LT8471 View Datasheet(PDF) - Linear Technology
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LT8471 Datasheet PDF : 34 Pages
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LT8471
Applications Information
Internal Undervoltage Lockouts
The LT8471 monitors VIN1 and VIN2 supply voltages in case
either drops below a minimum operating level (typically
about 2.35V and 2.25V, respectively).
When VIN1 is detected low, all power switches are de-
activated, and while sufficient VIN1 voltage persists, the
soft-start capacitors for both SS1 and SS2 are discharged.
After VIN1 is detected high, the channel 1 power switch is
re-enabled and SS1 begins charging.
When VIN2 is detected low, the channel 2 power switch is
deactivated, and while sufficient VIN1 voltage persists, the
soft-start capacitor for SS2 is discharged. After both VIN1
and VIN2 are detected high, the channel 2 power switch is
re-enabled and SS2 begins charging.
Driving SYNC high for an extended period of time effectively
stops the operating clock and prevents latches SR11 and
SR12 from becoming set (see the Block Diagram). As a
result, the switching operation of the LT8471 stops, and
all the power switches are turned off.
The duty cycle of the SYNC signal must be between 35%
and 65% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
1. SYNC may not toggle outside the frequency range of
100kHz to 2MHz unless it is set low to enable the free-
running oscillator.
2. The SYNC frequency can always be higher than the
free-running oscillator frequency, fOSC, but should not
be less than 25% below fOSC.
Oscillator
The internal free-running oscillator can set the operating
frequency of the LT8471. When the SYNC pin is driven low
(< 0.4V), the frequency of operation is set by a resistor
from RT to ground. An internally trimmed timing capacitor
resides inside the IC. The oscillator frequency is calculated
using the following formula:
fOSC
=
85.5
RT + 1
where fOSC is in MHz and RT is in kΩ. Conversely, RT
(in kΩ) can be calculated from the desired frequency (in
MHz) using:
RT
=
85.5
fOSC
–1
Operating Frequency Selection
There are several considerations in selecting the operat-
ing frequency of the converter. The first is staying clear
of sensitive frequency bands, which cannot tolerate any
spectral noise. For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz, and in
that case, a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size
of the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade-off is efficiency, since the switching losses due to
NPN base charge (see the Power and Thermal Calculation
section), Schottky diode charge, and other capacitive loss
terms increase proportionally with frequency.
Clock Synchronization
Soft-Start
The operating frequency of the LT8471 can be synchro-
nized to an external clock source. To synchronize to the
external source, simply provide a digital clock signal into
the SYNC pin. The LT8471 will operate at the SYNC clock
frequency. The LT8471 will revert to the internal free-
running oscillator clock after SYNC is driven low for a few
free-running clock cycles.
The LT8471 contains soft-start circuitry to limit peak
switch currents during start-up. High start-up current is
inherent in switching regulators since the feedback loop
is saturated due to VOUT being far from its final value. The
regulator tries to charge the output capacitor as quickly as
possible, which results in large peak currents.
The start-up current can be limited by connecting external
capacitors (typically 100nF to 1μF) to the SS1 and SS2 pins.
The capacitors are slowly charged to ~2.15V by internal
For more information www.linear.com/8471
8471fc
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