LTC3856_15 Datasheet, PDF (26/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
Soft-Start and Tracking
The LTC3856 has the ability to either soft-start by itself
with a capacitor or track the output of another external
supply. When the controller is configured to soft-start by
itself, a capacitor should be connected to its TK/SS pin.
The controller is in the shutdown state if its RUN pin volt-
age is below 1.22V and its TK/SS pin is actively pulled to
ground in this shutdown state.
Once the RUN pin voltage is above 1.22V, the controller
powers up. A soft-start current of 1.25µA then starts to
charge the TK/SS soft-start capacitor. Note that soft-start
or tracking is achieved not by limiting the maximum
output current of the controller but by controlling the
output ramp voltage according to the ramp rate on the
TK/SS pin. Current foldback is disabled during this phase
to ensure smooth soft-start or tracking. The soft-start or
tracking range is defined to be the voltage range from 0V
to 0.6V on the TK/SS pin. The total soft-start time can be
calculated as:
t_SOFTSTART
=
0.6
•
C_SS
1.25µA
Regardless of the mode selected by the MODE pin, the
controller always starts in discontinuous mode up to TK/SS
= 0.5V. Between TK/SS = 0.5V and 0.54V, it will operate in
forced continuous mode and revert to the selected mode
once TK/SS > 0.54V. The output ripple is minimized dur-
ing the 40mV forced continuous mode window ensuring
a clean PGOOD signal.
When the channel is configured 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 flowing, producing a
small offset error. To minimize this error, select the track-
ing 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 LTC3856 is
forced into continuous mode of operation as soon as V_FB
is below the undervoltage threshold of 0.54V regardless of
the setting on the MODE pin. However, the LTC3856 should
always be set in forced continuous mode tracking down
26
when there is no load. After TK/SS drops below 0.1V, the
controller operates in discontinuous mode.
The LTC3856 allows the user to program how its output
ramps up and down by means of the TK/SS pins. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 11. In the following discussions, V_OUT1 refers
to the LTC3856’s output as a master and V_OUT2 refers to
another supply output as a slave. To implement the coinci-
dent tracking in Figure 11a, connect an additional resistive
divider to V_OUT1 and connect its mid-point to the TK/SS pin
of the slave controller. The ratio of this divider should be
the same as that of the slave controller’s feedback divider
shown in Figure 12a. In this tracking mode, V_OUT1 must
be set higher than V_OUT2. To implement the ratiometric
tracking in Figure 11b, the ratio of the V_OUT2 divider should
be exactly the same as the master controller’s feedback
divider shown in Figure 12b . By selecting different resis-
tors, the LTC3856 can achieve different modes of tracking
including the two in Figure 11.
So, which mode should be programmed? While either
mode in Figure 11 satisfies most practical applications,
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. Under ratiometric tracking, when the master
controller’s output experiences dynamic excursion (under
load transient, for example), the slave controller output
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
INTV_CC (LDO) and EXTV_CC
The LTC3856 features a true PMOS LDO that supplies
power to INTV_CC from the V_IN supply. INTV_CC powers
the gate drivers and much of the LTC3856’s internal cir-
cuitry. The LDO regulates the voltage at the INTV_CC pin
to 5V when V_IN is greater than 5.5V. EXTV_CC connects
to INTV_CC through a P-channel MOSFET and can 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 4.7µF
ceramic capacitor or other low ESR capacitor. No matter
what type of bulk capacitor is used, an additional 0.1µF
ceramic capacitor placed directly adjacent to the INTV_CC
and PGND pins is highly recommended. Good bypassing
3856f