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Número de pieza 10MV5600AX
Descripción N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages
Fabricantes National Semiconductor 
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June 2003
LM2727/LM2737
N-Channel FET Synchronous Buck Regulator Controller
for Low Output Voltages
General Description
The LM2727 and LM2737 are high-speed, synchronous,
switching regulator controllers. They are intended to control
currents of 0.7A to 20A with up to 95% conversion efficien-
cies. The LM2727 employs output over-voltage and under-
voltage latch-off. For applications where latch-off is not de-
sired, the LM2737 can be used. Power up and down
sequencing is achieved with the power-good flag, adjustable
soft-start and output enable features. The LM2737 and
LM2737 operate from a low-current 5V bias and can convert
from a 2.2V to 16V power rail. Both parts utilize a fixed-
frequency, voltage-mode, PWM control architecture and the
switching frequency is adjustable from 50kHz to 2MHz by
adjusting the value of an external resistor. Current limit is
achieved by monitoring the voltage drop across the on-
resistance of the low-side MOSFET, which enhances low
duty-cycle operation. The wide range of operating frequen-
cies gives the power supply designer the flexibility to fine-
tune component size, cost, noise and efficiency. The adap-
tive, non-overlapping MOSFET gate-drivers and high-side
bootstrap structure helps to further maximize efficiency. The
high-side power FET drain voltage can be from 2.2V to 16V
and the output voltage is adjustable down to 0.6V.
Features
n Input power from 2.2V to 16V
n Output voltage adjustable down to 0.6V
n Power Good flag, adjustable soft-start and output enable
for easy power sequencing
n Output over-voltage and under-voltage latch-off
(LM2727)
n Output over-voltage and under-voltage flag (LM2737)
n Reference Accuracy: 1.5% (0˚C - 125˚C)
n Current limit without sense resistor
n Soft start
n Switching frequency from 50 kHz to 2 MHz
n TSSOP-14 package
Applications
n Cable Modems
n Set-Top Boxes/ Home Gateways
n DDR Core Power
n High-Efficiency Distributed Power
n Local Regulation of Core Power
Typical Application
© 2003 National Semiconductor Corporation DS200494
20049410
www.national.com

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10MV5600AX pdf
Typical Performance Characteristics
Efficiency (VO = 1.5V)
FSW = 300kHz, TA = 25˚C
Efficiency (VO = 3.3V)
FSW = 300kHz, TA = 25˚C
20049412
VCC Operating Current vs Temperature
FSW = 600kHz, No-Load
20049413
Bootpin Current vs Temperature for BOOTV = 12V
FSW = 600kHz, Si4826DY FET, No-Load
20049414
Bootpin Current vs Temperature with 5V Bootstrap
FSW = 600kHz, Si4826DY FET, No-Load
20049415
PWM Frequency vs Temperature
for RFADJ = 43.2k
20049416
5
20049417
www.national.com

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10MV5600AX arduino
Application Information (Continued)
DESIGN CONSIDERATIONS
The following is a design procedure for all the components
needed to create the circuit shown in Figure 3 in the Ex-
ample Circuits section, a 5V in to 1.2V out converter, capable
of delivering 10A with an efficiency of 85%. The switching
frequency is 300kHz. The same procedures can be followed
to create the circuit shown in Figure 3, Figure 4, and to
create many other designs with varying input voltages, out-
put voltages, and output currents.
INPUT CAPACITOR
The input capacitors in a Buck switching converter are sub-
jected to high stress due to the input current waveform,
which is a square wave. Hence input caps are selected for
their ripple current capability and their ability to withstand the
heat generated as that ripple current runs through their ESR.
Input rms ripple current is approximately:
The power dissipated by each input capacitor is:
Here, n is the number of capacitors, and indicates that power
loss in each cap decreases rapidly as the number of input
caps increase. The worst-case ripple for a Buck converter
occurs during full load, when the duty cycle D = 50%.
In the 5V to 1.2V case, D = 1.2/5 = 0.24. With a 10A
maximum load the ripple current is 4.3A. The Sanyo
10MV5600AX aluminum electrolytic capacitor has a ripple
current rating of 2.35A, up to 105˚C. Two such capacitors
make a conservative design that allows for unequal current
sharing between individual caps. Each capacitor has a maxi-
mum ESR of 18mat 100 kHz. Power loss in each device is
then 0.05W, and total loss is 0.1W. Other possibilities for
input and output capacitors include MLCC, tantalum,
OSCON, SP, and POSCAPS.
INPUT INDUCTOR
The input inductor serves two basic purposes. First, in high
power applications, the input inductor helps insulate the
input power supply from switching noise. This is especially
important if other switching converters draw current from the
same supply. Noise at high frequency, such as that devel-
oped by the LM2727 at 1MHz operation, could pass through
the input stage of a slower converter, contaminating and
possibly interfering with its operation.
An input inductor also helps shield the LM2727 from high
frequency noise generated by other switching converters.
The second purpose of the input inductor is to limit the input
current slew rate. During a change from no-load to full-load,
the input inductor sees the highest voltage change across it,
equal to the full load current times the input capacitor ESR.
This value divided by the maximum allowable input current
slew rate gives the minimum input inductance:
In the case of a desktop computer system, the input current
slew rate is the system power supply or "silver box" output
current slew rate, which is typically about 0.1A/µs. Total input
capacitor ESR is 9m, hence V is 10*0.009 = 90 mV, and
the minimum inductance required is 0.9µH. The input induc-
tor should be rated to handle the DC input current, which is
approximated by:
In this case IIN-DC is about 2.8A. One possible choice is the
TDK SLF12575T-1R2N8R2, a 1.2µH device that can handle
8.2Arms, and has a DCR of 7m.
OUTPUT INDUCTOR
The output inductor forms the first half of the power stage in
a Buck converter. It is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple. (Io) The inductance is chosen by
selecting between tradeoffs in efficiency and response time.
The smaller the output inductor, the more quickly the con-
verter can respond to transients in the load current. As
shown in the efficiency calculations, however, a smaller in-
ductor requires a higher switching frequency to maintain the
same level of output current ripple. An increase in frequency
can mean increasing loss in the FETs due to the charging
and discharging of the gates. Generally the switching fre-
quency is chosen so that conduction loss outweighs switch-
ing loss. The equation for output inductor selection is:
Plugging in the values for output current ripple, input voltage,
output voltage, switching frequency, and assuming a 40%
peak-to-peak output current ripple yields an inductance of
1.5µH. The output inductor must be rated to handle the peak
current (also equal to the peak switch current), which is (Io +
0.5*Io). This is 12A for a 10A design. The Coilcraft D05022-
152HC is 1.5µH, is rated to 15Arms, and has a DCR of 4m.
OUTPUT CAPACITOR
The output capacitor forms the second half of the power
stage of a Buck switching converter. It is used to control the
output voltage ripple (Vo) and to supply load current during
fast load transients.
In this example the output current is 10A and the expected
type of capacitor is an aluminum electrolytic, as with the
input capacitors. (Other possibilities include ceramic, tanta-
lum, and solid electrolyte capacitors, however the ceramic
type often do not have the large capacitance needed to
supply current for load transients, and tantalums tend to be
more expensive than aluminum electrolytic.) Aluminum ca-
pacitors tend to have very high capacitance and fairly low
ESR, meaning that the ESR zero, which affects system
stability, will be much lower than the switching frequency.
The large capacitance means that at switching frequency,
the ESR is dominant, hence the type and number of output
capacitors is selected on the basis of ESR. One simple
formula to find the maximum ESR based on the desired
output voltage ripple, Vo and the designed output current
ripple, Io, is:
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