LM2735XSDEVAL National Semiconductor, LM2735XSDEVAL Datasheet - Page 11

• 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: -

significant change in capacitance at the operating input volt-
age and the operating temperature. The ESL of an input
capacitor is usually determined by the effective cross sec-
tional area of the current path. At the operating frequencies
of the LM2735, certain capacitors may have an ESL so large
that the resulting impedance (2
required to provide stable operation. As a result, surface
mount capacitors are strongly recommended. Multilayer ce-
ramic capacitors (MLCC) are good choices for both input and
output capacitors and have very low ESL. For MLCCs it is
recommended to use X7R or X5R dielectrics. Consult capac-
itor manufacturer datasheet to see how rated capacitance
varies over operating conditions.
OUTPUT CAPACITOR
The LM2735 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. The output ca-
pacitor is selected based upon the desired output ripple and
transient response. The initial current of a load transient is
provided mainly by the output capacitor. The output
impedance will therefore determine the maximum voltage
perturbation. The output ripple of the converter is a function
of the capacitor’s reactance and its equivalent series resis-
tance (ESR):
When using MLCCs, the ESR is typically so low that the ca-
pacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action .
Given the availability and quality of MLCCs and the expected
output voltage of designs using the LM2735, there is really no
need to review any other capacitor technologies. Another
benefit of ceramic capacitors is their ability to bypass high
frequency noise. A certain amount of switching edge noise
will couple through parasitic capacitances in the inductor to
the output. A ceramic capacitor will bypass this noise while a
tantalum will not. Since the output capacitor is one of the two
external components that control the stability of the regulator
control loop, most applications will require a minimum at 4.7
µF of output capacitance. Like the input capacitor, recom-
mended multilayer ceramic capacitors are X7R or X5R.
Again, verify actual capacitance at the desired operating volt-
age and temperature.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the following equation where
R1 is connected between the FB pin and GND, and R2 is
connected between V
OUT
and the FB pin.
π
fL) will be higher than that
11
A good value for R1 is 10kΩ.
COMPENSATION
The LM2735 uses constant frequency peak current mode
control. This mode of control allows for a simple external
compensation scheme that can be optimized for each appli-
cation. A complicated mathematical analysis can be complet-
ed to fully explain the LM2735’s internal & external compen-
sation, but for simplicity, a graphical approach with simple
equations will be used. Below is a Gain & Phase plot of a
LM2735 that produces a 12V output from a 5V input voltage.
The Bode plot shows the total loop Gain & Phase without ex-
ternal compensation.
One can see that the Crossover frequency is fine, but the
phase margin at 0dB is very low (22°). A zero can be placed
just above the crossover frequency so that the phase margin
will be bumped up to a minimum of 45°. Below is the same
application with a zero added at 8 kHz.
FIGURE 6. LM2735 Without External Compensation
FIGURE 5. Setting Vout
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