LM3431MHX National Semiconductor, LM3431MHX Datasheet - Page 20

• Manufacturer Part Number: LM3431MHX
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

www.national.com
During startup (and re-start), the THM monitor is active.
Therefore, the thermistor temperature must be below the
restart threshold for the LM3431 to startup.
TSD
If the LM3431 internal junction temperature increases above
160°C TSD is activated. This is a Type 3 fault condition. De-
vice temperature rise is determined by internal power dissi-
pation primarily in the LG and NDRVx drivers. The power
dissipation can be estimated as follows:
Where I
Where REFIN+Vbe is the NDRV voltage and I
culated previously in the NDRV section. For the case of open
LEDs, I
mA. The LM3431 power dissipation will be highest in open
LED conditions at 100% dimming duty. If NFETs are used for
regulation, P
can be calculated as:
Where I
as external op-amp power, or THM voltage divider.
The LG power dissipation, PG is given in the NFET section.
Temperature rise can then be calculated as:
Where θ
(Refer to the PCB Layout section).
Although the TSD threshold is 160°C, the LM3431 may not
operate within specification at temperatures above the maxi-
mum 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 tem-
perature. To maintain optimum headroom control and mini-
mum 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 connec-
tion. 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 volt-
age, 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 am-
bient 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 1 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
P
NDRV
NDRV
VCC
Q
JA
is 4.0 mA typically.
= (V
is typically 32°C/W and varies with pcb copper area
is any current being drawn from the VCC pin, such
on the open string will be at the maximum of 15
NDRV
IN
P
P
D
- REFIN - Vbe) x I
NDRV_FET
will be a function of dimming frequency and
P
= P
VCC
IQ
T
= (V
P
RISE
+ P
IQ
= f dim x Q
= V
NDRV
IN
= P
- V
IN
D
CC
+ P
x I
x θ
NDRV
) x I
Q
JA
VCC
g
VCC
x V
x D
+ P
IN
DIM
G
NDRV
x #Strings
was cal-
20
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 appli-
cation 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
current during the LED-off cycle. A value of at least 1k is rec-
ommended. 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 inter-
nal 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 ac-
curacy, and bipolar gain variation:
Where A
voltage accuracy, A
% accuracy, 5 mV is the maximum SNS amp offset voltage
(use 3 mV for LM3431A), β is the gain of NPN transistor, and
Δβ is the specified range of gain in the NPN. 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
are assuming the sense resistors have the same accuracy
rating. If FETs are used, the β term can be ignored in both
equations. The LED current in each string will be within
±Acc_single% of the set current. And the difference between
any two strings will be within ±Acc_s-s% of each other.
PCB LAYOUT
Good PCB layout is critical in all switching regulator designs.
A poor layout can cause EMI problems, excess switching
noise, and improper device operation. The following key
points should be followed to ensure a quality layout.
Traces carrying large AC currents should be as wide and
short as possible to minimize trace inductance and associat-
ed noise spikes.
These areas, shown hatched in Figure 15, are:
- The connection between the output capacitor and diode
- The PGND area between the output capacitor, R3 sense
resistor, and bulk input capacitor
- The switch node
The current sensing circuitry in current mode controllers can
be easily affected by switching noise. Although the LM3431
imposes 170ns of blanking time at the beginning of every cy-
OS_DELTA
R10
over temperature, use 4 mV for LM3431A) and we
is the sense resistor % accuracy, 2% is the REF
R7
and A
R8
are the REFIN setting resistors
CC