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| LTC3899_15 Datasheet, PDF (26/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller | |||
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LTC3899
Applications Information
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 10μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3899 will regulate its feedback voltage (and hence
VOUT) according to the voltage on the TRACK/SS pin, al-
lowing VOUT to rise smoothly from 0V (VIN for the boost)
to its final regulated value. The total soft-start time will
be approximately:
tSS
_ BUCK
=
CSS
â¢
0.8V
10µA
tSS
_ BOOST
=
CSS
â¢
1.2V
10µA
Alternatively, the TRACK/SS1 and TRACK/SS2 pins
for the two buck controllers can be used to track two
(or more) supplies during start-up, as shown qualita-
tively in Figures 7a and 7b. To do this, a resistor divider
should be connected from the master supply (VX) to the
TRACK/SS pin of the slave supply (VOUT), as shown in
Figure 8. During start-up VOUT will track VX according to
the ratio set by the resistor divider:
VX
VOUT
=
RA
RTRACKA
⢠RTRACKA
RA
+ RTRACKB
+ RB
For coincident tracking (VOUT = VX during start-up),
RA = RTRACKA
RB = RTRACKB
DRVCC and INTVCC Regulators (OPTI-DRIVE)
The LTC3899 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the DRVCC pin from either the VBIAS supply pin or the
EXTVCC pin depending on the connections of the EXTVCC
and DRVSET pins. A third P-channel LDO supplies power
at the INTVCC pin from the DRVCC pin. DRVCC powers the
gate drivers whereas INTVCC powers much of the LTC3899âs
internal circuitry. The VBIAS LDO and the EXTVCC LDO
regulate DRVCC between 5V to 10V, depending on how
the DRVSET pin is set. Each of these LDOs can supply a
LTC3899
TRACK/SS
CSS
GND
3899 F07
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
VX(MASTER)
VOUT(SLAVE)
TIME
(7a) Coincident Tracking
3889 F08a
VX(MASTER)
VOUT(SLAVE)
TIME
(7b) Ratiometric Tracking
3899 F08b
Figure 7. Two Different Modes of Output Voltage Tracking
VOUT
RB
RA
VX
RTRACKB
RTRACKA
VFB1,2
LTC3899
TRACK/SS1,2
3899 F09
Figure 8. Using the TRACK/SS Pin for Tracking
3899f
26
For more information www.linear.com/LTC3899
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