Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB Scheda Tecnica
Codici prodotto
LM3431EVAL/NOPB
R4 =
(IL
(LIM)
x R3) + (D x 75 mV)
40
P
A
d
R3
d
50 mV
IL
(AVE_MIN)
200 mV
IL
(AVE_MAX)
L
(MIN)
=
R3 x (V
OUT
- 2 x VIN
(MIN)
)
f
SW
x 75 mV x 2
IL
PEAK
= IL
(AVE)
+
'
i
L
2
'
i
L
=
V
IN
x D
L x f
SW
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
The resulting peak to peak inductor current is:
(10)
And the resulting peak inductor current is:
(11)
Peak inductor current will occur at minimum V
IN
.
The inductor must be rated to handle both the average current and peak current, which is the same as the peak
switch current. As switching frequency increases, less inductance is required. However, some minimum
inductance value is required to ensure stability at duty cycles greater than 50%. The minimum inductance
required for stability can be calculated as:
switch current. As switching frequency increases, less inductance is required. However, some minimum
inductance value is required 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.
(12)
Although the inductor must be large enough to meet both the stability and the
Δ
i
L
requirements, a value close to
minimum 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:
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:
(13)
Since I
L(AVE)
will vary with input voltage, R3 should be determined based on the full input voltage range, although
the resulting value may extend somewhat outside the recommended range.
Current Limit
Current limit occurs when the voltage across the sense resistor (measured at the CS pin) equals the current limit
threshold voltage. The current limit threshold is set by R4. This value can be calculated as follows:
threshold voltage. The current limit threshold is set by R4. This value can be calculated as follows:
(14)
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.
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
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
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 specification.
minimum specification.
In a boost switcher, there is a direct current path between input and output. Therefore, although the LM3431 will
shutdown in a shorted output condition, there are no means to limit the current flowing from input to output.
shutdown 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.
has occurred.
14
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