Datasheet

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
CSH
R
CSH
LM3424
V
CC
R
BIAS
R
MAX
Q6
Q7
R
ADJ
Q8
R
ADJ
Variable Current Source
Variable
Resistance
V
S
I
LED
= (I
CSH
- I
TF
) x
R
HSP
R
SNS
¸
¹
·
¨
©
§
LM3424
www.ti.com SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
Since the NTC is not usually local to the controller, a bypass capacitor (C
NTC
) is suggested from TSENSE to
GND. If a capacitor is used at TSENSE, then a capacitor (C
REF
) of equal or greater value should be placed from
TREF to GND in order to ensure the controller does not start-up in foldback. Alternatively, a smaller C
REF
can be
used to create a fade-up function at start-up (see APPLICATIONS INFORMATION section).
Thermal foldback is simply analog dimming according to a specific profile, therefore any method of controlling the
differential voltage between TREF and TSENSE can be use to analog dim the LED current. The corresponding
LED current for any V
DIF
> 0V is defined:
(10)
The CSH pin can also be used to analog dim the LED current by adjusting the current sense voltage (V
SNS
),
similar to thermal foldback. There are several different methods to adjust V
SNS
using the CSH pin:
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
.
Figure 24. Analog Dimming Circuitry
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 24 shows how both CSH 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. Like
thermal foldback, it simply sources current into the CSH pin, decreasing the amount of signal current that is
necessary. This method 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. 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:
(11)
The corresponding I
LED
for a specific I
ADD
is:
(12)
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