Datasheet

LTC3863
19
3863f
For more information www.linear.com/3863
Figure 5a. Two Different Modes of Output Tracking
Figure 5b. Setup for Ratiometric and Coincident Tracking
TIME
Coincident Tracking
EXTERNAL
SUPPLY
EXTERNAL
SUPPLY
–V
OUT
(V
OUT
< 0V)
–V
OUT
(V
OUT
< 0V)
VOLTAGE
TIME
3863 F05a
Ratiometric Tracking
VOLTAGE
R1 = R
FB1
– R
FB2
|V
OUT
| > 0.8V
EXT. V
R2 = R
FB2
Coincident Tracking Setup
TO SS
R
FB1
V
OUT
TO V
FBN
TO FB TO FB
R
FB2
R1
EXT. V
R2
R1+ R2
R2
TO SS
R
FB1
V
OUT
TO V
FBN
R
FB2
3863 F05b
Ratiometric Tracking Setup
0.8V
EXT. V
APPLICATIONS INFORMATION
Soft-start is enabled by connecting a capacitor from
the SS pin to ground. An internal 10µA current source
charges the capacitor, providing a linear ramping voltage
at the SS pin that causes V
OUT
to rise smoothly from 0V
to its final regulated value. The total soft-start time will
be approximately:
t
SS
= C
SS
0.8V
10µA
When the LTC3863 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the SS pin to scale the ramp rate appropriately. Two com-
mon implementations of tracking as shown in Figure5a
are coincident and ratiometric. For coincident tracking,
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:
I
LIMIT(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