LTC3811_15 Datasheet, PDF (31/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
use type III compensation effectively removes constraints
on output capacitor ESR. The impedance characteristics
of each capacitor type are significantly different than an
ideal capacitor and therefore require accurate modeling
and bench evaluation during design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo and the Panasonic SP
surface mount types have the lowest (ESR)(size) product
of any aluminum electrolytic at a somewhat higher price.
An additional ceramic capacitor in parallel with OS-CON
type capacitors is recommended to reduce inductance
effects.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. New special polymer
surface mount capacitors offer very low ESR also but
have much lower capacitive density per unit volume. In
the case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. Several
excellent choices are the AVX TPS, AVX TPSV or the KEMET
T510 series of surface mount tantalums, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON, Nichicon PL series and Sprague
595D series. Consult the manufacturer for other specific
recommendations. A combination of capacitors will often
result in maximizing performance and minimizing overall
cost and size.
Tracking and Soft-Start (SS/TRACK Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective SS/TRACK pin. When the voltage on the
SS/TRACK pin is less than the internal 0.6V reference,
the LTC3811 regulates the V_FB pin voltage to the voltage
on the SS/TRACK pin instead of 0.6V. The SS/TRACK 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 simply connecting a capacitor from
the SS/TRACK pin to ground, as shown in Figure 18. An
internal 2.5μA current source charges up the capacitor,
providing a linear ramping voltage at the SS/TRACK pin.
The LTC3811 will regulate the V_FB pin (and hence V_OUT)
according to the voltage on the SS/TRACK pin, allowing
V_OUT to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
0.6V
t_SS = C_SS • _2.5μA
Alternatively, the SS/TRACK pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 19a and 19b. To do this, a resistor divider
should be connected from the master supply (V_X) to the
SS/TRACK pin of the slave supply (V_OUT), as shown in
Figure 20. During start-up V_OUT will track V_X according
to the ratio set by the resistor divider:
V_OUT
V_X
=
R_TRACKA
R_A
•
R_A
R_TRACKA
+ R_B
+ R_TRACKB
For coincident tracking (V_OUT = V_X during start-up),
R_A = R_TRACKA
R_B = R_TRACKB
Note that the small SS/TRACK charging current is always
flowing, producing a small offset error. To minimize this
error, select the tracking resistor divider values to be small
enough to make this error negligible.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors C_B connected to the BOOST
and SW pins supply the gate drive voltages for the topside
MOSFETs. Capacitor C_B in the Functional Diagram is
charged though external diode D_B from DRV_CC when the
SW pin is low. When one of the topside MOSFETs is to
be turned on, the driver places the C_B voltage across the
gate-source of the desired MOSFET. This enhances the
MOSFET and turns on the topside switch. The switch
LTC3811
SS/TRACK
C_SS
SGND
3811 F18
Figure 18. Using the SS/TRACK pin to Program Soft-Start
3811f
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