LM3481 Datasheet, PDF(15/22 Page) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LM3481 Datasheet, PDF (15/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

◁

PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT

CURRENT

The output voltage can be programmed using a resistor di-

vider between the output and the feedback pins, as shown in

Figure 14. The resistors are selected such that the voltage at

the feedback pin is 1.275V. RF1 and RF2 can be selected using

the equation,

Isw(peak) x RSEN = VSENSE

The peak current through the switch is equal to the peak in-

ductor current.

Isw(peak) = IL(max) + ÎiL

Therefore for a boost converter

A 100 pF 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, RSEN. Current

limit occurs when the voltage that is generated across the

sense resistor equals the current sense threshold voltage,

VSENSE. Limits for VSENSE have been specified in the electrical

characteristics section. This can be expressed as:

Combining the two equations yields an expression for RSEN

Evaluate RSEN at the maximum and minimum VIN values and

choose the smallest RSEN calculated.

20136520

FIGURE 14. Adjusting the Output Voltage

CURRENT LIMIT WITH ADDITIONAL SLOPE

COMPENSATION

If an external slope compensation resistor is used (see Figure

5) the internal control signal will be modified and this will have

an effect on the current limit.

If RSL is used, then this will add to the existing slope compen-

sation. The command voltage, VCS, will then be given by:

VCS = VSENSE â ÎVSL

Where VSENSE is a defined parameter in the electrical char-

acteristics section and ÎVSL is the additional slope compen-

sation generated as discussed in the Slope Compensation

Ramp section. This changes the equation for RSEN to:

Note that since ÎVSL = RSL x K as defined earlier, RSLcan be

used to provide an additional method for setting the current

limit. In some designs RSL can also be used to help filter noise

to keep the ISEN pin quiet.

POWER DIODE SELECTION

Observation of the boost converter circuit shows that the av-

erage current through the diode is the average load current,

and the peak current through the diode is the peak current

through the inductor. The diode should be rated to handle

more than the inductor peak current. The peak diode current

can be calculated using the formula:

ID(Peak) = [IOUT/ (1âD)] + ÎiL

In the above equation, IOUT is the output current and ÎiL has

been defined in Figure 13.

The peak reverse voltage for a boost converter is equal to the

regulator output voltage. The diode must be capable of han-

dling this peak reverse voltage. To improve efficiency, a low

forward drop Schottky diode is recommended.

www.national.com

▷