LM2731YMFX/NOPB National Semiconductor, LM2731YMFX/NOPB Datasheet - Page 12

• Manufacturer Part Number: LM2731YMFX/NOPB
• Description: IC BOOST ADJ 1.8A SOT23-5
• Manufacturer: National Semiconductor
• Type: Step-Up (Boost)r

Specifications of LM2731YMFX/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 22 V
• Current - Output: 1.8A
• Frequency - Switching: 600kHz
• Voltage - Input: 2.7 ~ 14 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• For Use With: LM2731YEVAL - BOARD EVALUATION LM2731Y
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2731YMFX

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DUTY CYCLE
The maximum duty cycle of the switching regulator deter-
mines the maximum boost ratio of output-to-input voltage that
the converter can attain in continuous mode of operation. The
duty cycle for a given boost application is defined as:
This applies for continuous mode operation.
INDUCTANCE VALUE
The first question we are usually asked is: “How small can I
make the inductor?” (because they are the largest sized com-
ponent and usually the most costly). The answer is not simple
and involves trade-offs in performance. Larger inductors
mean less inductor ripple current, which typically means less
output voltage ripple (for a given size of output capacitor).
Larger inductors also mean more load power can be delivered
because the energy stored during each switching cycle is:
Where “lp” is the peak inductor current. An important point to
observe is that the LM2731 will limit its switch current based
on peak current. This means that since lp(max) is fixed, in-
creasing L will increase the maximum amount of power avail-
able to the load. Conversely, using too little inductance may
limit the amount of load current which can be drawn from the
output.
Best performance is usually obtained when the converter is
operated in “continuous” mode at the load current range of
interest, typically giving better load regulation and less output
ripple. Continuous operation is defined as not allowing the in-
ductor current to drop to zero during the cycle. It should be
noted that all boost converters shift over to discontinuous op-
eration as the output load is reduced far enough, but a larger
inductor stays “continuous” over a wider load current range.
To better understand these trade-offs, a typical application
circuit (5V to 12V boost with a 10 µH inductor) will be ana-
lyzed. We will assume:
V
Since the frequency is 1.6 MHz (nominal), the period is ap-
proximately 0.625 µs. The duty cycle will be 62.5%, which
means the ON time of the switch is 0.390 µs. It should be
noted that when the switch is ON, the voltage across the in-
ductor is approximately 4.5V.
Using the equation:
IN
= 5V, V
OUT
= 12V, V
E = L/2 X (lp)
DIODE
V = L (di/dt)
= 0.5V, V
2
SW
= 0.5V
Basic Application Circuit
12
We can then calculate the di/dt rate of the inductor which is
found to be 0.45 A/µs during the ON time. Using these facts,
we can then show what the inductor current will look like dur-
ing operation:
During the 0.390 µs ON time, the inductor current ramps up
0.176A and ramps down an equal amount during the OFF
time. This is defined as the inductor “ripple current”. It can also
be seen that if the load current drops to about 33 mA, the
inductor current will begin touching the zero axis which means
it will be in discontinuous mode. A similar analysis can be
performed on any boost converter, to make sure the ripple
current is reasonable and continuous operation will be main-
tained at the typical load current values.
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the current
limiter cuts in is dependent on duty cycle of the application.
This is illustrated in the graphs below which show typical val-
ues of switch current for both the "X" and "Y" versions as a
function of effective (actual) duty cycle:
5V–12V Boost (LM2731X)
10 µH Inductor Current,
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