The LM5000EP is a current mode PWM regulator. The signal

flow of this control scheme has two feedback loops, one that

senses switch current and one that senses output voltage.

To keep a current programmed control converter stable

above duty cycles of 50%, the inductor must meet certain

criteria. The inductor, along with input and output voltage,

will determine the slope of the current through the inductor

(see Figure 4 (a)). If the slope of the inductor current is too

great, the circuit will be unstable above duty cycles of 50%.

The LM5000EP provides a compensation pin (COMP) to

customize the voltage loop feedback. It is recommended that

a series combination of R

sation network, as shown in Figure 1. The series combina-

tion of R

following equations:

where R

850k

Ω. For most applications, performance can be opti-

mized by choosing values within the range 5k

Ω≤ R

C

≤ 20kΩ

and 680pF

≤ C

C

≤ 4.7nF.

COMPENSATION

This section will present a general design procedure to help

insure a stable and operational circuit. The designs in this

datasheet are optimized for particular requirements. If differ-

ent conversions are required, some of the components may

need to be changed to ensure stability. Below is a set of

general guidelines in designing a stable circuit for continu-

ous conduction operation (loads greater than 100mA), in

most all cases this will provide for stability during discontinu-

ous operation as well. The power components and their

effects will be determined first, then the compensation com-

ponents will be chosen to produce stability.

INDUCTOR SELECTION

To ensure stability at duty cycles above 50%, the inductor

must have some minimum value determined by the mini-

mum input voltage and the maximum output voltage. This

equation is:

where fs is the switching frequency, D is the duty cycle, and

R

equation is only good for duty cycles greater than 50%

The inductor ripple current is important for a few reasons.

One reason is because the peak switch current will be the

average inductor current (input current) plus

∆i

be taken to make sure that the switch will not reach its

current limit during normal operation. The inductor must also

be sized accordingly. It should have a saturation current

rating higher than the peak inductor current expected. The

output voltage ripple is also affected by the total ripple cur-

rent.

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 LM5000EP, choosing a crossover point well

below where the right half plane zero is located will ensure

sufficient phase margin. A discussion of the right half plane

zero and checking the crossover using the DC gain will

follow.

OUTPUT CAPACITOR SELECTION

The choice of output capacitors is somewhat more arbitrary.

It is recommended that low ESR (Equivalent Series Resis-

tance, denoted R

polymer electrolytic, or low ESR tantalum. Higher ESR ca-

pacitors may be used but will require more compensation

which will be explained later on in the section. The ESR is

also important because it determines the output voltage

ripple according to the approximate equation:

∆V

OUT

) 2

∆i

After choosing the output capacitor you can determine a

pole-zero pair introduced into the control loop by the follow-

ing equations:

Where R

the maximum load current. The zero created by the ESR of

the output capacitor is generally very high frequency if the

ESR is small. If low ESR capacitors are used it can be

neglected. If higher ESR capacitors are used see the High

Output Capacitor ESR Compensation section.

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

frequency of the RHP zero or less. This zero occurs at a

frequency of:

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