Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB Datenbogen
Produktcode
LM3431EVAL/NOPB
f
P1
=
KD
2
S
x R
L
x C
OUT
f
Z1
=
1
2
S
x ESR x C
OUT
RHP
Z
=
2
S
x I
OUT
x L
V
OUT
x (
'¶
)
2
fpn =
2
f
SW
I
RMS_OUT
=
D
'¶
2
+
'
iL
2
12
D
¶
x I
OUT
2
x
I
RMS_IN
=
'
i
L
12
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
Fast switching FETs can cause noise spikes at the SW node which may affect performance. To reduce these
spikes a drive resistor up to 10
spikes a drive resistor up to 10
Ω
can be placed between LG and the NFET gate.
Input Capacitor Selection
Because the inductor is at the input of a boost converter, the input current waveform is continuous and triangular.
The inductor ensures that the input capacitor sees relatively low ripple currents. The rms current in the input
capacitor is given by:
The inductor ensures that the input capacitor sees relatively low ripple currents. The rms current in the input
capacitor is given by:
(20)
The input capacitor must be capable of handling this rms current. 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 tripping 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.
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 tripping 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 output 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:
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:
(21)
Additionally, the ESR of the output capacitor affects the output ripple and has an effect on transient response
during dimming. For low output ripple voltage, low ESR ceramic capacitors are recommended. Although not a
critical parameter, excessive output ripple can affect LED current.
during dimming. For low output ripple voltage, low ESR ceramic capacitors 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 minimum value of 4 µF is recommended, at lower dimming frequencies, the longer LED-off times will typically
require more capacitance to reduce output voltage transients.
a minimum value of 4 µF is recommended, at lower dimming frequencies, 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
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
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 effective 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.
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 characteristics and compensation can be determined. The
COMP pin is provided to ensure stable operation and optimum transient performance over a wide range of
applications. The following equations define the control-to-output or power stage of the loop:
COMP pin is provided to ensure stable operation and optimum transient performance over a wide range of
applications. The following equations define the control-to-output or power stage of the loop:
(22)
Where R
L
is the load resistance corresponding to LED current, and K
f
is calculated as shown:
16
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