LTC3863 Datasheet, PDF (20/36 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller Wide Operating VIN Range: 3.5V to 60V

LTC3863
APPLICATIONS INFORMATION
frequency, f, will fold back to a minimum value of 0.18 • f
when V_FB reaches 0V. Both current limit and frequency
foldback are active in all modes of operation. In a short-
circuit fault condition, the output current is first limited
by current limit and then further reduced by folding back
the operating frequency as the short becomes more se-
vere. The worst-case fault condition occurs when V_OUT
is shorted to ground.
Short-Circuit Recovery and Internal Soft-Start
An internal soft-start feature guarantees a maximum posi-
tive output voltage slew rate in all operational cases. In a
short-circuit recovery condition for example, the output
recovery rate is limited by the internal soft-start so that
output voltage overshoot and excessive inductor current
buildup is prevented.
The internal soft-start voltage and the external SS pin
operate independently. The output will track the lower of
the two voltages. The slew rate of the internal soft-start
voltage is roughly 1.2V/ms, which translates to a total
soft-start time of 650µs. If the slew rate of the SS pin is
greater than 1.2V/ms the output will track the internal soft-
start ramp. To assure robust fault recovery, the internal
soft-start feature is active in all operational cases. If a
short-circuit condition occurs which causes the output to
drop significantly, the internal soft-start will assure a soft
recovery when the fault condition is removed.
The internal soft-start assures a clean soft ramp-up from
any fault condition that causes the output to droop, guar-
anteeing a maximum ramp rate in soft-start, short-circuit
fault release. Figure 6 illustrates how internal soft-start
controls the output ramp-up rate under varying scenarios.
V_IN Undervoltage Lockout (UVLO)
The LTC3863 is designed to accommodate applications
requiring widely varying power input voltages from 3.5V
to 60V. To accommodate the cases where V_IN drops sig-
nificantly once in regulation, the LTC3863 is guaranteed
to operate down to a V_IN of 3.5V over the full temperature
range.
V_IN
––V_OUT
INTERNAL SOFT-START INDUCED START-UP
(NO EXTERNAL SOFT-START CAPACITOR)
~650µs
TIME
(6a)
–V_OUT
SHORT-CIRCUIT _{INTERNAL SOFT-START}
INDUCED RECOVERY
TIME
(6b)
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Figure 6. Internal Soft-Start (6a) Allows Soft-Start without an
External Soft-Start Capacitor and Allows Soft Recovery from
(6b) a Short-Circuit
The implications of both the UVLO rising and UVLO falling
specifications must be carefully considered for low V_IN
operation. The UVLO threshold with V_IN rising is typi-
cally 3.5V (with a maximum of 3.8V) and UVLO falling is
typically 3.25V (with a maximum of 3.5V). The operating
input voltage range of the LTC3863 is guaranteed to be
3.5V to 60V over temperature, but the initial V_IN ramp
must exceed 3.8V to guarantee start-up.
Minimum On-Time Considerations
The minimum on-time, t_ON(MIN), is the smallest time
duration that the LTC3863 is capable of turning on the
power MOSFET, and is typically 220ns. It is determined
by internal timing delays and the gate charge required
to turn on the MOSFET. Low duty cycle applications may
approach this minimum on-time limit, so care should be
taken to ensure that:
( ^{( )} ) ^{tON(MIN) < f •}
| V_OUT | + V_D
V_IN(MAX) + | V_OUT | + V_D
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