LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 10

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

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SLOPE COMPENSATION RAMP
The LM3488 uses a current mode control scheme. The main
advantages of current mode control are inherent cycle-by-cy-
cle current limit for the switch, and simpler control loop char-
acteristics. 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 cy-
cles greater than 50%, as shown in Figure 2. In Figure 2, a
small increase in the load current causes the switch current
to increase by ΔI
O
. The effect of this load change, ΔI
FIGURE 1. Basic Operation of the PWM comparator
FIGURE 2. Sub-Harmonic Oscillation for D>0.5
1
, is :
10
From the above equation, when D > 0.5, ΔI
than ΔI
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,
O
. In other words, the disturbance is divergent. So a
1
10138809
will be greater
10138807