LM3478MM datasheet(15/21 Pages) NSC

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Typical Applications (Continued)

If V

L(t) is constant, diL(t)/dt must be constant. Hence, for a

given input voltage and output voltage, the current in the

inductor changes at a constant rate.

The important quantities in determining a proper inductance

value are I

L (the average inductor current) and

∆i

L (the

inductor current ripple). If

∆i

L is larger than IL, the inductor

current will drop to zero for a portion of the cycle and the

converter will operate in discontinuous conduction mode. If

∆i

L is smaller than IL, the inductor current will stay above

zero and the converter will operate in continuous conduction

mode. All the analysis in this datasheet assumes operation

in continuous conduction mode. To operate in continuous

conduction mode, the following conditions must be met:

L >

∆i

Choose the minimum I

OUT to determine the minimum L. A

common choice is to set

∆i

L to 30% of IL. Choosing an

appropriate core size for the inductor involves calculating the

average and peak currents expected through the inductor. In

a boost converter,

and I

L_peak =IL(max) +

∆i

L(max),

where

A core size with ratings higher than these values should be

chosen. If the core is not properly rated, saturation will

dramatically reduce overall efficiency.

The LM3478 can be set to switch at very high frequencies.

When the switching frequency is high, the converter can be

operated with very small inductor values. With a small induc-

tor value, the peak inductor current can be higher than the

output currents, especially under light load conditions.

The LM3478 senses the peak current through the switch.

The peak current through the switch is the same as the peak

current calculated above.

PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT

CURRENT

The output voltage can be programmed using a resistor

divider between the output and the feedback pins, as shown

in Figure 10. The resistors are selected such that the voltage

at the feedback pin is 1.26V. R

F1 and RF2 can be selected

using the equation,

A 100pF capacitor may be connected between the feedback

and ground pins to reduce noise.

The maximum amount of current that can be delivered at the

output can be controlled by the sense resistor, R

SEN. Current

limit occurs when the voltage that is generated across the

sense resistor equals the current sense threshold voltage,

SENSE. Limits for VSENSE have been specified in the elec-

trical characteristics. This can be expressed as:

sw(peak) * RSEN =VSENSE

SENSE represents the maximum value of the control signal

as shown in Figure 2. This control signal, however, is not a

constant value and changes over the course of a period as a

result of the internal compensation ramp (see Figure 3).

Therefore the current limit will also change as a result of the

internal compensation ramp. The actual command signal,

CS, can be better expressed as a function of the sense

voltage and the internal compensation ramp:

CS =VSENSE −(D * VSL)

SL is defined as the internal compensation ramp voltage,

limits are specified in the electrical characteristics.

The peak current through the switch is equal to the peak

inductor current.

sw(peak) =IL +

∆i

Therefore for a boost converter

Combining the three equation yields an expression for R

SEN

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