LM2735XSDEVAL National Semiconductor, LM2735XSDEVAL Datasheet - Page 12

• 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: -

www.national.com
The simplest method to determine the compensation compo-
nent value is as follows.
Set the output voltage with the following equation.
Where R1 is the bottom resistor and R2 is the resistor tied to
the output voltage. The next step is to calculate the value of
C3. The internal compensation has been designed so that
when a zero is added between 5 kHz & 10 kHz the converter
will have good transient response with plenty of phase margin
for all input & output voltage combinations.
Lower output voltages will have the zero set closer to 10 kHz,
and higher output voltages will usually have the zero set clos-
er to 5 kHz. It is always recommended to obtain a Gain/Phase
plot for your actual application. One could refer to the Typical
applications section to obtain examples of working applica-
tions and the associated component values.
Pole @ origin due to internal gm amplifier:
Pole due to output load and capacitor:
This equation only determines the frequency of the pole for
perfect current mode control (CMC). I.e, it doesn’t take into
account the additional internal artificial ramp that is added to
the current signal for stability reasons. By adding artificial
ramp, you begin to move away from CMC to voltage mode
control (VMC). The artifact is that the pole due to the output
load and output capacitor will actually be slightly higher in fre-
quency than calculated. In this example it is calculated at 650
Hz, but in reality it is around 1 kHz.
The zero created with capacitor C3 & resistor R2:
FIGURE 7. LM2735 With External Compensation
F
P-ORIGIN
20215832
12
There is an associated pole with the zero that was created in
the above equation.
It is always higher in frequency than the zero.
A right-half plane zero (RHPZ) is inherent to all boost con-
verters. One must remember that the gain associated with a
right-half plane zero increases at 20dB per decade, but the
phase decreases by 45° per decade. For most applications
there is little concern with the RHPZ due to the fact that the
frequency at which it shows up is well beyond crossover, and
has little to no effect on loop stability. One must be concerned
with this condition for large inductor values and high output
currents.
There are miscellaneous poles and zeros associated with
parasitics internal to the LM2735, external components, and
the PCB. They are located well over the crossover frequency,
and for simplicity are not discussed.
PCB Layout Considerations
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most impor-
tant consideration when completing a Boost Converter layout
is the close coupling of the GND connections of the C
pacitor and the LM2735 PGND pin. The GND ends should be
close to one another and be connected to the GND plane with
at least two through-holes. There should be a continuous
ground plane on the bottom layer of a two-layer board except
under the switching node island. The FB pin is a high
impedance node and care should be taken to make the FB
trace short to avoid noise pickup and inaccurate regulation.
The feedback resistors should be placed as close as possible
to the IC, with the AGND of R1 placed as close as possible to
the GND (pin 5 for the LLP) of the IC. The V
should be routed away from the inductor and any other traces
that are switching. High AC currents flow through the V
FIGURE 8. Setting External Pole-Zero
OUT
20215829
trace to R2
OUT
IN
, SW
ca-