LM3488MM datasheet(21/33 Pages) TI

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LM3488

LM3488-Q1

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SNVS089M – JULY 2000 – REVISED MARCH 2013

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 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 directly controls the efficiency. The critical parameters for selection of 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) 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. RDS(ON) is needed to estimate the conduction losses. The conduction loss,

PCOND, is the I

2R loss across the MOSFET. The maximum conduction loss is given by:

where

•

DMAX is the maximum duty cycle.

(26)

(27)

The turn-on and turn-off transitions of a MOSFET require times of tens of nano-seconds. CRSS and Qg are

needed to estimate the large instantaneous power loss that occurs during these transitions.

The amount of gate current required to turn the MOSFET on can be calculated using the formula:

IG = Qg.FS

(28)

The required gate drive power to turn the MOSFET on is equal to the switching frequency times the energy

required to deliver the charge to bring the gate charge voltage to VDR (see Electrical Characteristics and Typical

Performance Characteristics for the drive voltage specification).

PDrive = FS.Qg.VDR

(29)

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 33. 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 by:

(30)

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 10µF to 20µF. If a value lower than 10µF 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

Ω resistor at the input to provide a 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 35). A 0.1µF or 1µF ceramic

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

of the resistor with the input power supply.

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