Datasheet

V
FB
2
R
SET
P
RSET
=
D
2
xx
=
R
DSON
¸
¸
¹
·
¨
¨
©
§
D
'
P
NFET-COND
I
LED
Isw-rms = I
IND
D
1 +
'i
I
IND
1
3
2
x
D
I
IND
|
t
'i
I
IN
I
SW(t)
LM3410, LM3410Q
SNVS541G –OCTOBER 2007–REVISED MAY 2013 www.ti.com
Figure 23. LM3410 Switch Current
(27)
(small ripple approximation)
P
COND-NFET
= I
IN
2
x R
DSON
x D (28)
Or
(29)
The value for R
DSON
should be equal to the resistance at the junction temperature you wish to analyze. As an
example, at 125°C and R
DSON
= 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:
P
SWR
= 1/2(V
OUT
x I
IN
x f
SW
x t
RISE
) (30)
P
SWF
= 1/2(V
OUT
x I
IN
x f
SW
x t
FALL
) (31)
P
SW
= P
SWR
+ P
SWF
(32)
Table 4. Typical Switch-Node Rise and Fall Times
V
IN
V
OUT
t
RISE
t
FALL
3V 5V 6nS 4nS
5V 12V 6nS 5nS
3V 12V 8nS 7nS
5V 18V 10nS 8nS
Quiescent Power Losses
I
Q
is the quiescent operating current, and is typically around 1.5 mA.
P
Q
= I
Q
x V
IN
(33)
R
SET
Power Loss
(34)
Example Efficiency Calculation:
Operating Conditions:
5 x 3.3V LEDs + 190mV
REF
≊ 16.7V (35)
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