ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 22

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3853
APPLICATIONS INFORMATION
time. The total RMS power lost is lower when more than
one controller is 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 or triple 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 3-phase system. The overall benefi t of
a multiphase design will only be fully realized when the
source impedance of the power supply/battery is included
in the effi ciency testing. The sources of the top MOSFETs
should be placed within 1cm of each other and share a
common C
duce undesirable voltage and current resonances at V
A small (0.1μF to 1μF) bypass capacitor between the chip
V
suggested. A 2.2Ω to 10Ω resistor placed between C
(C1) and the V
the channels.
The selection of C
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
Setting Output Voltage
The LTC3853 output voltages are each set by an external
feedback resistive divider carefully placed across the
output, as shown in Figure 9. The regulated output volt-
age is determined by:
22
IN
Δ
V
pin and ground, placed close to the LTC3853, is also
OUT
V
OUT
RIPPLE
=
IN
≈
0 8
(s). Separating the sources and C
.
I
increases with input voltage.
RIPPLE
IN
V
•
OUT
RIPPLE
pin provides further isolation between
⎛
⎝ ⎜
OUT
1
⎛
⎝ ⎜
) is approximated by:
+
ESR
R
R
is driven by the effective series
is the ripple current in the induc-
B
A
⎞
⎠ ⎟
+
8
fC
1
OUT
⎞
⎠ ⎟
OUT
is the output
IN
may pro-
IN
IN
.
To improve the frequency response, a feed-forward ca-
pacitor, C
route the V
inductor or the SW line.
Fault Conditions: Current Limit and Current Foldback
The LTC3853 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 LTC3853 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
and inductor value:
The resulting short-circuit current is:
Phase-Locked Loop and Frequency Synchronization
The LTC3853 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the top MOSFET of
I
ΔI
SC
L SC
ON(MIN)
(
=
1 3
FF
)
/
, may be used. Great care should be taken to
FB
=
V
t
of the LTC3853 (≈ 90ns), the input voltage
R
line away from noise sources, such as the
Figure 9. Setting Output Voltage
ON MIN
SENSE MAX
1/3 LTC3853
SENSE
(
(
)
V
FB
•
V
L
)
IN
–
V
2
1
OUT
Δ
R
R
3853 F09
B
A
I
L SC
(
)
C
FF
3853f