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PDF LM20242 Data sheet ( Hoja de datos )
| Número de pieza | LM20242 | |
| Descripción | 2A PowerWise Adjustable Frequency Synchronous Buck Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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www.DataSheet4U.com
November 1, 2007
LM20242
36V, 2A PowerWise® Adjustable Frequency Synchronous
Buck Regulator
General Description
The LM20242 is a full featured 1MHz capable synchronous
buck regulator capable of delivering up to 2A of load current.
The current mode control loop is externally compensated with
only two external components, offering both high perfor-
mance and ease of use. The device is optimized to work over
the input voltage range of 4.5V to 36V making it well suited
for high voltage systems.
The device features internal Over Voltage Protection (OVP)
and Over Current Protection (OCP) circuits for increased sys-
tem reliability. A precision Enable pin and integrated UVLO
allows the turn on of the device to be tightly controlled and
sequenced. Startup inrush currents are limited by both an in-
ternally fixed and externally adjustable soft-start circuit. Fault
detection and supply sequencing are possible with the inte-
grated PGOOD circuit.
The LM20242 is designed to work well in multi-rail power
supply architectures. The output voltage of the device can be
configured to track a higher voltage rail using the SS/TRK pin.
If the output of the LM20242 is pre-biased at startup it will not
sink current to pull the output low until the internal soft-start
ramp exceeds the voltage at the feedback pin.
The LM20242 is offered in an exposed pad 20 pin TSSOP
package that can be soldered to the PCB, eliminating the
need for bulky heatsinks.
Features
■ 2A Output Current, 3.7A peak current
■ 130 mΩ/110 mΩ integrated power MOSFETs
■ 1.5% output voltage accuracy
■ Current Mode Control, selectable compensation
■ Resistor programmed, 1MHz capable oscillator
■ Synchronous rectifier with diode emulation
■ Adjustable output voltage down to 0.8V
■ Compatible with pre-biased loads
■ Programmable soft-start with external capacitor
■ Precision enable pin with hysteresis
■ OVP, UVLO inputs and PGOOD output
■ Internally protected with peak current limit, thermal
shutdown and restart
■ Accurate current limit with frequency foldback
■ Non-linear current mode slope compensation
■ eTSSOP-20 exposed pad package
Applications
■ Simple to design, high efficiency point of load regulation
from a 4.5V to 36V bus
■ High Performance DSPs, FPGAs, ASICs and
Microprocessors
■ Communications Infrastructure, Automotive
Simplified Application Circuit
PowerWise® is a registered trademark of National Semiconductor Corporation.
© 2007 National Semiconductor Corporation 300314
30031401
www.national.com
1 page  
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Error Amplifier Gain
Error Amplifier Phase
Line Regulation
30031405
VCC vs. VIN
30031406
Non-Switching IQ vs. VIN
30031407
Shutdown IQ vs. VIN
30031408
30031409
5
30031410
www.national.com
5 Page 
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Design Guide
This section walks the designer through the steps necessary
to select the external components to build a fully functional
power supply. As with any DC-DC converter numerous trade-
offs are possible to optimize the design for efficiency, size, or
performance. These will be taken into account and highlight-
ed throughout this discussion. To facilitate component selec-
tion discussions the circuit shown in Figure 2 below may be
used as a reference. Unless otherwise indicated all formulas
assume units of amps (A) for current, farads (F) for capaci-
tance, henries (H) for inductance and volts (V) for voltages.
FIGURE 2. Typical Application Circuit
The first equation to calculate for any buck converter is duty-
cycle. Ignoring conduction losses associated with the FETs
and parasitic resistances it can be approximated by:
30031429
INDUCTOR SELECTION (L)
The inductor value is determined based on the operating fre-
quency, load current, ripple current and duty cycle.
The inductor selected should have a saturation current rating
greater than the peak current limit of the device. Keep in mind
the specified current limit does not account for delay of the
current limit comparator, therefore the current limit in the ap-
plication may be higher than the specified value. To optimize
the performance and prevent the device from entering current
limit at maximum load, the inductance is typically selected
such that the ripple current, ΔiL, is not greater than 30% of the
rated output current. Figure 3 illustrates the switch and in-
ductor ripple current waveforms. Once the input voltage, out-
put voltage, operating frequency and desired ripple current
are known, the minimum value for the inductor can be calcu-
lated by the formula shown below:
30031467
FIGURE 3. Switch and Inductor Current Waveforms
If needed, slightly smaller value inductors can be used, how-
ever, the peak inductor current, IOUT + ΔiL/2, should be kept
below the peak current limit of the device. In general, the in-
ductor ripple current, ΔiL, should be more than 10% of the
rated output current to provide adequate current sense infor-
mation for the current mode control loop. If the ripple current
in the inductor is too low, the control loop will not have suffi-
cient current sense information and can be prone to instability.
OUTPUT CAPACITOR SELECTION (COUT)
The output capacitor, COUT, filters the inductor ripple current
and provides a source of charge for transient load conditions.
A wide range of output capacitors may be used with the
LM20242 that provide excellent performance. The best per-
formance is typically obtained using ceramic, SP or OSCON
type chemistries. Typical trade-offs are that the ceramic ca-
pacitor provides extremely low ESR to reduce the output
ripple voltage and noise spikes, while the SP and OSCON
capacitors provide a large bulk capacitance in a small volume
for transient loading conditions.
When selecting the value for the output capacitor, the two
performance characteristics to consider are the output volt-
age ripple and transient response. The output voltage ripple
can be approximated by using the following formula.
11 www.national.com
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| PDF Descargar | [ Datasheet LM20242.PDF ] | |
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