LM3477AMM datasheet(18/23 Pages) NSC

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Power MOSFET Selection (Continued)

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 input voltage. V

DS(MAX) of the MOSFET must be

greater than the input 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:

where D

MAX is the maximum operating duty cycle:

The turn-on and turn-off transition times of a MOSFET from

the MOSFET specifications require tens of nano-seconds.

RSS and Qg are needed from the MOSFET specifications to

estimate the large instantaneous power loss that occurs

during these transitions.

The average amount of gate current required to turn the

MOSFET on can be calculated using the formula:

G =Qg.FS

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 V

(see ELECTRICAL CHARACTERISTICS and TYPICAL

PERFORMANCE CHARACTERISTICS for the drive voltage

specification).

Drive =FS.Qg.VDR

It is sometimes helpful or necessary to slow down the turn on

transition of the FET so that less switching noise appears at

the I

SEN pin. This can be done by inserting a drive resistor

DR in series with the boot-strap capacitor (see Figure 6).

This can help reduce sensing noise that may be preventing

designs from operating at or near the LM3477/A’s minimum

duty cycle limit. Gate drive resistors from 2.2

Ω to 51Ω are

recommended.

Power Diode Selection

The output current commutates through the diode when the

external MOSFET turns off. The three most important pa-

rameters for the diode are the peak current, peak inverse

voltage, and average power dissipation. Exceeding these

ratings can cause damage to the diode. The average current

through the diode is given by:

D(AVG) =IOUT x (1-D)

where D is the duty cycle and I

OUT is the output current. The

diode must be rated to handle this current.

The off-state voltage across the diode in a buck converter is

approximately equal to the input voltage. The peak inverse

voltage rating of the diode must be greater than the off-state

voltage of the diode. To improve efficiency, a low forward

drop schottky diode is recommended.

Input Capacitor Selection

In a buck converter, the high side switch draws large ripple

currents from the input capacitor. The input capacitor must

be rated to handle this RMS current.

The power dissipated in the input capacitor is given by:

D(CIN)=IRMS_CIN

ESR_CIN,

where R

ESR_CIN is the ESR of the input capacitor. The input

capacitor must be selected to handle the rms current and

must be able to dissipate the power. P

D(CIN) must be lower

than the rated power dissipation of the selected input capaci-

tor. In many cases, several capacitors have to be paralleled

to handle the rms current. In that case, the power dissipated

in each capacitor is given by:

D(CIN) =(I

RMS_CINRESR_CIN)/n

2, where n is the total num-

ber of capacitors paralled at the input.

A 0.1µF or 1µF ceramic bypass capacitor is also recom-

mended on the V

IN pin (pin 8) of the IC. This capacitor must

be connected very close to pin 8.

Compensation

The LM3477/A is a current mode controller, therefore the

control block diagram representation involves 2 feedback

loops (see Figure 12). The inner feedback loop derives its

feedback from the sensed inductor current, while the outer

loop monitors the output voltage. This section will not give a

rigorous analysis of current mode control, but rather a sim-

plified but accurate method to determine the compensation

network. The first part reveals the results of the model, giving

expressions for solving for component values in the compen-

sation network.

The compensation network is designed around the power

components, or the power stage. An isolated schematic of

the error amplifier and the various compensation compo-

nents is shown in Figure 13. The error amplifier in conjunc-

tion with the compensation network makes up the compen-

sator block in Figure 12. The purpose of the compensator

block is to stabilize the control loop and achieve high perfor-

mance in terms of the transient response, audio susceptibil-

ity and output impedance.

20003391

FIGURE 12. Control Block Diagram of a Current Mode

Controlled Buck Converter

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