LM3488_05 datasheet(11/24 Pages) NSC

Teilenummer	LM3488

Bauteilbeschribung	High Efficiency Low-Side N-Channel Controller for Switching Regulators

PDF	24 Pages

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Hersteller	NSC [National Semiconductor (TI)]

Direct Link	http://www.national.com

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11 / 24 page

Functional Description (Continued)

SLOPE COMPENSATION RAMP

The LM3488 uses a current mode control scheme. The main

advantages of current mode control are inherent cycle-by-

cycle current limit for the switch, and simpler control loop

characteristics. It is also easy to parallel power stages using

current mode control since as current sharing is automatic.

Current mode control has an inherent instability for duty

cycles greater than 50%, as shown in Figure 2.In Figure 2,

a small increase in the load current causes the switch cur-

rent to increase by

∆I

O. The effect of this load change,

∆I

1,is

From the above equation, when D > 0.5,

∆I

1 will be greater

than

∆I

O. In other words, the disturbance is divergent. So a

very small perturbation in the load will cause the disturbance

to increase.

To prevent the sub-harmonic oscillations, a compensation

ramp is added to the control signal, as shown in Figure 3.

With the compensation ramp,

10138807

FIGURE 1. Basic Operation of the PWM comparator

10138809

FIGURE 2. Sub-Harmonic Oscillation for D>0.5

LM3488 Datenblatt(PDF) 11 Page - National Semiconductor (TI)