Datasheet
di
L
dt
di
L
=
(V
IN
- V
OUT
)
dt
Lf
DOWN
=
V
OUT
Lf
UP
I
Lpk
= I
OUT
+
= 4A + = 4.63A
'i
L
2
2
1.25A
'i
L
=
1.5 PH x 500 kHz
1.2V
1
-
1.2V
5.5V
= 1.25A
D
=
V
OUT
V
IN
Lf =
'i
L
f
SW
V
OUT
(1-D)
'i
L
f
SW
=
V
OUT
1
-
V
OUT
V
IN
Component Selection
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2. Connect the load with a 4A capability to the VOUT and GND terminals
3. The EN terminal can be left open for normal operation as there is an on-board pull-up resistor.
4. Set V
IN
to 3.0V with no load applied. V
OUT
should be in regulation with a nominal 1.2V output.
5. Slowly increase the load while monitoring the output voltage. V
OUT
should remain in regulation with a
nominal 1.2V output as the load is increased up to 4A.
6. Slowly sweep the input voltage from 2.95V to 5.5V. V
OUT
should remain in regulation with a nominal
1.2V output.
7. Temporarily short the EN terminal to GND to check the shutdown function.
8. Increase the load beyond the normal range to check current limit. The output current should limit at
approximately 5.6A. Short the VOUT and GND terminals to verify short circuit protection.
7 Component Selection
7.1 Filter Inductor
The selection of the output filter components, Lf and Co1, are intrinsically linked as both of these
parameters affect the stability of the system and various characteristics of the output voltage.
First, a 1.5 µH inductor is chosen to allow stable operation (per datasheet recommendations) over the
entire input voltage range from 2.95V to 5.5V. The inductance also directly affects the amplitude of the
inductor current ripple which flows in the output capacitor. The filter inductance is given by
(1)
where the variable D refers to the duty cycle and can be approximated by
(2)
From this, it follows that the inductor ripple current, Δi
L
, reaches a maximum when duty cycle is minimum
or input voltage is maximum, i.e. V
IN
= 5.5V. Under these conditions, the inductor peak to peak ripple
current is given by
(3)
or approximately 31% of full load current. It follows that the peak inductor current at full load is
(4)
and this level is adequately below the peak inductor current associated with current limit. This implies that
an inductor must be selected with a saturation current higher than 4.63A to ensure that the inductor will
never saturate during normal operating conditions.
Another concern related to the inductance value is that it governs the maximum inductor current slew rate
during a load on or off transient respectively as follows
(5)
Thus, a smaller inductance value allows the control loop to slew the inductor current more rapidly.
This evaluation board uses the Vishay IHLP2525 series 1.5 µH inductor to provide the necessary current
handling capability with low DC resistance in a relatively small footprint and profile .
4
AN-1786 LM2854 500 kHz Buck Regulator Evaluation Board SNVA323B–March 2008–Revised May 2013
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