Datasheet
I
LIM
=
245 mV
R
LIM
LM3421/23
I
T
PWM
COMP
IS
R
LIM
Q1
GATE
LEB
PGND
0.245V
0.8V
R
DS-ON
Sensing
R
LIM
Sensing
I
LED
= (I
CSH
- I
ADD
) x
R
HSP
R
SNS
¸
¹
·
¨
©
§
R
ADJ
x V
REF
I
ADD
=
R
ADJ
+ R
MAX
- V
BE-Q6
¸
¹
·
¨
©
§
R
BIAS
LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com
SNVS574E –JULY 2008–REVISED MAY 2013
Method 2 provides a complete dimming range and better noise performance, though it is more complex. It
consists of a PNP current mirror and a bias network consisting of an NPN, 2 resistors and a potentiometer
(R
ADJ
), where R
ADJ
controls the amount of current sourced into the CSH pin. A higher resistance value will source
more current into the CSH pin causing less regulated signal current through R
HSP
, effectively dimming the LEDs.
V
REF
should be a precise external voltage reference, while Q7 and Q8 should be a dual pair PNP for best
matching and performance. The additional current (I
ADD
) sourced into the CSH pin can be calculated:
(12)
The corresponding I
LED
for a specific I
ADD
is:
(13)
CURRENT SENSE/CURRENT LIMIT
The LM3421/23 achieves peak current mode control using a comparator that monitors the main MosFET (Q1)
transistor current, comparing it with the COMP pin voltage as shown in Figure 20. Further, it incorporates a
cycle-by-cycle over-current protection function. Current limit is accomplished by a redundant internal current
sense comparator. If the voltage at the current sense comparator input (IS) exceeds 245 mV (typical), the on
cycle is immediately terminated. The IS input pin has an internal N-channel MosFET which pulls it down at the
conclusion of every cycle. The discharge device remains on an additional 210 ns (typical) after the beginning of a
new cycle to blank the leading edge spike on the current sense signal. The leading edge blanking (LEB)
determines the minimum achievable on-time (t
ON-MIN
).
Figure 20. Current Sense / Current Limit Circuitry
There are two possible methods to sense the transistor current. The R
DS-ON
of the main power MosFET can be
used as the current sense resistance because the IS pin was designed to withstand the high voltages present on
the drain when the MosFET is in the off state. Alternatively, a sense resistor located in the source of the MosFET
may be used for current sensing, however a low inductance (ESL) type is suggested. The cycle-by-cycle current
limit (I
LIM
) can be calculated using either method as the limiting resistance (R
LIM
):
(14)
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