LTC3855_15 Datasheet, PDF (26/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

LTC3855
Applications Information
can also be used for C_IN. Always consult the manufacturer
if there is any question.
The benefit of the LTC3855 2-phase operation can be cal-
culated by using the equation above for the higher power
controller 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 control-
lers 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 sources of the top MOSFETs should be placed within
1cm of each other and share a common C_IN(s). Separating
the sources 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 LTC3855, is also
suggested. A 2.2Ω 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 (∆V_OUT) is approximated by:
∆V_OUT
≈ I_RIPPLE
 _ESR
+
1
8fC_OUT


where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I_RIPPLE increases with input voltage.
Setting Output Voltage
The LTC3855 output voltages are each set by an external
feedback resistive divider carefully placed across the
output, as shown in Figure 11. The regulated output
voltage is determined by:
V_OUT
=
0.6V
•


1+
R_B
R_A


To improve the frequency response, a feed-forward 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 LTC3855
V_FB
R_B
C_FF
R_A
3855 F11
Figure 11. Setting Output Voltage
Fault Conditions: Current Limit and Current Foldback
The LTC3855 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put falls below 50% of its nominal output level, then the
maximum sense voltage is progressively lowered from its
maximum programmed value to one-third of the maximum
value. Foldback current limiting is disabled during the
soft-start or tracking up. Under short-circuit conditions
with very low duty cycles, the LTC3855 will begin cycle
skipping in order to limit the short-circuit current. In this
situation the bottom MOSFET will be dissipating most of
the power but less than in normal operation. The short-
circuit ripple current is determined by the minimum on-
time t_ON(MIN) of the LTC3855 (≈ 90ns), the input voltage
and inductor value:
∆I_L(SC)
=
t_ON(MIN)
•
V_IN
L
The resulting short-circuit current is:
I_SC
=
1/3
V_{SENSE(MAX )}
R_SENSE
–
1
2
∆I_L(SC)
3855f
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