LTC3890-2 Datasheet, PDF (21/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-2
APPLICATIONS INFORMATION
The benefit of the LTC3890-2 2-phase operation can be
calculated by using Equation 1 for the higher power control-
ler and then calculating the loss that would have resulted
if both controller channels switched on at the same time.
The total RMS power lost is lower when both controllers
are operating due to the reduced overlap of current pulses
required through the input capacitor’s ESR. This is why
the input capacitor’s requirement calculated above for the
worst-case controller is adequate for the dual controller
design. Also, the input protection fuse resistance, battery
resistance, and PC board trace resistance losses are also
reduced due to the reduced peak currents in a 2-phase
system. The overall benefit of a multiphase design will
only be fully realized when the source impedance of the
power supply/battery is included in the efficiency testing.
The drains of the top MOSFETs should be placed within
1cm of each other and share a common C_IN(s). Separating
the drains and C_IN may produce undesirable voltage and
current resonances at V_IN.
A small (0.1μF to 1μF) bypass capacitor between the chip
V_IN pin and ground, placed close to the LTC3890-2, is
also suggested. A 10Ω resistor placed between C_IN (C1)
and the V_IN pin provides further isolation between the
two channels.
The selection of C_OUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (ΔV_OUT) is approximated by:
ΔV_OUT
≈
⎛
ΔI_L
⎜ESR
⎝
+
8
•
f
1
• C_OUT
⎞
⎟
⎠
where f is the operating frequency, C_OUT is the output
capacitance and ΔI_L is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage.
Setting Output Voltage
The LTC3890-2 output voltages are each set by an exter-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 5. The regulated output voltage
is determined by:
V_OUT
=
⎛
0.8V _⎜1+
⎝
R_B
R_A
⎞
⎟
⎠
To improve the frequency response, a feedforward ca-
pacitor, C_FF, may be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
inductor or the SW line.
V_OUT
1/2 LTC3890-2
V_FB
R_B
C_FF
R_A
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Figure 5. Setting Output Voltage
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3890-2 regulates the V_FB pin voltage to the voltage on
the TRACK/SS pin instead of 0.8V. The TRACK/SS pin can
be used to program an external soft-start function or to
allow V_OUT to track another supply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 1μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3890-2 will regulate the V_FB pin (and hence V_OUT)
according to the voltage on the TRACK/SS pin, allowing
V_OUT to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
t_SS
=
C_SS
•
0.8V
1μA
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