LM3488MM datasheet(18/24 Pages) NSC

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Typical Applications (Continued)

CURRENT LIMIT WITH ADDITIONAL SLOPE

COMPENSATION

If an external slope compensation resistor is used (see

Figure 4) the internal control signal will be modified and this

will have an effect on the current limit. The control signal is

given by:

CS =VSENSE −(D * VSL)

Where V

SENSE and VSL are defined parameters in the elec-

trical characteristics section. If R

SL is used, then this will add

to the existing slope compensation. The command voltage

will then be given by:

CS =VSENSE −(D * (VSL +

∆V

SL))

Where

∆V

SL is the additional slope compensation generated

and can be calculated by use of Figure 5 or is equal to 40 x

−6 *R

SL. This changes the equation for RSEN to:

Therefore R

SL can be used to provide an additional method

for setting the current limit.

POWER DIODE SELECTION

Observation of the boost converter circuit shows that the

average current through the diode is the average load cur-

rent, and the peak current through the diode is the peak

current through the inductor. The diode should be rated to

handle more than its peak current. The peak diode current

can be calculated using the formula:

D(Peak) =IOUT/ (1−D) +

∆I

In the above equation, I

OUT is the output current and

∆I

L has

been defined in Figure 11

The peak reverse voltage for boost converter is equal to the

regulator output voltage. The diode must be capable of

handling this voltage. To improve efficiency, a low forward

drop schottky diode is recommended.

POWER MOSFET SELECTION

The drive pin of LM3488 must be connected to the gate of an

external MOSFET. In a boost topology, the drain of the

external N-Channel MOSFET is connected to the inductor

and the source is connected to the ground. The drive pin

(DR) voltage depends on the input voltage (see typical per-

formance characteristics). In most applications, a logic level

MOSFET can be used. For very low input voltages, a sub-

logic level MOSFET should be used.

The selected MOSFET directly controls the efficiency. The

critical parameters for selection of a MOSFET are:

Minimum threshold voltage, V

TH(MIN)

On-resistance, R

DS(ON)

Total gate charge, Q

Reverse transfer capacitance, C

RSS

Maximum drain to source voltage, V

DS(MAX)

The off-state voltage of the MOSFET is approximately equal

to the output voltage. V

DS(MAX) of the MOSFET must be

greater than the output voltage. The power losses in the

MOSFET can be categorized into conduction losses and ac

switching or transition losses. R

DS(ON) is needed to estimate

the conduction losses. The conduction loss, P

COND,isthe

2R loss across the MOSFET. The maximum conduction loss

is given by:

10138820

FIGURE 12. Adjusting the Output Voltage

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