Texas Instruments LM3423 Evaluation Board LM3423BBLSCSEV/NOPB LM3423BBLSCSEV/NOPB Datenbogen
Produktcode
LM3423BBLSCSEV/NOPB
CSH
R
CSH
LM3421/23
V
CC
R
BIAS
R
MAX
Q6
Q7
R
ADJ
Q8
R
ADJ
Variable Current Source
Variable
Resistance
Resistance
V
REF
V
O
= 76V - V
IN
V
HSP
< 76V
V
HSN
> 3.5V
SNVS574E – JULY 2008 – REVISED MAY 2013
The selection of the three resistors (R
SNS
, R
CSH
, and R
HSP
) is not arbitrary. For matching and noise performance,
the suggested signal current I
CSH
is approximately 100 µA. This current does not flow in the LEDs and will not
affect either the off-state LED current or the regulated LED current. I
CSH
can be above or below this value, but
the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize the effect of the
high-side amplifier voltage offset on LED current accuracy, the minimum V
high-side amplifier voltage offset on LED current accuracy, the minimum V
SNS
is suggested to be 50 mV. Finally,
a resistor (R
HSN
= R
HSP
) should be placed in series with the HSN pin to cancel out the effects of the input bias
current (~10 µA) of both inputs of the high-side sense amplifier.
The sense resistor (R
SNS
) can be placed anywhere in the series string of LEDs as long as the voltage at the HSN
and HSP pins (V
HSP
and V
HSN
) satisfies the following conditions.
(10)
Typically, for a buck-boost configuration, R
SNS
is placed at the bottom of the string (LED-) which allows for
greater flexibility of input and output voltage. However, if there is substantial input voltage ripple allowed, it can
help to place R
help to place R
SNS
at the top of the string (LED+) which limits the output voltage of the string to:
(11)
Note that he CSH pin can also be used as a low-side current sense input regulated to 1.24V. The high-side
sense amplifier is disabled if HSP and HSN are tied to AGND (or V
sense amplifier is disabled if HSP and HSN are tied to AGND (or V
HSN
> V
HSP
) .
ANALOG DIMMING
The CSH pin can be used to analog dim the LED current by adjusting the current sense voltage (V
SNS
). 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 19. 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.
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.
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.
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.
14
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