LTC3866 Datasheet, PDF (21/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Applications Information
available in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POSCAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturers for other specific recommendations.
Differential Amplifier
The LTC3866 has true remote voltage sense capability.
The sense connections should be returned from the load,
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier rejects common mode signals capacitively or
inductively radiated into the feedback PC traces as well
as ground loop disturbances. The LTC3866 diffamp has
80kΩ input impedance on DIFFP. It is designed to be con-
nected directly to the output. The output of the diffamp
connects to the V_FB pin through a voltage divider, setting
the output voltage.
External Soft-Start and Tracking
The LTC3866 has the ability to either soft-start by itself
or track the output of another channel or external supply.
When the controller is configured to soft-start by itself, a
capacitor may be connected to its TK/SS pin or the internal
soft-start may be used. The controller is in the shutdown
state if its RUN pin voltage is below 1.14V and its TK/SS
pin is actively pulled to ground in this shutdown state. If
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 volt-
age 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/PLLIN 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 during the 40mV forced continuous mode
window, ensuring a clean PGOOD signal. When the chan-
nel 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 volt-
age ramp rate on this pin is determined by the ramp rate
of the other supply’s voltage. It is only possible to track
another supply that is slower than the internal soft-start
ramp. Note that the small soft-start capacitor charging
current is always flowing, 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 LTC3866 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/PLLIN pin. However, the LTC3866 should
always be set in forced continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, the
controller operates in discontinuous mode.
The LTC3866 allows the user to program how its output
ramps up and down by means of the TK/SS pin. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 8. In the following discussions, V_OUT2 refers to the
LTC3866’s output as a slave and V_OUT1 refers to another
supply output as a master. To implement the coincident
tracking in Figure 8a, connect an additional resistive di-
vider 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 9a. In this tracking mode, V_OUT1 must
be set higher than V_OUT2. To implement the ratiometric
tracking in Figure 8b, the ratio of the V_OUT2 divider should
be exactly the same as the master controller’s feedback
divider shown in Figure 9b . By selecting different resis-
tors, the LTC3866 can achieve different modes of tracking
including the two in Figure 8.
So which mode should be programmed? While either
mode in Figure 8 satisfies most practical applications,
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