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| 型号： | ISL9440CEVAL1Z |
| 厂商： | Intersil |
| 文件页数： | 22/24页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD 1 FOR ISL9440C |
| 产品培训模块： | Solutions for Industrial Control Applications |
| 标准包装： | 1 |
| 系列： | * |
ISL9440B, ISL9440C
( L O ) ( I TRAN )
C OUT = -----------------------------------------------------------
2 ( V IN – V O ) ( DV OUT )
( I O ) ( r DS ( ON ) ) ( V OUT )
( I O ) ( V IN ) ( t SW ) ( F SW )
P UPPER = --------------------------------------------------------------- + ------------------------------------------------------------
V IN
The power dissipation includes two loss components;
conduction loss and switching loss. These losses are
distributed between the upper and lower MOSFETs
according to duty cycle (see Equations 11 and 12). The
conduction losses are the main component of power
dissipation for the lower MOSFETs. Only the upper MOSFET
has significant switching losses, since the lower device turns
on and off into near zero voltage. The equations assume
linear voltage-current transitions and do not model power
loss due to the reverse-recovery of the lower MOSFETs
body diode (see Equations 11 and 12).
2
2
(EQ. 11)
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.
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:
2
(EQ. 14)
where, C OUT is the output capacitor(s) required, L O is the
output inductor, I TRAN is the transient load current step, V IN
is the input voltage, V O is output voltage, and DV OUT is the
drop in output voltage allowed during the load transient.
( I O ) ( r DS ( ON ) ) ( V IN – V OUT )
P LOWER = -------------------------------------------------------------------------------
2
V IN
(EQ. 12)
High frequency capacitors initially supply the transient
current and slow the load rate-of-change seen by the bulk
A large gate-charge increases the switching time, t SW , which
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.
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
Output Inductor Selection
The PWM converters require output inductors. The output
Equation 15:
V RIPPLE = Δ I L ( ESR )
(EQ. 15)
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 22 and the ripple current is approximated
by Equation 13:
Where, Δ I L is calculated in the “Output Inductor Selection”
on page 22.
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
( V IN – V OUT ) ( V OUT )
( f S ) ( L ) ( V IN )
Δ I L = ----------------------------------------------------------
(EQ. 13)
components. Consult with the manufacturer of the load
circuitry for specific decoupling requirements.
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
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 Z ) be between 1.2kHz and
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:
C OUT = ------------------------------------
slew to its new level. The ISL9440B and ISL9440C will
provide either 0% or maximum duty cycle in response to a
load transient.
1
2 π ( ESR ) ( f Z )
(EQ. 16)
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
22
In conclusion, the output capacitors must meet three criteria:
1. They must have sufficient bulk capacitance to sustain the
output voltage during a load transient while the output
inductor current is slewing to the value of the load
transient.
FN6799.3
June 24, 2010
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