LM3402HV datasheet(28/38 Pages) TI

28 / 38 page

LM3402, LM3402HV

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

www.ti.com

In expectation that more capacitance will be needed to prevent power supply interaction a 2.2 µF ceramic

capacitor rated to 100V with X7R dielectric in an 1812 case size will be used. From the Design Considerations

section, input rms current is:

IIN-RMS = 0.35 x Sqrt(0.82 x 0.18) = 134 mA

(66)

Ripple current ratings for 1812 size ceramic capacitors are typically higher than 2A, more than enough for this

design.

RECIRCULATING DIODE

The input voltage of 60V ±5% requires Schottky diodes with a reverse voltage rating greater than 60V. Some

manufacturers provide Schottky diodes with ratings of 70, 80 or 90V; however the next highest standard voltage

rating is 100V. Selecting a 100V rated diode provides a large safety margin for the ringing of the switch node and

also makes cross-referencing of diodes from different vendors easier.

The next parameters to be determined are the forward current rating and case size. In this example the high duty

cycle (D = 49.2 / 60 = 82%) places less thermals stress on D1 and more on the internal power MOSFET of the

LM3402. The estimated average diode current is:

ID = 0.361 x 0.18 = 65 mA

(67)

A Schottky with a forward current rating of 0.5A would be adequate, however at 100V the majority of diodes have

a minimum forward current rating of 1A. To determine the proper case size, the dissipation and temperature rise

in D1 can be calculated as shown in the Design Considerations section. VD for a small case size such as SOD-

123F in a 100V, 1A Schottky diode at 350 mA is approximately 0.65V and the

θJA is 88°C/W. Power dissipation

and temperature rise can be calculated as:

PD = 0.065 x 0.65 = 42 mW TRISE = 0.042 x 88 = 4°C

(68)

CB AND CF

The bootstrap capacitor CB should always be a 10 nF ceramic capacitor with X7R dielectric. A 25V rating is

appropriate for all application circuits. The linear regulator filter capacitor CF should always be a 100 nF ceramic

capacitor, also with X7R dielectric and a 25V rating.

EFFICIENCY

To estimate the electrical efficiency of this example the power dissipation in each current carrying element can

be calculated and summed. Electrical efficiency,

η, should not be confused with the optical efficacy of the circuit,

which depends upon the LEDs themselves.

Total output power, PO, is calculated as:

PO = IF x VO = 0.361 x 49.2 = 17.76W

(69)

Conduction loss, PC, in the internal MOSFET:

PC = (IF

2 x R

DSON) x D = (0.361

2 x 1.5) x 0.82 = 160 mW

(70)

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 + 3 x 105 x 3 x 10-9) x 60 = 90 mW

(71)

Switching loss, PS, in the internal MOSFET:

PS = 0.5 x VIN x IF x (tR + tF) x fSW PS = 0.5 x 60 x 0.361 x 40 x 10

-9 x 3 x 105 = 130 mW

(72)

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

PCIN = IIN(rms)

2 x ESR = (0.134)2 x 0.006 = 0.1 mW (negligible)

(73)

DCR loss, PL, in the inductor

PL = IF

2 x DCR = 0.352 x 1.1 = 135 mW

(74)

Recirculating diode loss, PD = 42 mW

Current Sense Resistor Loss, PSNS = 69 mW

Electrical efficiency,

η = PO / (PO + Sum of all loss terms) = 17.76 / (17.76 + 0.62) = 96%

Temperature Rise in the LM3402HV IC is calculated as:

TLM3402 = (PC + PG + PS) x θJA = (0.16 + 0.084 + 0.13) x 200 = 74.8°C

(75)

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