LM3431AQMH/NOPB National Semiconductor, LM3431AQMH/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM3431AQMH/NOPB
• Description: IC LED DRVR HP CONS CURR 28TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3431AQMH/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 3
• Internal Driver: No
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 651kHz ~ 1.1MHz
• Voltage - Supply: 4.5 V ~ 36 V
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 200mA
• Internal Switch(s): Yes
• Efficiency: 88%
• Led Driver Application: Display Backlighting, Automotive Lighting
• No. Of Outputs: 3
• Output Current: 200mA
• Output Voltage: 40V
• Input Voltage: 5V To 37V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3431AQMH

Where I
ming. I
Then determine the minimum L based on Δi
ing equation:
A good starting point is to set Δi
(AVE)
mended Δi
in normal operation. In general a smaller inductor (higher rip-
ple current) will give a better dimming response due to the
higher dI/dt. This is shown graphically below.
The resulting peak to peak inductor current is:
And the resulting peak inductor current is:
Peak inductor current will occur at minimum V
The inductor must be rated to handle both the average current
and peak current, which is the same as the peak switch cur-
rent. As switching frequency increases, less inductance is
required. However, some minimum inductance value is re-
quired to ensure stability at duty cycles greater than 50%. The
minimum inductance required for stability can be calculated
as:
Where R3 is the sense resistor determined in the next section.
Although the inductor must be large enough to meet both the
and calculate using that value. The maximum recom-
FIGURE 5. Inductor Current During Dimming
L(AVE)
OUT
L
is the sum of all LED string currents at 100% dim-
is 200% of I
will be highest at the minimum input voltage.
L(AVE)
to maintain continuous current
L
to 150% of the minimum I
30041129
L
with the follow-
IN
.
L
13
stability and the Δi
will typically give the best performance.
CURRENT SENSING
Switch current is sensed via the sense resistor, R3, while the
switch is on and the inductor is charging. The sensed current
is used to control switching and to monitor current limit. To
optimize the control signal, a typical sense voltage between
50mV and 200mV is recommended. The sense resistor can
therefore be calculated by the following equation:
Since I
mined based on the full input voltage range, although the
resulting value may extend somewhat outside the recom-
mended range.
CURRENT LIMIT
Current limit occurs when the voltage across the sense re-
sistor (measured at the CS pin) equals the current limit thresh-
old voltage. The current limit threshold is set by R4. This value
can be calculated as follows:
Where 40 µA is the typical ILIM source current in current limit
and ILlim is the peak (not average) inductor current which
triggers current limit.
To avoid false triggering, current limit should be set safely
above the peak inductor current level. However, the current
limit resistor also has some effect on the control loop as seen
in the block diagram. For this reason, R4 should not be set
much higher than necessary. When current limit is activated,
the NFET will be turned off immediately until the next cycle.
Current limit will typically result in a drop in output and cathode
voltage. This will cause the COMP pin voltage to increase to
maximum, which will trigger a fault and start the DLY pin
source current (see LED Protection section). The LM3431 will
continue to operate in current limit with reduced on-time until
the DLY pin has reached its threshold. However, the current
limit cannot reduce the on-time below the minimum specifi-
cation.
In a boost switcher, there is a direct current path between
input and output. Therefore, although the LM3431 will shut-
down in a shorted output condition, there are no means to limit
the current flowing from input to output.
Note that if the maximum duty cycle of 85% (typical) is
reached, the LM3431 will behave as though current limit has
occurred.
VCC
The VCC pin is the output of the internal voltage regulator. It
must be bypassed to PGND with a minimum 4.7 µF ceramic
capacitor. Although VCC is capable of supplying up to 72 mA,
external loads will increase the power dissipation and tem-
perature rise within the LM3431. See the TSD section for
more detail. Above 72 mA, the VCC voltage will drop due to
current limit. Since the UVLO threshold is monitored at this
pin, UVLO may be enabled by a VCC over current event.
For input voltages between 4.5V and 5.5V, connect VCC to
V
LO threshold and allow operation at input voltages as low as
IN
through a 4.7Ω resistor. This will hold VCC above the UV-
L(AVE)
will vary with input voltage, R3 should be deter-
L
requirements, a value close to minimum
www.national.com