isl9440b Intersil Corporation, isl9440b Datasheet - Page 23

• Manufacturer Part Number: isl9440b
• Description: Triple Step-down Pwm And Single Linear Controller With Programmable Soft-start
• Manufacturer: Intersil Corporation

loss due to the reverse-recovery of the lower MOSFETs
body diode (see Equations 11 and 12).
A large gate-charge increases the switching time, t
increases the upper MOSFET switching losses. Ensure that
both MOSFETs are within their maximum junction
temperature at high ambient temperature by calculating the
temperature rise according to package thermal-resistance
specifications.
Output Inductor Selection
The PWM converters require output inductors. The output
inductor is selected to meet the output voltage ripple
requirements. The inductor value determines the converter’s
ripple current and the ripple voltage is a function of the ripple
current and output capacitor(s) ESR. The ripple voltage
expression is given beginning in the “Output Capacitor
Selection” on page 23 and the ripple current is approximated
by Equation 13:
Output Capacitor Selection
The output capacitors for each output have unique
requirements. In general, the output capacitors should be
selected to meet the dynamic regulation requirements
including ripple voltage and load transients. Selection of
output capacitors is also dependent on the output inductor,
so some inductor analysis is required to select the output
capacitors.
One of the parameters limiting the converter’s response to a
load transient is the time required for the inductor current to
slew to its new level. The ISL9440B and ISL9440C will
provide either 0% or maximum duty cycle in response to a
load transient.
The response time is the time interval required to slew the
inductor current from an initial current value to the load
current level. During this interval the difference between the
inductor current and the transient current level must be
supplied by the output capacitor(s). Minimizing the response
time can minimize the output capacitance required. Also, if
the load transient rise time is slower than the inductor
response time, as in a hard drive or CD drive, it reduces the
requirement on the output capacitor.
P
P
ΔI
UPPER
LOWER
L
=
(
--------------------------------------------------------- -
V
IN
=
=
( ) L ( ) V
–
(
-------------------------------------------------------------- -
f
(
------------------------------------------------------------------------------ -
I
S
I
O
V
O
OUT
2
2
) r
) r
(
(
(
DS ON
DS ON
) V
(
IN
V
(
(
OUT
)
IN
V
)
)
) V
IN
) V
)
(
(
OUT
IN
23
–
)
V
+
OUT
(
----------------------------------------------------------- -
I
O
)
) V
(
IN
) t
(
2
SW
) F
(
SW
ISL9440B, ISL9440C
SW
(EQ. 13)
(EQ. 11)
(EQ. 12)
, which
)
The maximum capacitor value required to provide the full,
rising step, transient load current during the response time of
the inductor is shown in Equation 14:
where, C
output inductor, I
is the input voltage, V
drop in output voltage allowed during the load transient.
High frequency capacitors initially supply the transient
current and slow the load rate-of-change seen by the bulk
capacitors. The bulk filter capacitor values are generally
determined by the ESR (Equivalent Series Resistance) and
voltage rating requirements as well as actual capacitance
requirements.
The output voltage ripple is due to the inductor ripple current
and the ESR of the output capacitors as defined by
Equation 15:
Where, ΔI
on page 23.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load
circuitry for specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications at 300kHz
(ISL9440B)/600kHz (ISL9440C) for the bulk capacitors. In
most cases, multiple small-case electrolytic capacitors
perform better than a single large-case capacitor.
The stability requirement on the selection of the output
capacitor is that the ‘ESR zero’ (f
30kHz. This range is set by an internal, single compensation
zero at 6kHz. The ESR zero can be a factor of five on either
side of the internal zero and still contribute to increased
phase margin of the control loop. Therefore:
In conclusion, the output capacitors must meet three criteria:
C
V
C
1. They must have sufficient bulk capacitance to sustain the
2. The ESR must be sufficiently low to meet the desired
3. The ESR zero should be placed, in a rather large range,
OUT
RIPPLE
OUT
output voltage during a load transient while the output
inductor current is slewing to the value of the load
transient.
output voltage ripple due to the output inductor current.
to provide additional phase margin.
=
=
----------------------------------- -
2π ESR
OUT
---------------------------------------------------------- -
2 V
=
L
(
(
is calculated in the “Output Inductor Selection”
ΔI
(
IN
is the output capacitor(s) required, L
L
L
1
O
(
–
ESR
) I
TRAN
) f
V
(
( )
O
TRAN
Z
) DV
)
(
O
is the transient load current step, V
is output voltage, and DV
)
OUT
2
)
Z
) be between 1.2kHz and
January 15, 2009
OUT
O
is the
(EQ. 14)
(EQ. 15)
(EQ. 16)
is the
FN6799.0
IN