ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 17

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.74V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.74V. The output ripple is
minimized during the 100mV forced continuous mode
window ensuring a clean PGOOD signal.
When the channel is confi gured to track another supply,
the feedback voltage of the other supply is duplicated by
a resistor divider and applied to the TK/SS pin. Therefore,
the voltage ramp rate on this pin is determined by the
ramp rate of the other supply’s voltage. Note that the
small soft-start capacitor charging current is always
fl owing, producing a small offset error. To minimize this
error, select the tracking resistive divider value to be small
enough to make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3850-2 is forced into
continuous mode of operation as soon as V
undervoltage threshold of 0.74V regardless of the setting
on the MODE/PLLIN pin. However, the LTC3850-2 should
always be set in force continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, its
channel will operate in discontinuous mode.
Output Voltage Tracking
The LTC3850-2 allows the user to program how its out-
put ramps up and down by means of the TK/SS pins.
Through these pins, the output can be set up to ei-
ther coincidentally or ratiometrically track another
supply’s output, as shown in Figure 5. In the following
discussions, V
master channel and V
2 as a slave channel. In practice, though, either phase can
be used as the master. To implement the coincident track-
ing in Figure 5a, connect an additional resistive divider to
V
slave channel. The ratio of this divider should be the same
as that of the slave channel’s feedback divider shown in
Figure 6a. In this tracking mode, V
than V
of the V
master channel’s feedback divider. By selecting different
resistors, the LTC3850-2 can achieve different modes of
tracking including the two in Figure 5.
OUT1
OUT2
and connect its midpoint to the TK/SS pin of the
OUT2
. To implement the ratiometric tracking, the ratio
divider should be exactly the same as the
OUT1
refers to the LTC3850-2’s output 1 as a
OUT2
refers to the LTC3850-2’s output
OUT1
must be set higher
FB
is below the
So which mode should be programmed? While either
mode in Figure 5 satisfi es most practical applications,
some tradeoffs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. This can be better understood with the help
of Figure 7. At the input stage of the slave channel’s error
amplifi er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
is used to match the shifted common mode voltage. The
top two current sources are of the same amplitude. In the
coincident mode, the TK/SS voltage is substantially higher
than 0.8V at steady state and effectively turns off D1. D2
and D3 will therefore conduct the same current and offer
tight matching between V
0.8V reference. In the ratiometric mode, however, TK/SS
equals 0.8V at steady state. D1 will divert part of the bias
current to make V
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
channel’s output experiences dynamic excursion (under
load transient, for example), the slave channel output will
be affected as well. For better output regulation, use the
coincident tracking mode instead of ratiometric.
INTV
The LTC3850-2 features an NPN linear regulator that sup-
plies power to INTV
the gate drivers and much of the LTC3850-2’s internal
circuitry. The linear regulator regulates the voltage at the
INTV
connects to INTV
supply the needed power when its voltage is higher than
4.7V. Each of these can supply a peak current of 100mA
and must be bypassed to ground with a minimum of 1μF
ceramic capacitor or low ESR electrolytic capacitor. No mat-
ter what type of bulk capacitor is used, an additional 0.1μF
ceramic capacitor placed directly adjacent to the INTV
and PGND pins is highly recommended. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between the channels.
CC
CC
Regulators and EXTV
pin to 5V when V
CC
FB2
CC
through a P-channel MOSFET and can
slightly lower than 0.8V.
from the V
IN
FB2
is greater than 6.5V. EXTV
CC
and the internal precision
IN
supply. INTV
LTC3850-2
CC
17
powers
38502f
CC
CC