LM3429_14 Datasheet, PDF (13/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
www.ti.com
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
1. External variable resistance : Adjust a potentiometer placed in series with R_CSH to vary V_SNS.
2. External variable current source: Source current (0 µA to I_CSH) into the CSH pin to adjust V_SNS.
In general, analog dimming applications require a lower switching frequency to minimize the effect of the leading
edge blanking circuit. As the LED current is reduced, the output voltage and the duty cycle decreases.
Eventually, the minimum on-time is reached. The lower the switching frequency, the wider the linear dimming
range. Figure 6 shows how both methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path which is a simple addition to the existing circuitry.
However, the LEDs cannot dim completely because there is always some resistance causing signal current to
flow. This method is also susceptible to noise coupling at the CSH pin since the potentiometer increases the size
of the signal current loop.
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:
§
¨
R_ADJ
x
©^{RADJ +}
V_{REF ¸·}
R_{MAX ¹}
-
V_BE-Q6
I_ADD =
R_BIAS
(10)
The corresponding I_LED for a specific I_ADD is:
( ) ^{§ · RHSP}
¨© ¸¹ ^{ILED = ICSH - IADD x} _RSNS
(11)
Variable Current Source
V_REF
Q7
Q8
V_CC ^LM3429
R_MAX
Q6
R_ADJ
R_BIAS
CSH
R_CSH
R_ADJ
Variable
Resistance
Figure 6. 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 Figure 7. 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_ON-MIN).
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