lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 9

• Manufacturer Part Number: lm2735xsdx
• Description: 520khz/1.6mhz - Space-efficient Boost And Sepic Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

CURRENT LIMIT
The LM2735 uses cycle-by-cycle current limiting to protect
the internal NMOS switch. It is important to note that this cur-
rent limit will not protect the output from excessive current
during an output short circuit. The input supply is connected
to the output by the series connection of an inductor and a
diode. If a short circuit is placed on the output, excessive cur-
rent can damage both the inductor and diode.
Design Guide
ENABLE PIN / SHUTDOWN MODE
The LM2735 has a shutdown mode that is controlled by the
Enable pin (EN). When a logic low voltage is applied to EN,
the part is in shutdown mode and its quiescent current drops
to typically 80 nA. Switch leakage adds up to another 1 µA
from the input supply. The voltage at this pin should never
exceed V
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
160°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to ap-
proximately 150°C.
SOFT-START
This function forces V
ing start up. During soft-start, the error amplifier’s reference
voltage ramps to its nominal value of 1.255V in approximately
4.0ms. This forces the regulator output to ramp up in a more
IN
+ 0.3V.
FIGURE 3. Typical Waveforms
OUT
to increase at a controlled rate dur-
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9
linear and controlled fashion, which helps reduce inrush cur-
rent.
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
Therefore:
Power losses due to the diode (D1) forward voltage drop, the
voltage drop across the internal NMOS switch, the voltage
drop across the inductor resistance (R
losses must be included to calculate a more accurate duty
cycle (See Calculating Efficiency and Junction Temperature
for a detailed explanation). A more accurate formula for cal-
culating the conversion ratio is:
Where η equals the efficiency of the LM2735 application.
The inductor value determines the input ripple current. Lower
inductor values decrease the size of the inductor, but increase
the input ripple current. An increase in the inductor value will
decrease the input ripple current.
A good design practice is to design the inductor to produce
10% to 30% ripple of maximum load. From the previous equa-
tions, the inductor value is then obtained.
Where: 1/T
S
= F
FIGURE 4. Inductor Current
SW
= switching frequency
O
) to input voltage (V
DCR
) and switching
IN
):
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