LM3431MH National Semiconductor, LM3431MH Datasheet - Page 14

• Manufacturer Part Number: LM3431MH
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

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INPUT CAPACITOR SELECTION
Because the inductor is at the input of a boost converter, the
input current waveform is continuous and triangular. The in-
ductor ensures that the input capacitor sees relatively low
ripple currents. The rms current in the input capacitor is given
by:
The input capacitor must be capable of handling this rms cur-
rent. Input ripple voltage increases with increasing ESR as
well as decreasing input capacitance. A typical value of 10 µF
will work well for most applications. For low input voltages,
additional input capacitance may be required to prevent trip-
ping the UVLO. Additionally, a ceramic capacitor of 1 µF or
larger should be placed close to the VIN pin to prevent noise
from interfering with normal device operation.
OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the out-
put current when the switch is on and the inductor is charging.
As a result, the output capacitor sees relatively large ripple
currents. The output capacitor must be capable of handling
more than the rms current, which can be estimated as:
Additionally, the ESR of the output capacitor affects the output
ripple and has an effect on transient response during dim-
ming. For low output ripple voltage, low ESR ceramic capac-
itors are recommended. Although not a critical parameter,
excessive output ripple can affect LED current.
The output capacitance requirement is somewhat arbitrary
and depends mostly on dimming frequency. Although a min-
imum value of 4 µF is recommended, at lower dimming fre-
quencies, the longer LED-off times will typically require more
capacitance to reduce output voltage transients.
When ceramic capacitors are used, audible noise may be
generated during LED dimming. Audible noise increases with
the amplitude of output voltage transients. To minimize this
noise, use the smallest case sizes and if possible, use a larger
number of capacitors in parallel to reduce the case size of
each. Output transients are also minimized via the FF pin
(See Setting FF section). Setting the dimming frequency
above 18 kHz or below 500 Hz will also help eliminate the
audible effects of output voltage transients.
When selecting an output capacitor, always consider the ef-
fective capacitance at the output voltage, which can be less
than 50% of the capacitance specified at 0V. Use this effective
capacitance value for the compensation calculations below.
COMPENSATION
Once the output capacitor is selected, the control loop char-
acteristics and compensation can be determined. The COMP
pin is provided to ensure stable operation and optimum tran-
sient performance over a wide range of applications. The
following equations define the control-to-output or power
stage of the loop:
14
Where R
rent, and K
Since the control-to-output response will shift with input volt-
age, the compensation should be calculated at both the min-
imum and maximum input voltage.
The zero created by the ESR of the output capacitor, f
generally at a very high frequency if the ESR is small. If low
ESR capacitors are used f
capacitors are used, C
A current mode control boost regulator has an inherent right
half plane zero, RHPz. This has the effect of a zero in the gain
plot, causing a +20dB/decade increase, but has the effect of
a pole in the phase, subtracting 90° in the phase plot. This
can cause instability if the control loop is influenced by this
zero. To ensure the RHP zero does not cause instability, the
control loop must be designed to have a bandwidth of less
than one third the frequency of the RHP zero. The regulator
also has a double pole, fpn, at one half the switching frequen-
cy. The control loop bandwidth must be lower than 1/5 of fpn.
A typical control-to-output gain response is shown in Figure
6 below.
Once the control-to-output response has been determined,
the compensation components are selected. A series combi-
FIGURE 6. Typical Control-to-Output Bode Plot
L
is the load resistance corresponding to LED cur-
f
is calculated as shown:
C2
can be added (see below).
z1
can be neglected and if high ESR
30041140
z1
, is