LTC3863_15 Datasheet, PDF (20/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
choose the divider ratio for the external supply as shown
in Figure 5b. Ratiometric tracking could be achieved by
using a different ratio than the feedback (Figure 5b).
Note that the soft-start capacitor charging current is always
flowing, producing a small offset error. To minimize this
error, select the tracking resistive divider values to be small
enough to make this offset error negligible.
Short-Circuit Faults: Current Limit and Foldback
The inductor current limit is inherently set in a current mode
controller by the maximum sense voltage and R_SENSE. In
the LTC3863, the maximum sense voltage is 95mV, mea-
sured across the inductor sense resistor, R_SENSE, placed
across the V_IN and SENSE pins. The output current limit
is approximately:
( ) ^ILIMIT(MIN)

= _
95mV
R_SENSE
–
∆I_L
2


•
V_IN(MIN)
V_IN(MIN)+ | V_OUT | +V_D
The current limit must be chosen to ensure that I_LIMIT(MIN)
> I_OUT(MAX) under all operating conditions. The inductor
current limit should be greater than the inductor current
required to produce maximum output power at worst-case
efficiency. For the LTC3863, both minimum and maximum
V_IN cases should be checked to determine the worst-case
efficiency.
Short-circuit fault protection is assured by the combination
of current limit and frequency foldback. When the output
feedback voltage, V_FB, drops below 0.4V, the operating
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
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)
3863 F06
Figure 6. Internal Soft-Start (6a) Allows Soft-Start without an
External Soft-Start Capacitor and Allows Soft Recovery from (6b)
a Short-Circuit
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.
3863fa
20
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