LM3478MASLASHNOPB datasheet(22/43 Pages) TI2

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RdrOn

tLH =

Vdr - Vgsth

Qgs

Qgd +

PSW =

ILmax x Vout

x fSW x (tLH + tHL)

LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

Thermally the diode must be able to handle the maximum average current delivered to the output. The peak

reverse voltage 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.

8.2.1.2.7

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 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)

The off-state voltage of the MOSFET is approximately equal to the output voltage. Vds(max) must be greater

than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and

switching losses. RDS(ON) is needed to estimate the conduction losses, Pcond:

Pcond = I

2 x R

DS(ON) x D

(28)

The temperature effect on the RDS(ON) usually is quite significant. Assume 30% increase at hot.

For the current I in Equation 28 the average inductor current may be used.

Especially at high switching frequencies the switching losses may be the largest portion of the total losses.

The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation.

Often the individual MOSFET's data sheet does not give enough information to yield a useful result. Equation 29

and Equation 30 give a rough idea how the switching losses are calculated:

(29)

(30)

8.2.1.2.8

Input Capacitor Selection

Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous

and triangular as shown in Figure 29. The inductor ensures that the input capacitor sees fairly low ripple currents.

However, as the input capacitor gets smaller, the input ripple goes up. The RMS current in the input capacitor is

given using Equation 31.

(31)

The input capacitor should be capable of handling the RMS current. Although the input capacitor is not as critical

in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should

be chosen in the range of 10 µF to 20 µF. If a value lower than 10 µF is used, then problems with impedance

interactions or switching noise can affect the LM3478. To improve performance, especially with Vin below 8 volts,

it is recommended to use a 20 Ohm resistor at the input to provide an RC filter. The resistor is placed in series

with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 31). A 0.1-µF or 1-µF

ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the

other side of the resistor at the input power supply.

LM3478MA/NOPB Datasheet(PDF) 22 Page - Texas Instruments

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