LM3410XMFE datasheet(20/49 Pages) TI

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VFB

RSET

PRSET =

RDSON

¸¸

¨¨

NFET

COND

ILED

Isw-rms = IIND D

1 +

IIND

IIN

ISW(t)

LM3410, LM3410Q

SNVS541G – OCTOBER 2007 – REVISED MAY 2013

www.ti.com

Figure 23. LM3410 Switch Current

(27)

(small ripple approximation)

PCOND-NFET = IIN

2 x R

DSON x D

(28)

(29)

The value for RDSON should be equal to the resistance at the junction temperature you wish to analyze. As an

example, at 125°C and RDSON = 250 mΩ (See typical graphs for value).

Switching losses are also associated with the internal power switch. They occur during the switch on and off

transition periods, where voltages and currents overlap resulting in power loss.

The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the

switch at the switch node:

PSWR = 1/2(VOUT x IIN x fSW x tRISE)

(30)

PSWF = 1/2(VOUT x IIN x fSW x tFALL)

(31)

PSW = PSWR + PSWF

(32)

Table 4. Typical Switch-Node Rise and Fall Times

VIN

VOUT

tRISE

tFALL

6nS

4nS

12V

6nS

5nS

12V

8nS

7nS

18V

10nS

8nS

Quiescent Power Losses

IQ is the quiescent operating current, and is typically around 1.5 mA.

PQ = IQ x VIN

(33)

RSET Power Loss

(34)

Example Efficiency Calculation:

Operating Conditions:

5 x 3.3V LEDs + 190mVREF ≊ 16.7V

(35)

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