ISL85415FRZ-T 查看數據表（PDF） - Intersil
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ISL85415FRZ-T Datasheet PDF : 24 Pages
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ISL85415
limit. At this point the controller will turn off both FET’s and wait for
COMP to indicate return to normal operation. During this time, the
controller will apply a 100Ω load from PHASE to PGND and
attempt to discharge the output. Negative current limit is a
pulse-by-pulse style operation and recovery is automatic. Negative
current limit protection is disabled in PFM operating mode
because reverse current is not allowed to build due to the diode
emulation behavior of the lower FET.
Over-Temperature Protection
Over-temperature protection limits maximum junction
temperature in the ISL85415. When junction temperature (TJ)
exceeds +150°C, both FET’s are turned off and the controller
waits for temperature to decrease by approximately 20°C.
During this time PG is pulled low. When temperature is within an
acceptable range, the controller will initiate a normal soft-start
sequence. For continuous operation, the +125°C junction
temperature rating should not be exceeded.
Boot Undervoltage Protection
If the Boot capacitor voltage falls below 1.8V, the Boot
undervoltage protection circuit will turn on the lower FET for
400ns to recharge the capacitor. This operation may arise during
long periods of no switching such as PFM no load situations. In
PWM operation near dropout (VIN near VOUT), the regulator may
hold the upper FET on for multiple clock cycles. To prevent the
boot capacitor from discharging, the lower FET is forced on for
approximately 200ns every 10 clock cycles.
Application Guidelines
Simplifying the Design
While the ISL85415 offers user programmed options for most
parameters, the easiest implementation with fewest
components involves selecting internal settings for SS, COMP
and FS. Table 1 on page 4 provides component value selections
for a variety of output voltages and will allow the designer to
implement solutions with a minimum of effort.
Operating Frequency
The ISL85415 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 4.
RFS[kΩ] = 108.75kΩ∗(t – 0.2μs ) ⁄ 1μs
Where:
t is the switching period in µs.
(EQ. 4)
300
200
100
0
500 750 1000 1250 1500 1750 2000
FS (kHz)
FIGURE 59. RFS SELECTION vs FS
Synchronization Control
The frequency of operation can be synchronized up to 2MHz by
an external signal applied to the SYNC pin. The rising edge on the
SYNC triggers the rising edge of PHASE. To properly sync, the
external source must be at least 10% greater than the
programmed free running IC 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. A reasonable starting point is 30% of
total load current. The inductor value can then be calculated
using Equation 5:
L=
VIN - VOUT
VOUT
x
FS x DI
VIN
(EQ. 5)
Increasing the value of inductance reduces the ripple current and
thus, the 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. For typical ISL85415 applications,
inductor values generally lies in the 10µH to 47µH range. In
general, higher VOUT will mean higher inductance.
Buck Regulator Output Capacitor Selection
An output capacitor is required to filter the inductor current. The
current mode control loop allows the use of low ESR ceramic
capacitors and thus supports very small circuit implementations
on the PC board. Electrolytic and polymer capacitors may also be
used.
While ceramic capacitors 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
19
FN8373.2
September 26, 2013
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