LM2735XMFSLASHNOPB datasheet(13/59 Pages) TI

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( ) R

Load

RHPZERO

POLE -

25((R1 R2) x C3)

(

)

R2 xC3

2 5

ZERO -

LOAD

(RLoadCOUT)

2 5

F RC

P-

LM2735, LM2735-Q1

www.ti.com

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

Feature Description (continued)

Lower output voltages will have the zero set closer to 10 kHz, and higher output voltages will usually have the

zero set closer to 5 kHz. TI recommends obtaining a Gain and Phase plot for your actual application. See

Application and Implementation to obtain examples of working applications and the associated component

values.

Pole at origin due to internal GM amplifier:

FP-ORIGIN

(3)

Pole due to output load and capacitor:

(4)

This equation only determines the frequency of the pole for perfect current mode control (CMC). That is, 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 frequency

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 17. Setting External Pole-Zero

(5)

There is an associated pole with the zero that was created in the above equation.

(6)

It is always higher in frequency than the zero.

A right-half plane zero (RHPZ) is inherent to all boost converters. One must remember that the gain associated

with a right-half plane zero increases at 20 dB 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.

(7)

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.

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