tance loss due to capacitor voltage coefficient. See the

capacitor manufacturer’s datasheet for DC bias perfor-

mance.

COMPENSATION

DC Gain and Open-Loop Gain

Since the control stage of the converter forms a complete

feedback loop with the power components, it forms a closed-

loop system that must be stabilized to avoid positive feed-

back and instability. A value for open-loop DC gain will be

required, from which you can calculate, or place, poles and

zeros to determine the crossover frequency and the phase

margin. A high phase margin (greater than 45˚) is desired for

the best stability and transient response. For the purpose of

stabilizing the LM3551/2, choosing a crossover point well

below where the right half plane zero is located will ensure

sufficient phase margin.

RHP zero, calculate the open-loop DC gain, A

value is known, you can calculate the crossover visually by

placing a −20dB/decade slope at each pole, and a +20dB/

decade slope for each zero. The point at which the gain plot

crosses unity gain, or 0dB, is the crossover frequency. If the

margin should be high enough for stability. The equation for

A

calculation:

mc ) 0.072 x fs (in V/s)

where R

frequency, V

amplifier transconductance and R

on-resistance. The value for g

the Electrical Characteristics table.

Right Half Plane Zero

A current mode control boost regulator has an inherent right

half plane zero (RHP zero). This zero has the effect of a zero

in the gain plot, causing an imposed +20dB/decade on the

rolloff, but has the effect of a pole in the phase, subtracting

another 90˚ in the phase plot. This can cause undesirable

effects if the control loop is influenced by this zero. To ensure

the RHP zero does not cause instability issues, the control

the frequency of the RHP zero. This zero occurs at a fre-

quency of:

where I

Compensation Components

The LM3551 and LM3552 provide a compensation pin (V

to customize the voltage loop feedback. It is recommended

that a series combination of R

compensation network, as shown in the typical application

circuit. For any given application, there exists a unique com-

bination of R

the LM3551/2 circuit in terms of its transient response. The

series combination of R

The frequency of the pole created is determined by the

equation:

where R

approximately 900k

Ω. Since R

R

neglected until a value is chosen to set the zero f

created to cancel the pole created by the output capacitor,

f

currents as shown by the equation, so setting the zero is not

exact. Determine the range of f

and then set the zero f

middle. The frequency of this zero is determined by:

Now R

Check to make sure that the pole f

500Hz range, and change each value slightly if needed to

ensure both component values are in the recommended

range. For both typical applications circuits shown on the

front page, the Recommended value for C

R

Ω for Lumiled applications. 10nF and 27kΩ are

recommended for Sharp applications.

RECOMMENDED MINIMUM COMPONENT

SPECIFICATIONS

Component

Value

Ratings

L1

4.7µH

2.0A 30% I

SAT

Rating

C

IN

4.7µF (Sharp)

10V X5R or

X7R

10µF (Lumiled)

C

OUT

4.7µF (Sharp)

16V X5R or

X7R

10uF (Lumiled)

C

FTO

User

Determined

6.3V X5R or

X7R

www.national.com

12