Datasheet

LTC3864
17
3864f
APPLICATIONS INFORMATION
The current limit must be chosen to ensure that I
LIMIT(MIN)
> I
OUT(MAX)
under all operating conditions. The minimum
current limit value should be greater than the inductor
current required to produce maximum output power at
worst case efficiency. Worst-case efficiency typically oc-
curs at the highest V
IN
.
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 severe.
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
Figure 5(a). Two Different Modes of Output Tracking
TIME
Coincident Tracking
EXTERNAL
SUPPLY
EXTERNAL
SUPPLY
V
OUT
VOLTAGE
V
OUT
TIME
3864 F05a
Ratiometric Tracking
VOLTAGE
Figure 5(b): Setup for Ratiometric and Coincident Tracking
R
FB2
EXT. V
R
FB1
Coincident Tracking Setup
TO SS
R
FB2
V
OUT
TO V
FB
R
FB1
R1
EXT. V
R2
R1+ R2
≥
R2
TO SS
R
FB2
V
OUT
TO V
FB
R
FB1
3864 F05b
Ratiometric Tracking Setup
0.8V
EXT. V