LM3488MM National Semiconductor, LM3488MM Datasheet - Page 10

• Manufacturer Part Number: LM3488MM
• Description: IC, N-CH CONTROLLER, MSOP-8
• Manufacturer: National Semiconductor

Specifications of LM3488MM

• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 1A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Termination Type: SMD
• Input Voltage Primary Min: 2.95V
• Output Voltage Min: 1.26V
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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
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
FIGURE 2. Sub-Harmonic Oscillation for D>0.5
2. In
Figure
1
, is :
2, a
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
With the compensation ramp,
O
. In other words, the disturbance is divergent. So a
1
10138809
will be greater
Figure
10138807
3.