Datasheet

6 mV x 100
REFIN
+
'E x100
2 x E
2
2 x A
R10
2
+
Acc_s-s% = r
2
5 mV x 100
REFIN
+
'E x 100
2 x E
2
Acc_single% = r A
R10
+ 2% + A
R7
+ A
R8
+
LM3431
www.ti.com SNVS547G –NOVEMBER 2007–REVISED MAY 2013
Although the TSD threshold is 160°C, the LM3431 may not operate within specification at temperatures above
the maximum rating of 125°C. Power dissipation should be limited to ensure that device temperature stays within
this limit.
Temperature Coefficients
Several device specifications are designed to vary with temperature. To maintain optimum headroom control and
minimum NPN power dissipation, CFB regulation has a tempco of -2.6 mV/°C. This is matched to the typical
tempco of the small signal diodes used for the cathode feedback connection. Although the CFB voltage will vary
with temperature, the cathode voltage will remain stable. The SS/SH pin rises to 1.85V typically during soft start.
This voltage has a tempco of approximately -2.2 mV/°C, which is designed to follow the tempco of the LED
strings. At then end of soft start, the anode voltage will be greater than the maximum LED forward voltage,
regardless of operating temperature. To avoid false errors, the AFB overvoltage threshold has a tempco of -1.4
mV/°C. Of course, these temperature monitoring features are most effective with the LM3431 mounted within the
same ambient temperature as the LEDs.
LEDOFF: Adding Additional channels
Although the LM3431 has three internal current controllers, more channels can easily be added. A fourth LED
string is shown in Figure 14 connected to VC4.
For additional channels, the sense resistor should be the same value as the main three channels. During startup
and dimming off time, LEDOFF rises to 5V, which quickly turns off the external driver. While the LED strings are
on, the LEDOFF signal is low, allowing normal regulation. If LEDOFF is used to add additional channels, it
cannot be used to enable auto-restart mode.
All additional channels must also be connected through diodes to the SC and CFB pins as shown in the typical
application schematic. The op-amp used to drive the additional channel current regulator must be fast enough to
drive the regulator fully on within the DLY blanking time. A slew rate of 5V/µsec is typically sufficient. Also, the
op-amp output must be capable of completely turning off the NPN regulator, which requires a drive voltage no
greater than the REFIN voltage. A rail-to-rail type op-amp is recommended.
Finally, the R14 resistor should be large enough to limit V
CC
current during the LED-off cycle. A value of at least
1k is recommended. Any additional channels will have a longer turn-on delay time than channels 1-3. An
additional delay time of 250 ns is typical. The added delay can affect dimming linearity at on times less than 1 µs.
LED Current Accuracy
LED string current accuracy is affected by factors both internal and external to the LM3431. For any single string
the maximum deviation from ideal is simply the sum of the sense resistor, offset error, REF voltage, REFIN
resistor divider accuracy, and bipolar gain variation:
Where
• A
R10
is the sense resistor % accuracy
• 2% is the REF voltage accuracy, A
R7
• A
R8
are the REFIN setting resistors % accuracy
• 5 mV is the maximum SNS amp offset voltage (use 3 mV for LM3431A)
• β is the gain of NPN transistor
• Δβ is the specified range of gain in the NPN (40)
The string-to-string accuracy is the maximum difference in current between any two strings. It is best calculated
using the RSS method:
Where
• 6 mV is the maximum SNS amp delta offset voltage (V
OS_DELTA
over temperature, use 4 mV for LM3431A) and
we are assuming the sense resistors have the same accuracy rating (41)
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