LTC3863_15 Datasheet, PDF (19/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
Discontinuous and Continuous Operation
The LTC3863 operates in discontinuous conduction (DCM)
until the load current is high enough for the inductor
current to be positive at the end of the switching cycle.
The output load current at the continuous/discontinuous
boundary, I_OUT(CDB), is given by the following equation:
( ) _{IOUT(CDB) =}
V_IN(MAX)2 • | V_OUT | +V_D
2
( ) ^{2 •L • f • V}IN(MAX)^{+ | V}_OUT ^{| +V}D
The continuous/discontinuous boundary is inversely
proportional to the inductor value. Therefore, if required,
I_OUT(CDB) can be reduced by increasing the inductor value.
External Soft-Start and Output Tracking
Start-up characteristics are controlled by the voltage on
the SS pin. When the voltage on the SS pin is less than
the internal 0.8V reference, the LTC3863 regulates the
V_FB pin voltage to the voltage on the SS pin. When the SS
pin is greater than the internal 0.8V reference, the V_FB pin
voltage regulates to the 0.8V internal reference. The SS
pin can be used to program an external soft-start function
or to allow V_OUT to track another supply during start-up.
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,
EXTERNAL
SUPPLY
–V_OUT
(V_OUT < 0V)
EXTERNAL
SUPPLY
–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
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