LM3478MM National Semiconductor, LM3478MM Datasheet - Page 10

• Manufacturer Part Number: LM3478MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3478MM

• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Features: High Efficiency
• Package / Case: 8-MSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Functional Description
SLOPE COMPENSATION RAMP
The LM3478 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 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
FIGURE 1. Basic Operation of the PWM comparator
FIGURE 2. Sub-Harmonic Oscillation for D
(Continued)
1
, is
10
From the above equation, when D
than ∆I
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
>
0.5
10135509
>
0.5, ∆I
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1
will be greater