VLCF4020-6R8 ONSEMI [ON Semiconductor], VLCF4020-6R8 Datasheet - Page 12

• Manufacturer Part Number: VLCF4020-6R8
• Description: Up to 500 mA, High Efficiency Synchronous Step-Down DC-DC Converter
• Manufacturer: ONSEMI [ON Semiconductor]

Component Selection
Input Capacitor Selection
with large switching noise. Using an input bypass capacitor
reduces the peak current transients drawn from the input
supply
significantly. The capacitance needed for the input bypass
capacitor depends on the source impedance of the input
supply. The RMS capacitor current is calculated as:
with maximum output current, which is I
for most of the cases. For effective bypass results, the input
capacitor should be placed as close as possible to the V
pin.
Inductor Value Selection
desired ripple current. The relationship between the
inductance and the inductor ripple current is given by the
equation below.
to the maximum load current plus half the ripple current to
prevent core saturation. For NCP1511, the compensation is
internally fixed and a fixed 6.8 mH inductor is needed for
most of the applications. For better efficiency, choose a low
DC resistance inductor.
Output Capacitor Selection
desired output ripple voltage. Ceramic capacitors with low
ESR values will have the lowest output ripple voltage and
are strongly recommended. The output ripple voltage is
given by:
In PWM operating mode, the input current is pulsating
where:
D = duty cycle, which equals V
The maximum RMS current occurs at 50% duty cycle
A low profile ceramic capacitor of 10 mF should be used
Selecting the proper inductor value is based on the
The DC current of the inductor should be at least equal
Selecting the proper output capacitor is based on the
The RMS output capacitor current is given by:
source,
DV c + Di L @ ESR )
I RMS ( C out ) +
Di L +
thereby
I RMS [ I O D @ D
V out
Lf s
reducing
V O @ ( 1 * D )
1 *
2 3 @ L @ f s
out
V out
V in
4f s C out
/V
1
in
, and D’ = 1 − D.
switching
O,max
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(eq. 1)
(eq. 2)
(eq. 3)
(eq. 4)
noise
CC
NCP1511
12
effective series resistance of the output capacitor. A low
ESR, 22 mF ceramic capacitor is recommended for
NCP1511 in most of applications. For example, with TDK
C2012X5R0J226 output capacitor, the output ripple is less
than 10 mV at 300 mA.
Design Example
in a single lithium−ion battery application. The input
voltage, V
1.5 V with a typical load current of 120 mA and a maximum
of 300 mA. For NCP1511, the inductor has a predetermined
value, 6.8 mH. The inductor ESR will factor into the overall
efficiency of the converter. The inductor needs to be
selected by the required peak current.
describes the voltage across the inductor. The inductance
value determines the slope of the current of the inductor.
current for the on−time of the converter in Continuous
Conduction Mode.
is calculated using the following worst−case conditions.
V in, max + 4.2 V, V out + 1.5 V, f s + 1 MHz−20%,
L + 6.8 mH−10%, i L, pk−pk + 197 mA, i L, max + 399 mA
exceeding 405 mA.
input and output capacitors as well as the inductor have a
predetermined value too: C
ESR capacitors are needed for best performance.
Therefore, ceramic capacitors are recommended.
Where f
As a design example, assume that the NCP1511 is used
Equation 5 is the basic equation for an inductor and
Equation 5 is rearranged to solve for the change in
Utilizing Equations 6, the peak−to−peak inductor current
Therefore, the inductor must have a maximum current
Since the compensation is fixed internally in the IC, the
i L, pk−pk +
in
s
, is 3.0 V to 4.2 V. Output condition is V
is the switching frequency and ESR is the
i L, max + I O, max )
+
( V in * V out )
( V in * V out )
V L
L
L
L
in
+
= 10 mF and C
di L
d t
@
@ DT s
V out
Di L, pk−pk
V in
2
@
f s
1
out
= 22 mF. Low
(eq. 5)
(eq. 6)
out
at