ISL85001 查看數據表（PDF） - Renesas Electronics
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ISL85001 Datasheet PDF : 16 Pages
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ISL85001
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
Equation 3:
I = -V----I--Nf--S---–-W---V-----O-L---U----T-- V----V-O---I-U-N---T--
VOUT = I 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.
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
ISL85001 will provide either 0% or 80% 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. Equation 4 gives the approximate
response time interval for application and removal of a transient
load:
tRISE = -V---L-I--N-----I–--T--V-R----OA----UN----T--
tFALL = L-----V---I-O-T----UR----TA----N--
(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 Equation 4 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 shown in
Equation 5:
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.
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 largest RMS current required by the
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
conservative guideline. For most cases, the RMS current rating
requirement for the input capacitor of a buck regulator is
approximately 1/2 the DC load current.
The maximum RMS current required by the regulator may be
closely approximated through Equation 6:
IRMS_MAX =
V----V-O---I--U-N----T--
V-----I--N-----V-–----IV--N--O------U----T--
IO
U
T
M
A
2
X
+
1--1--2--
V-----I--N-----V-–----IV--N--O------U----T--
L--V----O--f--S-U----W-T--- 2
(EQ. 6)
For a through-hole design, several electrolytic capacitors may be
needed. For surface mount designs, solid tantalum capacitors
can be used, but caution must be exercised with regard to the
capacitor surge current rating. These capacitors must be capable
of handling the surge-current at power-up. Some capacitor series
available from reputable manufacturers are surge current tested.
FN6769 Rev.3.00
Apr 14, 2017
Page 12 of 16
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