LM3478_09 Datasheet, PDF(15/22 Page) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

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LM3478_09 Datasheet, PDF (15/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

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PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT

CURRENT

The output voltage can be programmed using a resistor di-

vider between the output and the FB pin. The resistors are

selected such that the voltage at the FB pin is 1.26V. Pick

RF1 (the resistor between the output voltage and the feedback

pin) and RF2 (the resistor between the feedback pin and

ground) can be selected using the following equation,

RF2 = (1.26V x RF1) / (Vout - 1.26V)

A 100pF capacitor may be connected between the feedback

and ground pins to reduce noise.

CURRENT LIMIT WITH ONLY THE INTERNAL SLOPE

COMPENSATION

The maximum amount of current that can be delivered at the

output is 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 char-

acteristics section. This can be expressed as:

VSENSE = ISW(PEAK) x RSEN

VSENSE represents the maximum value of the control signal

VCS as shown in Figure 10. 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. Therefore the

current limit will also change as a result of the internal com-

pensation ramp. The actual VCS can be better expressed as

a function of the sense voltage (VSENSE) and the internal com-

pensation ramp:

VCS = VSENSE â (D x VSL)

10135552

FIGURE 10. Current Sense Voltage vs Duty Cycle

Figure 10 shows how VCS changes with duty cycle. The curve

shows the ramp Se which is defined by the voltage VSENSE (at

0% dutycycle) and by the internally generated slope VSL

which changes VCS with duty cycle. The dotted line shows

VSENSE. At 100% duty cycle, the current sense voltage will be

VSENSE minus VSL.

The graph also shows the increased current limit of 343 mV

(typical) during the 325 ns (typical) blank out time. For differ-

ent frequencies this fixed blank out time obviously occupies

more duty cycle, percentage wise.

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

ductor current.

The expression for RSEN is:

The numerator of the above equation is the VCS and the de-

nominator is the peak current.

CURRENT LIMIT WITH THE INTERNAL SLOPE

COMPENSATION PLUS ADDITIONAL EXTERNAL SLOPE

COMPENSATION

If an external slope compensation resistor RSL is used, the

internal command signal will be modified and this will have an

effect on the current limit. The command signal then includes

the external slope ÎVSL:

VCS = VSENSE â (D x (VSL + ÎVSL))

Where ÎVSL is the additional slope compensation generated

as discussed in the section slope compensation ramp. This

changes the equation for RSEN to:

Note that RSL which defines ÎVSL is an additional way of set-

ting the current limit.

In some designs RSL can also help to 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 its peak current. The peak diode current can be

calculated using the formula:

ID(Peak) = IOUT/ (1âD) + ÎIL

Thermally the diode must be able to handle the maximum av-

erage current delivered to the output. The peak reverse volt-

age for boost converters is equal to the regulated 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 the LM3478 must be connected to the gate

of an external MOSFET. The drive pin (DR) voltage depends

on the input voltage (see typical performance 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 has a great influence on the

system efficiency. The critical parameters for selecting a

MOSFET are:

1. Minimum threshold voltage, VTH(MIN)

2. On-resistance, RDS(ON)

3. Total gate charge, Qg

4. Reverse transfer capacitance, CRSS

5. Maximum drain to source voltage, VDS(MAX)

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