LM2735XSDEVAL National Semiconductor, LM2735XSDEVAL Datasheet - Page 10

• Manufacturer Part Number: LM2735XSDEVAL
• Description: BOARD EVAL LM2735 1.6MHZ 6LLP
• Manufacturer: National Semiconductor

Specifications of LM2735XSDEVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 500mA
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Boost
• Frequency - Switching: 1.6MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2735
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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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
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.
IN
+ 0.3V.
OUT
O
to increase at a controlled rate dur-
) to input voltage (V
DCR
) and switching
IN
):
10
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
One must also ensure that the minimum current limit (2.1A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (I
lated by:
or
When selecting an inductor, make sure that it is capable of
supporting the peak input current without saturating. Inductor
saturation will result in a sudden reduction in inductance and
prevent the regulator from operating correctly. Because of the
speed of the internal current limit, the peak current of the in-
ductor need only be specified for the required maximum input
current. For example, if the designed maximum input current
is 1.5A and the peak current is 1.75A, then the inductor should
be specified with a saturation current limit of >1.75A. There is
no need to specify the saturation or peak current of the in-
ductor at the 3A typical switch current limit.
Because of the operating frequency of the LM2735, ferrite
based inductors are preferred to minimize core losses. This
presents little restriction since the variety of ferrite-based in-
ductors is huge. Lastly, inductors with lower series resistance
(DCR) will provide better operating efficiency. For recom-
mended inductors see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 10 µF to 44 µF de-
pending on the application. The capacitor manufacturer
specifically states the input voltage rating. Make sure to check
any recommended deratings and also verify if there is any
S
= F
FIGURE 4. Inductor Current
SW
= switching frequency
IL
pk
IL
= I
pk
OUT
= I
IN
LPK
/ D' + ΔI
+ ΔI
) in the inductor is calcu-
L
L
IN
does not
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