LM3477MM National Semiconductor, LM3477MM Datasheet - Page 10

• Manufacturer Part Number: LM3477MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3477MM

• Input Voltage Primary Min: 2.95V
• Mounting Type: Surface Mount
• Topology: Buck (Step Down)
• Control Mode: Current
• Duty Cycle Max: 93%
• Input Voltage Primary Max: 35V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Functional Block Diagram
Functional Description
General Description
The LM3477/A is a switching regulator controller for topolo-
gies incorporating a high side switch. The most common of
these topologies is the step-down, or buck, converter. Other
topologies such as the inverting (buck-boost) and inverse
SEPIC (zeta) converters can be realized. This datasheet will
focus on buck converter applications.
The LM3477/A employs current mode control architecture.
Among the many benefits of this architecture are superior
line and load regulation, cycle-by-cycle current limiting, and
simple loop compensation. The LM3477/A features a pat-
ented adjustable slope compensation scheme to enable flex-
ible inductor selection. The LM3477/A has a combination of
features that allow its use in a wide variety of applications.
The input voltage can range from 2.97V to 35V, with the
output voltage being positive or negative depending on the
topology. The current limit can be scaled to safely drive a
wide range of loads. An internal soft-start is provided to limit
initial in-rush current. Output over voltage and input under
voltage protection ensure safe operation of the LM3477/A.
Regions of Operation
Pulse width modulation (PWM) is the normal mode of opera-
tion. In PWM, the output voltage is well regulated and has a
ripple frequency equal to the switching frequency (500kHz).
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In low load conditions, the part operates in hysteretic mode.
In this mode, the output voltage is regulated between a high
and low value that results in a higher ripple magnitude and
lower ripple frequency than in PWM mode (see OVER VOLT-
AGE PROTECTION section).
FIGURE 1. Operating Regions of the LM3477/A
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