ISL6558EVAL1Z Intersil, ISL6558EVAL1Z Datasheet - Page 7

• Manufacturer Part Number: ISL6558EVAL1Z
• Description: EVAL BOARD 1 FOR ISL6558
• Manufacturer: Intersil

Specifications of ISL6558EVAL1Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Other than being able to absorb the ripple current and
dissipate the power defined in Equation 19, the input
capacitors should be able to hold the input voltage within
its operational limits during normal and dynamic modes.
The required input capacitance for normal mode
operation is defined in Equation 20.
The ΔV
contributed by the amount of input capacitance, of which
is the input capacitors that filter most of pulsating
currents. The peak-to-peak input ripple current through
the input capacitors is defined in Equation 21.
FIGURE 7. INPUT-CAPACITOR RMS CURRENT
Cin
P
I
INCAP PP
CIN
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.30
0.25
0.20
0.15
0.10
0.05
0.00
=
=
Io N D
--------------------------------------------------------------------------- -
,
FIGURE 8. RAMP CURRENT MUTIPLIER
IN,CAP
I
(
0
IN RMS
0
2
Δ
,
V
=
•
IN CAP
MUTIPLIER
0.1 0.2 0.3 0.4
0.1 0.2
Io
---- -
N
,
–
is the acceptable input voltage ripple
+
m
•
4
3
I
----------------- -
ESR
Lo PP
+
•
4
N=1
2
1
,
N
2
) m N D
(
IN
2
0.3 0.4 0.5 0.6 0.7
DUTY CYCLE (D)
DUTY CYCLE (D)
•
–
Fsw
3
•
7
2
)
0.5 0.6 0.7 0.8 0.9
Application Note 1029
0.8 0.9
N=1
(EQ. 19)
(EQ. 20)
(EQ. 21)
1
1
The overall input voltage ripple induced by the ESR and
capacitance of the input capacitors (at Cin) can be
estimated with Equation 22. The spikes caused by the
ESL of the input capacitors are assumed to be decoupled
with low ESL ceramic capacitors.
For high speed applications with limited input current
slew rate (dI
the charge demanded by the converter due to load
transients (0% to 100% step). The input capacitance
should also meet the inequality relationship in
Equation 23 to prevent the input voltage dip from being
greater than ΔV
An input inductor, defined in Equation 24, is also required
to prevent the converter from drawing current at a
higher slew rate than what the input source can respond.
Thus, the input source will remain within its performance
specifications when the converter responds to a high
speed load transient.
In addition, the input inductor helps reduce the
peak-to-peak input ripple current reflected back to the
input source, as approximated in Equation 25, with the
assumption of negligible amount of
Equation 22.
LOWER MOSFET POWER CALCULATION
The power dissipated in the lower MOSFET is dominated
by the resistive loss due to currents conducted through
the channel resistance (R
An additional term can be added to the lower-MOSFET
loss equation to account for its body-diode conduction
loss during the dead time. This term is dependent on the
diode forward voltage at a current Io/N, V
switching frequency, Fsw, and the length of dead times,
t
MOSFET conduction intervals, respectively.
Cin
V
I
d1
Lin
IN RIPPLE
IN RIPPLE
P
,
LOW 1 ,
where
,
>
and t
>
----------------------------------------------------------------------------------------------------------------------------------- -
2η
---------
Cin
1
I
2
Q2 RMS
⎛
⎝
d2
⎛
⎜
⎜
⎜
⎝
=
ΔV
,
---------------------------------- -
πηV
≈
, at the beginning and the end of the lower-
=
R
IN TRAN
(
---------------------------------------------------------------------------------------------------- -
IN
ND m
Q2
Δ
2Po
,
IN
V
/dt), the input capacitors provide most of
=
•
IN CAP
IN,TRAN
•
–
I
dI
---------- -
2
Q2 RMS
,
dt
IN
+
⎛ ⎞
⎜ ⎟
⎝ ⎠
,
–
I
----
N
⎞
⎟
⎟
⎟
⎠
1
o
ESR
Lin N
2
)
+
2
•
Po
I
and out of operational limits.
+
INCAP PP
(
•
m ND
IN
Q2
I
---------------- -
2
Lo
2
⎛
⎝
–
12
2
Io
---- - I
), as defined in Equation 26.
N
,
PP
•
,
Fsw
–
)
IN
•
•
•
(
⎞
⎠
Io ESR
1 D
ESR
⎞
⎠
–
•
ΔV
•
V
IN,CAP
2
IN
)
IN
•
IN
dI
---------- -
D(ON)
dt
IN
in
(EQ. 22)
(EQ. 23)
(EQ. 24)
(EQ. 25)
July 31, 2009
, the
(EQ. 26)
AN1029.3