1 page  
DataSheet.in
Symbol Parameter
η Efficiency
η Efficiency
Conditions
VIN = 12V VO = 1.8V IO = 1A
VIN = 12V VO = 1.8V IO = 2A
Min
(Note 3)
Typ
(Note 4)
87
77
Max
(Note 3)
Units
%
%
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD-22-114.
Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
Note 4: Typical numbers are at 25°C and represent the most likely parametric norm.
Note 5: EN 55022:2006, +A1:2007, FCC Part 15 Subpart B: 2007. See AN-2024 and layout for information on device under test.
Note 6: Theta JA measured on a 1.705” x 3.0” four layer board, with one ounce copper, thirty five 12 mil thermal vias, no air flow, and 1W power dissipation.
Refer to PCB layout diagrams
Typical Performance Characteristics
Unless otherwise specified, the following conditions apply: VIN = 12V; Cin = 10uF X7R Ceramic; CO = 100uF X7R Ceramic; Tam-
bient = 25 C for efficiency curves and waveforms.
Efficiency 4.5V Input 25°C
Dissipation 4.5V Input @ 25°C
Efficiency 5V Input 25°C
30114751
30114752
Dissipation 5V Input @ 25°C
30114753
5
30114754
www.national.com
5 Page 
DataSheet.in
General Description
The LMZ12002 SIMPLE SWITCHER® power module is an
easy-to-use step-down DC-DC solution capable of driving up
to 2A load with exceptional power conversion efficiency, line
and load regulation, and output accuracy. The LMZ12002 is
available in an innovative package that enhances thermal
performance and allows for hand or machine soldering.
The LMZ12002 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 LMZ12002 only requires three
external resistors and four external capacitors to complete the
power solution. The LMZ12002 is a reliable and robust design
with the following protection features: thermal shutdown, in-
put under-voltage lockout, output over-voltage protection,
short-circuit protection, output current limit, and allows startup
into a pre-biased output. A single resistor adjusts the switch-
ing 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 RON. RON is
connected to VIN 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 LMZ12002
Application
The LMZ12002 is fully supported by Webench® and offers
the following: Component selection, electrical and thermal
simulations as well as the build-it board for a reduction in de-
sign time. The following list of steps can be used to manually
design the LMZ12002 application.
• Select minimum operating VIN with enable divider resistors
• Program VO with divider resistor selection
• Program turn-on time with soft-start capacitor selection
• Select CO
• Select CIN
• Set operating frequency with RON
• Determine module dissipation
• Layout PCB for required thermal performance
ENABLE DIVIDER, RENT 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 LMZ12002 output rail. The two resistors
should be chosen based on the following ratio:
RENT / RENB = (VIN UVLO / 1.18V) – 1 (1)
The LMZ12002 demonstration and evaluation boards use
11.8kΩ for RENB and 32.4kΩ for RENT resulting in a rising UV-
LO 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 VO 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:
VO = 0.8V * (1 + RFBT / RFBB) (2)
Rearranging terms; the ratio of the feedback resistors for a
desired output voltage is:
RFBT / 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 VO = 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 RFBT 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 RFBT , 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:
tSS = VREF * CSS / Iss = 0.8V * CSS / 8uA (4)
This equation can be rearranged as follows:
CSS = tSS * 8 μA / 0.8V (5)
Use of a 0.022μF 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.
Voltage levels between 0.8V and 3.8V have no effect on other
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| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LMZ12002.PDF ] | |
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