LTC3618 Datasheet, PDF (17/24 Pages) Linear Integrated Systems – Dual 4MHz, 3A Synchronous Buck Converter for DDR Termination

LTC3618
APPLICATIONS INFORMATION
Run and Soft-Start
The RUNx pins provide a means to shut down each chan-
nel of the LTC3618. Pulling both pins below 0.3V places
the LTC3618 in a low quiescent current shutdown state
(I_Q < 1μA).
After enabling the LTC3618 by bringing the RUNx pins
above the threshold, the enabled channels enter a soft-
start-up state. The type of soft-start behavior of V_DDQ is
set by the TRACK/SS1 pin. The soft-start cycle begins with
an initial discharge pulse pulling down the TRACK/SS1
pin to SGND and discharging the external capacitor C_SS
(see Figure 3).
The initial discharge is adequate to discharge capacitors
up to 33nF. If a larger capacitor is required, connect the
external soft-start resistor R_SS to the RUN pin to fully
discharge the capacitor.
1. Tying this pin to SV_IN selects the internal soft-start
circuit for V_DDQ to the final value within 1ms.
2. If a longer soft-start period is desired, it can be
set externally with a resistor and capacitor on the
TRACK/SS1 pin as shown in Figure 3. The voltage ap-
plied at the TRACK/SS1 pin sets the value of the internal
reference at V_FB1 until TRACK/SS1 is pulled above 0.6V.
The external soft-start duration can be calculated by
using the following equation:
t_SS1
=
R_SS
•
C_SS
•
I
n




SV_IN
SV_IN – 0.6V



3. The TRACK/SS1 pin can be used to track the output
voltage of another supply.
The VTTR voltage follows the soft-start behavior of
V_DDQ at the same rate and ramps up V_TT output voltage.
If RUN2 is pulled high later than RUN1, VTTR will follow
its internal soft-start, and ramps output voltage of V_TT
at a rate of approximately 850mV/ms.
Regardless of either the internal or external soft-start
state, the MODE/SYNC pin is ignored during start-up and
defaults to pulse-skipping mode. In addition, the PGOOD
pin is kept low, and the frequency foldback function is
disabled.
Output Voltage Tracking Input
In the run state, the TRACK/SS1 pin can be used to track
down/up the output voltage of another supply for V_DDQ.
If V_TRACK/SS1 again drops below 0.6V, the LTC3618 enters
the down-tracking state and V_DDQ is referenced to the
TRACK/SS1 voltage. If V_TRACK/SS1 reaches 0.1V value
the switching frequency is reduced by 4x to ensure that
the minimum duty cycle limit does not prevent the output
from following TRACK/SS1 pin. The run state will resume
if V_TRACK/SS1 again exceeds 0.6V and V_DDQ is referenced
to the internal reference.
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc.
are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses: V_IN quiescent current and I²R losses. The V_IN
quiescent current loss dominates the efficiency loss at
very low load currents whereas the I²R loss dominates
the efficiency loss at medium to high load currents. In a
typical efficiency plot, the efficiency curve at very low load
currents can be misleading since the actual power lost is
of little consequence.
1. The V_IN quiescent current is due to two components: the
DC bias current as given in the Electrical Characteristics
and the internal main switch and synchronous switch
gate charge currents. The gate charge current results
from switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
high to low to high again, a packet of charge dQ moves
from V_IN to ground. The resulting dQ/dt is the current
out of V_IN due to gate charge, and it is typically larger
than the DC bias current. Both the DC bias and gate
charge losses are proportional to V_IN, thus, their effects
will be more pronounced at higher supply voltages.
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