User manual
t
ON
= k x
R
ON
V
IN
R
SNS
=
V
IN
- V
OUT
2L
x t
ON
+
V
OUT
x t
D
L
(I
F
) -
0.20V
0.2V = R
SNS
V
IN
- V
OUT
2L
x t
ON
+
V
OUT
L
x t
D
I
F
-
i
TARGET
= I
F
-
V
IN
- V
OUT
2L
x t
ON
+
V
OUT
L
x t
D
V
OUT
L
'i
D
=
t
D
I
TARGET
= I
F
-
'i
L
2
+'i
D
=
V
IN
- V
OUT
2L
'i
2
x t
ON
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Standard On-Time Set Calculation
Where
I
F
= I
LED-Average
(3)
The delta of the inductor current is given by:
(4)
There is a 220 ns delay (t
D
) from the time that the current sense comparator trips to the time at which the
control MOSFET actually turns on. We can solve for i
TARGET
knowing there is a delay.
(5)
Δi
D
is the magnitude of current beyond the target current and equal to:
(6)
Therefore:
(7)
The point at which you want the current sense comparator to give the signal to turn on the FET equals:
i
TARGET
x R
SNS
= 0.20V (8)
Therefore:
(9)
Finally R
SNS
can be calculated.
(10)
7 Standard On-Time Set Calculation
The control MOSFET on-time is variable, and is set with an external resistor R
ON
(R2 from Figure 1). On-
time is governed by the following equation:
(11)
Where
k = 1.34 x 10
-10
(12)
At the conclusion of t
ON
the control MOSFET turns off for a minimum OFF time (t
OFF-MIN
) of 300 ns, and
once t
OFF-MIN
is complete the CS comparator compares V
SNS
and V
REF
again, waiting to begin the next cycle.
The LM3402/04 have minimum ON and OFF time limitations. The minimum on time (t
ON
) is 300 ns, and
the minimum allowed off time (t
OFF
) is 300 ns.
Designing for the highest switching frequency possible means that you will need to know when minimum
ON and OFF times are observed.
Minimum OFF time will be seen when the input voltage is at its lowest allowed voltage, and the output
voltage is at its maximum voltage (greatest number of series LEDs).
The opposite condition needs to be considered when designing for minimum ON time. Minimum ON time
is the point at which the input voltage is at its maximum allowed voltage, and the output voltage is at its
lowest value.
5
SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current
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