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LM20124 Schematic ( PDF Datasheet ) - National Semiconductor

Teilenummer LM20124
Beschreibung Synchronous Buck Regulator
Hersteller National Semiconductor
Logo National Semiconductor Logo 




Gesamt 20 Seiten
LM20124 Datasheet, Funktion
www.DataSheet4U.com
January 16, 2008
LM20124
4A, 1MHz PowerWise® Synchronous Buck Regulator
General Description
The LM20124 is a full featured 1 MHz synchronous buck reg-
ulator capable of delivering up to 4A of continuous output
current. The current mode control loop can be compensated
to be stable with virtually any type of output capacitor. For
most cases, compensating the device only requires two ex-
ternal components, providing maximum flexibility and ease of
use. The device is optimized to work over the input voltage
range of 2.95V to 5.5V making it suited for a wide variety of
low 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. Start-up inrush currents are limited by both an
internally fixed and externally adjustable Soft-Start circuit.
Fault detection and supply sequencing is possible with the
integrated power good circuit.
The LM20124 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 LM20124 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 LM20124 is offered in a 16-pin eTSSOP package with an
exposed pad that can be soldered to the PCB, eliminating the
need for bulky heatsinks.
Features
Input voltage range 2.95V to 5.5V
Accurate current limit minimizes inductor size
96% efficiency at 1 MHz switching frequency
32 mintegrated FET switches
Starts up into pre-biased loads
Output voltage tracking
Peak current mode control
Adjustable output voltage down to 0.8V
Adjustable Soft-Start with external capacitor
Precision enable pin with hysteresis
Integrated OVP, UVLO, power good and thermal
shutdown
eTSSOP-16 exposed pad package
Applications
Simple to design, high efficiency point of load regulation
from a 5V or 3.3V bus
High Performance DSPs, FPGAs, ASICs and
microprocessors
Broadband, Networking and Optical Communications
Infrastructure
Typical Application Circuit
PowerWise® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 300142
30014201
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LM20124 Datasheet, Funktion
www.DataSheet4U.com Shutdown Current vs. Temperature
Enable Threshold vs. Temperature
30014241
UVLO Threshold vs. Temperature
30014228
Peak Current Limit vs. Temperature
30014245
Peak Current Limit vs. VOUT
Peak Current Limit vs. VIN
30014242
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LM20124 pdf, datenblatt
www.DataShDeeet4sU.icgomn 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.
30014223
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:
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 less than 30% of the rated
output current. Figure 3, shown below illustrates the switch
and inductor ripple current waveforms. Once the input volt-
age, output voltage, operating frequency, and desired ripple
current are known, the minimum value for the inductor can be
calculated by the formula shown below:
30014209
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 greater 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
LM20124 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 per-
formance characteristics to consider are the output voltage
ripple and transient response. The output voltage ripple can
be approximated by using the formula shown below.
Where, ΔVOUT (V) is the amount of peak to peak voltage ripple
at the power supply output, RESR (Ω) is the series resistance
of the output capacitor, fSW(Hz) is the switching frequency,
and COUT (F) is the output capacitance used in the design.
The amount of output ripple that can be tolerated is applica-
tion specific; however a general recommendation is to keep
the output ripple less than 1% of the rated output voltage.
Keep in mind ceramic capacitors are sometimes preferred
because they have very low ESR; however, depending on
package and voltage rating of the capacitor the value of the
capacitance can drop significantly with applied voltage. The
output capacitor selection will also affect the output voltage
droop during a load transient. The peak droop on the output
voltage during a load transient is dependent on many factors;
however, an approximation of the transient droop ignoring
loop bandwidth can be obtained using the following equation.
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