LTC3864 Datasheet, PDF (18/28 Pages) Linear Technology – 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability

LTC3864
APPLICATIONS INFORMATION
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, or output recovery from drop out. Figure 6
illustrates how internal soft-start controls the output
ramp-up rate under varying scenarios.
V_IN
V_OUT
INTERNAL SOFT-START INDUCED START-UP
(NO EXTERNAL SOFT-START CAPACITOR)
~ 650µs
TIME
(a)
V_OUT
SHORT-CIRCUIT _{INTERNAL SOFT-START}
INDUCED RECOVERY
TIME
(b)
V_IN
V_IN
DROPOUT
V_OUT
INTERNAL SOFT-START
INDUCED RECOVERY
TIME
(c)
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Figure 6. Internal Soft-Start (a) Allows Soft Start-Up without
an External Soft-Start Capacitor and Allows Soft Recovery from
(b) a Short-Circuit or (c) a V_IN Dropout
V_IN Undervoltage Lockout (UVLO)
The LTC3864 is designed to accommodate applications
requiring widely varying power input voltages from 3.5V
to 60V. To accommodate the cases where V_IN drops
significantly once in regulation, the LTC3864 is
18
guaranteed to operate down to a V_IN of 3.5V over the full
temperature range.
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 LTC3864 is guaranteed to be
3.5V to 60V over temperature, but the initial V_IN ramp
must exceed 3.8V to guarantee start-up.
For example, Figure 7 illustrates LTC3864 operation when
an automotive battery droops during a cold crank condi-
tion. The typical automotive battery is 12V to 14V, which is
more than enough headroom above 3.8V for the LTC3864
to start up. Onboard electronics which are powered by a
DC/DC regulator require a minimum supply voltage for
seamless operation during the cold crank condition, and
the battery may droop close to these minimum supply
requirements during a cold crank. The DC/DC regulator
should not exacerbate the situation by having excessive
dropout between the already suppressed battery voltage
input and the output of the regulator which power these
electronics. As seen in Figure 7, the LTC3864’s 100%
duty cycle capability allows virtually no dropout (only the
I_OUT • (R_SENSE + R_DS(ON)) drop across the sense resistor
and P-MOSFET if there is a significant I_OUT) from the battery
to the output. The 3.5V guaranteed UVLO point assures
sufficient margin for continuous, uninterrupted operation in
extreme cold crank battery drooping conditions. However,
additional input capacitance or slower soft start-up time
may be required at low V_IN (e.g. 3.5V to 4.5V) in order to
limit V_IN droop caused by inrush currents, especially if
the battery or input source has a sufficiently large input
impedance.
12V
V_BATTERY
5V
V_OUT
LTC3864’s 100% DUTY CYCLE CAPABILITY ALLOWS
V_OUT TO RIDE V_IN WITHOUT SIGNIFICANT DROP-OUT
TIME
3864 F07
Figure 7. Typical Automotive Cold Crank
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