Datasheet

LM3488
LM3488-Q1
SNVS089M –JULY 2000–REVISED MARCH 2013 www.ti.com
Under extremely light load or no-load conditions, the energy delivered to the output capacitor when the external
MOSFET is on during the blank-out time is more than what is delivered to the load. An over-voltage comparator
inside the LM3488 prevents the output voltage from rising under these conditions. The over-voltage comparator
senses the feedback (FB pin) voltage and resets the RS latch under these conditions. The latch remains in reset
state till the output decays to the nominal value.
Figure 22. Basic Operation of the PWM comparator
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 23. In
Figure 23, a small increase in the load current causes the switch current to increase by ΔI
O
. The effect of this
load change, ΔI
1
, is :
(1)
From the above equation, when D > 0.5, ΔI
1
will be greater than ΔI
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 24.
With the compensation ramp,
(2)
Figure 23. Sub-Harmonic Oscillation for D>0.5
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