LTC3867_15 Datasheet, PDF (24/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
SP series of surface mount special polymer capacitors
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 LTC3867 has true remote voltage sense capability.
The sensing connections should be returned from the
load, back to the differential amplifier’s inputs through a
common, 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 LTC3867 diffamp has
high input impedance on DIFF⁺. It is designed to be used
with a feedback divider Kelvined to the load. The output
of the diffamp connects to the V_FB pin.
Nonlinear Control Loop
The LTC3867 features a unique control loop that can speed
up transient response dramatically. This feature is enabled
and programmed through the IFAST pin. When IFAST is
tied to INTV_CC, the nonlinear control loop is disabled. V_IFAST
is the voltage that can be programmed on the IFAST pin.
There is a precision 10µA flowing out of the IFAST pin.
Connecting a resistor to SGND sets the V_IFAST voltage.
When V_IFAST is set below 1.4V, the difference of 1.4V and
V_IFAST sets the threshold voltage that triggers nonlinear
control. Nonlinear control is only enabled when V_FB is
within the UV and OV window. It should be enabled only
for forced continuous mode of operation.
Once nonlinear control is enabled, the top gate of all chan-
nels will turn on if:
V_FB
=
V_REF
–
1.4 – V_IFAST
12.5
where V_REF is the reference voltage, normally at 0.6V, and
V_FB is the feedback voltage.
External Soft-Start and Tracking
The LTC3867 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 1.22V 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 pin, the
controller always starts in discontinuous mode up to
TK/SS = 0.5V. Between TK/SS = 0.5V and 0.565V, it will
operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.565V. The output ripple
is minimized during the 65mV 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
3867f
24