ISL8501 Ver la hoja de datos (PDF) - Renesas Electronics
Número de pieza
componentes Descripción
Lista de partido
ISL8501 Datasheet PDF : 19 Pages
| |||
ISL8501
optimum transient response. For stable operation, a ceramic
capacitor, with a minimum value of 10µF, is recommended for
both LDO outputs. There is no upper limit to the output
capacitor value. Larger capacitor can reduce noise and
improve load transient response, stability and PSRR. The
output capacitor should be located very close to VOUT pins to
minimize impact of PC board inductances and the other end of
the capacitor should be returned to a clean analog ground.
Buck Regulator Output Capacitor Selection
An output capacitor is required to filter the inductor current and
supply the load transient current. The filtering requirements are
a function of the switching frequency and the ripple current.
The load transient requirements are a function of the slew rate
(di/dt) and the magnitude of the transient load current. These
requirements are generally met with a mix of capacitors and
careful layout.
Embedded processor systems are capable of producing
transient load rates above 1A/ns. High frequency capacitors
initially supply the transient and slow the current load rate seen
by the bulk capacitors. The bulk filter capacitor values are
generally determined by the ESR (Effective Series Resistance)
and voltage rating requirements, rather than actual capacitance
requirements.
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 on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for switching-
regulator applications for the bulk capacitors. The bulk
capacitor’s ESR will determine the output ripple voltage and the
initial voltage drop after a high slew-rate transient. An aluminum
electrolytic capacitor’s ESR value is related to the case size with
lower ESR available in larger case sizes. However, the
Equivalent Series Inductance (ESL) of these capacitors
increases with case size and can reduce the usefulness of the
capacitor to high slew-rate transient loading. Unfortunately, ESL
is not a specified parameter. Work with your capacitor supplier
and measure the capacitor’s impedance with frequency to select
a suitable component. In most cases, multiple electrolytic
capacitors of small case size perform better than a single large
case capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage
ripple requirements and minimize the converter’s response
time to the load transient. The inductor value determines the
converter’s ripple current and the ripple voltage is a function of
the ripple current. The ripple voltage and current are
approximated by the following equations:
I = VIN - VOUT x VOUT
Fs x L
VIN
VOUT = I x ESR (EQ. 3)
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values reduce the
converter’s response time to a load transient. The
recommended I is 30% of the maximum output current.
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
ISL8501 will provide either 0% or 100% duty cycle in
response to a load transient. The response time is the time
required to slew the inductor current from an initial current
value to the transient current level. During this interval the
difference between the inductor current and the transient
current level must be supplied by the output capacitor.
Minimizing the response time can minimize the output
capacitance required.
The response time to a transient is different for the application
of load and the removal of load. The following equations give
the approximate response time interval for application and
removal of a transient load:
tRISE =
L x ITRAN
VIN - VOUT
tFALL =
L x ITRAN
VOUT
(EQ. 4)
where: ITRAN is the transient load current step, tRISE is the
response time to the application of load, and tFALL is the
response time to the removal of load. The worst case response
time can be either at the application or removal of load. Be
sure to check both of these equations at the minimum and
maximum output levels for the worst case response time.
Rectifier Selection
Current circulates from ground to the junction of the MOSFET
and the inductor when the high-side switch is off. As a
consequence, the polarity of the switching node is negative
with respect to ground. This voltage is approximately -0.5V (a
Schottky diode drop) during the off time. The rectifier's rated
reverse breakdown voltage must be at least equal to the
maximum input voltage, preferably with a 20% derating factor.
The power dissipation is:
PDW
=
IOUT
VD
1
–
V----V-O---I-U-N---T--
(EQ. 5)
where VD is the voltage of the Schottky diode = 0.5V to 0.7V
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic capacitors
for high frequency decoupling and bulk capacitors to supply the
current needed each time the switching MOSFET turns on.
Place the small ceramic capacitors physically close to the
MOSFET VIN pins (switching MOSFET drain) and the
Schottky diode anode.
FN6500 Rev 1.00
July 12, 2007
Page 16 of 19
Share Link: 