Texas Instruments LM3429 Evaluation Boards LM3429BKBSTEVAL/NOPB LM3429BKBSTEVAL/NOPB Datenbogen
Produktcode
LM3429BKBSTEVAL/NOPB
LM3429
I
T
PWM
COMP
IS
R
LIM
Q1
GATE
LEB
PGND
245 mV
800 mV
CSH
R
CSH
LM3429
V
CC
R
BIAS
R
MAX
Q6
Q7
V
REF
R
ADJ
Q8
R
ADJ
Variable Current Source
Variable
Resistance
Resistance
R
HSP
I
LED
= I
CSH
- I
ADD
x
R
SNS
¸
¹
·
¨
©
§
( )
R
ADJ
x V
REF
I
ADD
=
R
ADJ
+ R
MAX
- V
BE-Q6
¸
¹
·
¨
©
§
R
BIAS
SNVS616G – APRIL 2009 – REVISED MAY 2013
(10)
The corresponding I
LED
for a specific I
ADD
is:
(11)
Figure 18. Analog Dimming Circuitry
CURRENT SENSE/CURRENT LIMIT
The LM3429 achieves peak current mode control using a comparator that monitors the MosFET transistor
current, comparing it with the COMP pin voltage as shown in
current, comparing it with the COMP pin voltage as shown in
. 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 250 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
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 250 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 19. Current Sense / Current Limit Circuitry
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13
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