ISL6269B
13
FN6280.3
November 17, 2014
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Selection of the Input Capacitor
The important parameters for the bulk input capacitance are the
voltage rating and the RMS current rating. For reliable operation,
select bulk capacitors with voltage and current ratings above the
maximum input voltage and capable of supplying the RMS
current required by the switching circuit. Their voltage rating
should be at least 1.25x greater than the maximum input
voltage, while a voltage rating of 1.5x is a preferred rating.
Figure 5
is a graph of the input RMS ripple current, normalized
relative to output load current, as a function of duty cycle that is
adjusted for converter efficiency. The ripple current calculation is
written as shown in Equation 14
:
Where:
-I
MAX
is the maximum continuous I
LOAD
of the converter
- x is a multiplier (0 to 1) corresponding to the inductor
peak-to-peak ripple amplitude expressed as a percentage of
I
MAX
(0% to 100%)
- D is the duty cycle that is adjusted to take into account the
efficiency of the converter which is written as shown in
Equation 15
:
In addition to the bulk capacitance, some low ESL ceramic
capacitance is recommended to decouple between the drain of
the high-side MOSFET and the source of the low-side MOSFET.
MOSFET Selection and Considerations
Typically, a MOSFET cannot tolerate even brief excursions beyond
their maximum drain-to-source voltage rating. The MOSFETs used
in the power stage of the converter should have a maximum V
DS
rating that exceeds the sum of the upper voltage tolerance of the
input power source and the voltage spike that occurs when the
MOSFET switches off.
There are several power MOSFETs readily available that are
optimized for DC/DC converter applications. The preferred
high-side MOSFET emphasizes low switch charge so that the
device spends the least amount of time dissipating power in the
linear region. Unlike the low-side MOSFET, which has the drain to
source voltage clamped by its body diode during turn off, the
high-side MOSFET turns off with V
IN
-V
OUT
-V
L
across it. The
preferred low-side MOSFET emphasizes low r
DS(ON)
when fully
saturated to minimize conduction loss.
For the low-side MOSFET, (LS), the power loss can be assumed to
be conductive only and is written as:
For the high-side MOSFET, (HS), its conduction loss is written as:
For the high-side MOSFET, its switching loss is written as:
Where:
-I
VALLEY
is the difference of the DC component of the
inductor current minus 1/2 of the inductor ripple current
-I
PEAK
is the sum of the DC component of the inductor
current plus 1/2 of the inductor ripple current
-t
ON
is the time required to drive the device into saturation
-t
OFF
is the time required to drive the device into cut-off
Selecting The Bootstrap Capacitor
The selection of the bootstrap capacitor is written as:
Where:
-Q
g
is the total gate charge required to turn on the high-side
MOSFET
- V
BOOT
, is the maximum allowed voltage decay across the
boot capacitor each time the high-side MOSFET is switched
on
As an example, suppose the high-side MOSFET has a total gate
charge Q
g
, of 25nC at V
GS
= 5V and a V
BOOT
of 200mV. The
calculated bootstrap capacitance is 0.125µF. For a comfortable
margin select a capacitor that is double the calculated
capacitance, in this example 0.22µF will suffice. Use an X7R or
X5R ceramic capacitor.
(EQ. 14)
I
IN_RMS
I
MAX
2
DD
2
–xI
MAX
2
D
12
------
+
I
MAX
-----------------------------------------------------------------------------------------------------
=
D
V
OUT
V
IN
EFF
--------------------------
=
(EQ. 15)
FIGURE 5. NORMALIZED RMS INPUT CURRENT FOR x = 0.8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
DUTY CYCLE
NORMALIZED INPUT RMS RIPPLE CURRENT
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
x = 1
x = 0.75
x = 0.50
x = 0.25
x = 0
(EQ. 16)
P
CON_LS
I
LOAD
2
r
DS ON_LS
1D–
(EQ. 17)
P
CON_HS
I
LOAD
2
r
DS ON_HS
D=
(EQ. 18)
P
SW_HS
V
IN
I
VALLEY
t
ON
f
SW
2
-----------------------------------------------------------------
V
IN
I
PEAK
t
OFF
f
SW
2
-------------------------------------------------------------
+=
C
BOOT
Q
g
V
BOOT
------------------------
=
(EQ. 19)
