LM3404MASLASHNOPB datasheet(30/39 Pages) TI1

30 / 39 page

LM3404, LM3404HV

SNVS465F – OCTOBER 2006 – REVISED MAY 2013

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Conduction loss, PC, in the internal MOSFET:

PC = (IF

2 x R

DSON) x D = (0.5

2 x 0.8) x 0.73 = 146 mW

(75)

Gate charging and VCC loss, PG, in the gate drive and linear regulator:

PG = (IIN-OP + fSW x QG) x VIN PG = (600 x 10

-6 + 2.23 x 105 x 6 x 10-9) x 48 = 94 mW

(76)

Switching loss, PS, in the internal MOSFET:

PS = 0.5 x VIN x IF x (tR + tF) x fSW PS = 0.5 x 48 x 0.5 x 40 x 10

-9 x 2.23 x 105 = 107 mW

(77)

AC rms current loss, PCIN, in the input capacitor:

PCIN = IIN(rms)

2 x ESR = 0.2222 0.003 = 0.1 mW (negligible)

(78)

DCR loss, PL, in the inductor

PL = IF

2 x DCR = 0.52 x 0.56 = 140 mW

(79)

Recirculating diode loss, PD = 47 mW

Current Sense Resistor Loss, PSNS = 110 mW

Electrical efficiency,

η = PO / (PO + Sum of all loss terms) = 17.6 / (17.6 + 0.644) = 96%

Temperature Rise in the LM3404HV IC is calculated as:

TLM3404 = (PC + PG + PS) x θJA = (0.146 + 0.094 + 0.107) x 155 = 54°C

(80)

Layout Considerations

The performance of any switching converter depends as much upon the layout of the PCB as the component

selection. The following guidelines will help the user design a circuit with maximum rejection of outside EMI and

minimum generation of unwanted EMI.

COMPACT LAYOUT

Parasitic inductance can be reduced by keeping the power path components close together and keeping the

area of the loops that high currents travel small. Short, thick traces or copper pours (shapes) are best. In

particular, the switch node (where L1, D1, and the SW pin connect) should be just large enough to connect all

three components without excessive heating from the current it carries. The LM3404/04HV operates in two

distinct cycles whose high current paths are shown in Figure 32:

Figure 32. Buck Converter Current Loops

The dark grey, inner loop represents the high current path during the MOSFET on-time. The light grey, outer loop

represents the high current path during the off-time.

GROUND PLANE AND SHAPE ROUTING

The diagram of Figure 32 is also useful for analyzing the flow of continuous current vs. the flow of pulsating

currents. The circuit paths with current flow during both the on-time and off-time are considered to be continuous

current, while those that carry current during the on-time or off-time only are pulsating currents. Preference in

routing should be given to the pulsating current paths, as these are the portions of the circuit most likely to emit

EMI. The ground plane of a PCB is a conductor and return path, and it is susceptible to noise injection just as

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