LM3402HV datasheet(25/38 Pages) TI

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LM3402, LM3402HV

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SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

CO = 1/(2 x π x 0.157 x 4.68 x 10

5) = 2.18 µF

(34)

This calculation assumes that impedance due to the equivalent series resistance (ESR) and equivalent series

inductance (ESL) of CO is negligible. The closest 10% tolerance capacitor value is 2.2 µF. The capacitor used

should be rated to 10V or more and have an X7R dielectric. Several manufacturers produce ceramic capacitors

with these specifications in the 0805 case size. A typical value for ESR of 1 m

Ω can be read from the curve of

impedance vs. frequency in the product datasheet.

INPUT CAPACITOR

Following the calculations from the Input Capacitor section,

ΔvIN(MAX) will be 1%P-P = 240 mV. The minimum

required capacitance is:

CIN(MIN) = (0.35 x 300 x 10

-9) / 0.24 = 438 nF

(35)

In expectation that more capacitance will be needed to prevent power supply interaction a 1.0 µF ceramic

capacitor rated to 50V with X7R dielectric in a 1206 case size will be used. From the Design Considerations

section, input rms current is:

IIN-RMS = 0.35 x Sqrt(0.154 x 0.846) = 126 mA

(36)

Ripple current ratings for 1206 size ceramic capacitors are typically higher than 1A, more than enough for this

design.

RECIRCULATING DIODE

The first parameter for D1 which must be determined is the reverse voltage rating. Schottky diodes are available

at reverse ratings of 30V and 40V, often in the same package, with the same forward current rating. To account

for ringing a 40V Schottky will be used.

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

cycle (D = 3.7 / 24 = 15%) requires the recirculating diode D1 to carry the load current much longer than the

internal power MOSFET of the LM3402. The estimated average diode current is:

ID = 0.35 x 0.85 = 298 mA

(37)

Schottky diodes are available at forward current ratings of 0.5A, however the current rating often assumes a

25°C ambient temperature and does not take into account the application restrictions on temperature rise. A

diode rated for higher current may be needed to keep the temperature rise below 40°C.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-123 in a 40V, 0.5A Schottky diode at 350 mA is approximately

0.4V and the

θJA is 206°C/W. Power dissipation and temperature rise can be calculated as:

PD = 0.298 x 0.4 = 119 mW TRISE = 0.119 x 206 = 24.5°C

(38)

According to these calculations the SOD-123 diode will meet the requirements. Heating and dissipation are

among the factors most difficult to predict in converter design. If possible, a footprint should be used that is

capable of accepting both SOD-123 and a larger case size, such as SMA. A larger diode with a higher forward

current rating will generally have a lower forward voltage, reducing dissipation, as well as having a lower

θJA,

reducing temperature rise.

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. This term 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 = 0.35 x 3.7 = 1.295W

(39)

Conduction loss, PC, in the internal MOSFET:

PC = (IF

2 x R

DSON) x D = (0.35

2 x 1.5) x 0.154 = 28 mW

(40)

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