LTC3863 Datasheet, PDF (19/36 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller Wide Operating VIN Range: 3.5V to 60V

LTC3863
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.
EXTERNAL
SUPPLY
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
EXTERNAL
SUPPLY
–V_OUT
(V_OUT < 0V)
–V_OUT
(V_OUT < 0V)
TIME
Coincident Tracking
TIME
3863 F05a
Ratiometric Tracking
Figure 5a. Two Different Modes of Output Tracking
EXT. V
TO SS
|V_OUT| > 0.8V
R1 = R_FB1 – R_FB2
R2 = R_FB2
V_OUT
R_FB1
TO V_FBN
R_FB2
TO FB
Coincident Tracking Setup
EXT. V
TO SS
R1
R2
R1+ R2
≥
0.8V
EXT. V
R2
V_OUT
R_FB1
TO V_FBN
R_FB2
TO FB
3863 F05b
Ratiometric Tracking Setup
Figure 5b. Setup for Ratiometric and Coincident Tracking
For more information www.linear.com/3863
3863f
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