LM3431AQMHX/NOPB National Semiconductor, LM3431AQMHX/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM3431AQMHX/NOPB
• Description: IC LED DRVR HP CONS CURR 28TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3431AQMHX/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%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3431AQMHX

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Once the control-to-output response has been determined,
the compensation components are selected. A series combi-
nation of Rc and Cc is recommended for the compensation
network, shown as R9 and C4 in the typical application circuit.
The series combination of Rc and Cc introduces a pole-zero
pair according to the following equations:
where R
proximately 500 kΩ. The initial value of R
based on the required crossover frequency from the following
equations using the maximum input voltage:
Where B is the mid-frequency compensation gain (in v/v), R4
is the current limit setting resistor, Acm is the control-output
DC gain, and the gm values are given in the electrical char-
acteristic table. Fcross is the maximum allowable crossover
frequency, based on the calculated values of f
Any Rc value lower than the value calculated above can be
used and will ensure a low enough crossover frequency. Rc
should set the B value typically between 0.01v/v and 0.1v/v
(-20db to -40db). Larger values of R
bandwidth.
However, because the dynamic response of the LM3431 is
enhanced by the FF pin (See Setting FF section) the R
can be set conservatively. The typical range for R
300ohm and 3 kΩ. Next, select a value for Cc to set the com-
pensation zero, f
maximum calculated value of f
f
acceptable, choose a standard capacitor value smaller than
calculated. Confirm that fpc, the dominant low frequency pole
in the control loop, is less than 100 Hz and below f
typical range for C
pensation zero-pole pair is shown graphically below, along
with the total control loop, which is the sum of the compen-
sation and output-control response. Since the calculated
crossover frequency is an approximation, stability should al-
ways be verified on the bench.
p1
). Since an fzc value of up to a half decade above fp1 is
O
is the output impedance of the error amplifier, ap-
zc
C
, to a frequency greater or equal to the
is between 10 nF and 100 nF. The com-
p1
(f
zc
C
cancels the power pole,
will give a higher loop
C
is determined
pn
C
and RHPz.
is between
p1
C
value
. The
16
When using an output capacitor with a high ESR value, an-
other pole, f
by the ESR. This is accomplished by adding another capaci-
tor, C
placed at the same frequency as f
lated as:
To ensure this equation is valid, and that C
without negatively impacting the effects of R
must be at least 10 times greater than f
LED Current Regulator
SETTING LED CURRENT
LED current is independently regulated in each of 3 strings by
regulating the voltage at the SNS pins. Each SNS pin is con-
nected to a sense resistor, shown in the typical application
schematic as R10 - R13. The sense resistor value is calcu-
lated as follows:
Where I
regulated voltage at the REFIN pin, and INDRV is the NPN
base drive current. If using NFETs, INDRV can be ignored. A
minimum REFIN voltage of 100 mV is required, and 200mV
to 300mV is recommended for most applications. The REFIN
voltage is set with a resistor divider connected to the REF pin,
shown as R7 and R8 in the typical application schematic. The
resistor values are calculated as follows:
The sum of R7 and R8 should be approximately 100k to avoid
excessive loading on the REF pin.
FIGURE 7. Typical Compensation and Total Loop Bode
C2
, shown as C13 in the Figure 1. The pole should be
LED
pc2
is the current in each LED string, REF
, may be introduced to cancel the zero created
Plots
z1
. This pole can be calcu-
zc
.
C2
C
30041143
can be used
and C
IN
C
is the
, f
pc2