ISL85033IRTZ-T7A Intersil, ISL85033IRTZ-T7A Datasheet - Page 18

• Manufacturer Part Number: ISL85033IRTZ-T7A
• Description: IC REG BUCK ADJ DUAL 28TQFN
• Manufacturer: Intersil
• Type: Step-Down (Buck)r

Specifications of ISL85033IRTZ-T7A

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 V ~ 28 V
• Current - Output: 3A
• Frequency - Switching: 300kHz ~ 2MHz
• Voltage - Input: 4.5 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-WFQFN Exposed Pad
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ISL85033IRTZ-T7ATR

The internal overcurrent flag and counter are reset. A
normal soft-start cycle is attempted and normal
operation continues if the fault condition has cleared. If
the overcurrent fault counter overflows during soft-start,
the converter shuts down and this hiccup mode operation
repeats.
Thermal Overload Protection
Thermal overload protection limits maximum junction
temperature in the ISL85033. When the junction
temperature (T
sends a signal to the fault monitor.
The fault monitor commands the buck regulator to shut
down. When the junction temperature has decreased
by 20°C, the regulator will attempt a normal soft-start
sequence and return to normal operation. For
continuous operation, the +125°C junction temperature
rating should not be exceeded.
BOOT Undervoltage Protection
If the BOOT capacitor voltage falls below 2.5V, the
BOOT undervoltage protection circuit will pull the
phase pin low through a 1Ω switch for 400ns to
recharge the capacitor. This operation may arise during
long periods of no switching as in no load situations.
Application Guidelines
Operating Frequency
The ISL85033 operates at a default switching frequency
of 500kHz if FS is tied to VCC. Tie a resistor from FS to
GND to program the switching frequency from 300kHz to
2MHz, as shown in Equation 3.
Where:
t is the switching period in µs.
Synchronization Control
The frequency of operation can be synchronized up to
2MHz by an external signal applied to the SYNCIN pin.
R
FS
300
200
100
[
kΩ
0
]
=
122kΩ ∗ t (
FIGURE 43. R
500
J
) exceeds +150°C, a thermal sensor
–
750
0.17μs )
FS
1000
18
FS (kHz)
SELECTION vs FS
1250
1500
1750
(EQ. 3)
ISL85033
ISL85033
2000
The falling edge on the SYNCIN triggers the rising edge
of PHASE1/2. The switching frequency for each output is
half of the SYNCIN frequency.
Output Inductor Selection
The inductor value determines the converter’s ripple
current. Choosing an inductor current requires a
somewhat arbitrary choice of ripple current, ΔI
reasonable starting point is 30% of total load current.
The inductor value can then be calculated using
Equation 4:
Increasing the value of inductance reduces the ripple
current and thus ripple voltage. However, the larger
inductance value may reduce the converter’s response
time to a load transient. The inductor current rating
should be such that it will not saturate in overcurrent
conditions.
Buck Regulator Output Capacitor Selection
An output capacitor is required to filter the inductor
current. Output ripple voltage and transient response
are 2 critical factors when considering output
capacitance choice. The current mode control loop
allows the usage of low ESR ceramic capacitors and
thus smaller board layout. Electrolytic and polymer
capacitors may also be used.
Additional consideration applies to ceramic capacitors.
While they offer excellent overall performance and
reliability, the actual in-circuit capacitance must be
considered. Ceramic capacitors are rated using large peak-
to-peak voltage swings and with no DC bias. In the DC/DC
converter application, these conditions do not reflect reality.
As a result, the actual capacitance may be considerably
lower than the advertised value. Consult the manufacturers
data sheet to determine the actual in-application
capacitance. Most manufacturers publish capacitance vs DC
bias so that this effect can be easily accommodated. The
effects of AC voltage are not frequently published, but an
assumption of ~20% further reduction will generally
suffice. The result of these considerations can easily result
in an effective capacitance 50% lower than the rated value.
Nonetheless, they are a very good choice in many
applications due to their reliability and extremely low ESR.
The following equations allow calculation of the
required capacitance to meet a desired ripple voltage
level. Additional capacitance may be used.
For the ceramic capacitors (low ESR): =
where ΔI is the inductor’s peak to peak ripple current,
F
capacitor.
If using electrolytic capacitors then:
L=
V
V
SW
OUTripple
OUTripple
V
IN
Fs x ΔI
is the switching frequency and C
- V
OUT
=
=
ΔI*ESR
-------------------------------------- -
8 ∗ F
x
V
SW
V
OUT
IN
ΔI
∗ C
OUT
OUT
is the output
December 8, 2010
.
A
FN6676.2
(EQ. 4)
(EQ. 5)
(EQ. 6)