Texas Instruments 36V, 2A PowerWise® Adjustable Frequency Synchronous Buck Regulator Evaluation Module LM20242EVAL/NOPB LM20242EVAL/NOPB Datenbogen
Produktcode
LM20242EVAL/NOPB
'
V
OUT
=
'
i
L
x
1
8 x f
SW
x C
OUT
R
ESR
+
V
IN
I
L AVG
= I
OUT
'
I
L
Time
Time
IL
V
SW
L
MIN
=
(V
IN
- V
OUT
) x D
'
i
L
x f
SW
SNVS534E – OCTOBER 2007 – REVISED MARCH 2013
INDUCTOR SELECTION (L)
The inductor value is determined based on the operating frequency, load current, ripple current and duty cycle.
The inductor selected should have a saturation current rating greater than the peak current limit of the device.
Keep in mind the specified current limit does not account for delay of the current limit comparator, therefore the
current limit in the application may be higher than the specified value. To optimize the performance and prevent
the device from entering current limit at maximum load, the inductance is typically selected such that the ripple
current,
Keep in mind the specified current limit does not account for delay of the current limit comparator, therefore the
current limit in the application may be higher than the specified value. To optimize the performance and prevent
the device from entering current limit at maximum load, the inductance is typically selected such that the ripple
current,
Δ
i
L
, is not greater than 30% of the rated output current.
illustrates the switch and inductor
ripple current waveforms. Once the input voltage, output voltage, operating frequency and desired ripple current
are known, the minimum value for the inductor can be calculated by the formula shown below:
are known, the minimum value for the inductor can be calculated by the formula shown below:
(3)
Figure 10. Switch and Inductor Current Waveforms
If needed, slightly smaller value inductors can be used, however, the peak inductor current, I
OUT
+
Δ
i
L
/2, should
be kept below the peak current limit of the device. In general, the inductor ripple current,
Δ
i
L
, should be more
than 10% of the rated output current to provide adequate current sense information for the current mode control
loop. If the ripple current in the inductor is too low, the control loop will not have sufficient current sense
information and can be prone to instability.
loop. If the ripple current in the inductor is too low, the control loop will not have sufficient current sense
information and can be prone to instability.
OUTPUT CAPACITOR SELECTION (C
OUT
)
The output capacitor, C
OUT
, filters the inductor ripple current and provides a source of charge for transient load
conditions. A wide range of output capacitors may be used with the LM20242 that provide excellent performance.
The best performance is typically obtained using ceramic, SP or OSCON type chemistries. Typical trade-offs are
that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and noise spikes,
while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for transient loading
conditions.
The best performance is typically obtained using ceramic, SP or OSCON type chemistries. Typical trade-offs are
that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and noise spikes,
while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for transient loading
conditions.
When selecting the value for the output capacitor, the two performance characteristics to consider are the output
voltage ripple and transient response. The output voltage ripple can be approximated by using the following
formula.
voltage ripple and transient response. The output voltage ripple can be approximated by using the following
formula.
(4)
14
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