LTC3859_15 Datasheet, PDF (25/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
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). Separat-
ing 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 LTC3859, is also
suggested. A small (1Ω to 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 (DV_OUT) is approximated by:
ΔV_OUT
≈
ΔI_L
⎛
⎝⎜ ^ESR
+
1
8f C_OUT
⎞
⎠⎟
where f is the operating frequency, C_OUT is the output
capacitance and DI_L is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since DI_L increases with input voltage.
Setting Output Voltage
The LTC3859 output voltages are each set by an external
feedback resistor divider carefully placed across the output,
as shown in Figure 5. The regulated output voltages are
determined by:
V_OUT,
BUCK
=
0.8V
⎛
⎝⎜
1+
R_B
R_A
⎞
⎠⎟
V_OUT,
BOOST
=
1.2V
⎛
⎝⎜
1+
R_B
R_A
⎞
⎠⎟
V_OUT
1/3 LTC3859
V_FB
R_B
C_FF
3859 F05
R_A
Figure 5. Setting Output Voltage
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.
Tracking and Soft-Start
(TRACK/SS1, TRACK/SS2, SS3 Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective TRACK/SS pin (TRACK/SS1 for channel 1,
TRACK/SS2 for channel 2, SS3 for channel 3). When the
voltage on the TRACK/SS pin is less than the internal
0.8V reference (1.2V reference for the boost channel), the
LTC3859 regulates the V_FB pin voltage to the voltage on the
TRACK/SS pin instead of the internal reference. Likewise,
the TRACK/SS pin for the buck channels can be used to
program an external soft-start function or to allow V_OUT
to track another supply during start-up.
1/3 LTC3859
TRACK/SS
C_SS
SGND
3859 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
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 LTC3859 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 _BUCK}
= C_SS
•
0.8V
1µA
t_{SS _BOOST}
=
C_SS
•
1.2V
1µA
3859fa
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