ISL8500IRZ Intersil, ISL8500IRZ Datasheet - Page 12

• Manufacturer Part Number: ISL8500IRZ
• Description: IC PWM REG 2A BUCK 12-DFN
• Manufacturer: Intersil
• Type: Step-Down (Buck), PWM - Voltage Moder

Specifications of ISL8500IRZ

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 19 V
• Current - Output: 2A
• Frequency - Switching: 500kHz
• Voltage - Input: 4.5 ~ 25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: *
• Package / Case: 12-DFN
• Voltage - Supply: 4.5 V ~ 25 V
• Frequency-max: 550kHz
• Duty Cycle: 94%
• Pwm Type: Voltage Mode
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Rohs Compliant: YES
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 Equation 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
ISL8500 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.
ΔI =
V
IN
Fs x L
- V
OUT
x
V
V
OUT
IN
12
ΔV
OUT
= ΔI x ESR
(EQ. 3)
ISL8500
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:
where: I
response time to the application of load, and t
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:
where V
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 1.25 times greater than the maximum input voltage,
while a voltage rating of 1.5 times 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:
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
I
P
t
RMS
RISE
D
[
W
MAX
=
]
=
TRAN
D
=
I
V
is the voltage of the Schottky diode = 0.5V to 0.7V
OUT
L x I
IN
V
------------- -
V
- V
is the transient load current step, t
OUT
TRAN
⋅
IN
V
OUT
D
×
⋅
⎛
⎝
⎛
⎜
⎝
I
1
OUT
–
V
--------------- -
MAX
V
OUT
IN
2
t
⎞
⎟
⎠
FALL
+
----- -
12
1
×
=
⎛
⎝
V
---------------------------- -
L x I
IN
L
V
–
×
OUT
TRAN
V
f
OUT
s
FALL
December 10, 2007
×
RISE
V
------------- -
V
OUT
is the
IN
(EQ. 5)
(EQ. 6)
(EQ. 4)
is the
FN6611.0
⎞
⎠
2
⎞
⎠