LM3410, LM3410Q
SNVS541G –OCTOBER 2007–REVISED MAY 2013
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Di
IIN
ISW(t)
t
Figure 23. LM3410 Switch Current
2
Di
IIND
1
3
D x
Isw-rms = IIND
1 +
D
I
ö IND
(27)
(28)
(small ripple approximation)
PCOND-NFET = IIN2 x RDSON x D
Or
2
≈
’
ILED
P
x RDSON x D
=
∆
«
÷
÷
COND- NFET
'
D
◊
(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)
(31)
(32)
PSWF = 1/2(VOUT x IIN x fSW x tFALL
)
PSW = PSWR + PSWF
Table 4. Typical Switch-Node Rise and Fall Times
VIN
3V
5V
3V
5V
VOUT
5V
tRISE
6nS
tFALL
4nS
5nS
7nS
8nS
12V
12V
18V
6nS
8nS
10nS
Quiescent Power Losses
IQ is the quiescent operating current, and is typically around 1.5 mA.
PQ = IQ x VIN
(33)
(34)
(35)
RSET Power Loss
2
VFB
PRSET
=
RSET
Example Efficiency Calculation:
Operating Conditions:
5 x 3.3V LEDs + 190mVREF ≊ 16.7V
20
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