ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 21

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 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
the sources and C
current resonances at V
A small (0.1μF to 1μF) bypass capacitor between the chip
V
suggested. A 2.2Ω – 10Ω resistor placed between C
(C1) and the V
the two 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 LTC3850-2 output voltages are each set by an exter-
nal feedback resistive divider carefully placed across the
output, as shown in Figure 9. The regulated output voltage
is determined by:
IN
ΔV
V
pin and ground, placed close to the LTC3850-2, is also
OUT
OUT
RIPPLE
= 0.8V • 1+
≈I
RIPPLE
increases with input voltage.
IN
OUT
⎛
⎝ ⎜
RIPPLE
IN
pin provides further isolation between
OUT
⎛
⎝ ⎜
may produce undesirable voltage and
) is approximated by:
ESR +
R
R
A
B
is driven by the effective series
IN
is the ripple current in the induc-
⎞
⎠ ⎟
.
8fC
1
OUT
⎞
⎠ ⎟
OUT
IN
(s). Separating
is the output
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 LTC3850-2 includes current foldback to help limit
load current when the output is shorted to ground. If the
output 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 maxi-
mum value. Foldback current limiting is disabled during
the soft-start or tracking up. Under short-circuit condi-
tions with very low duty cycles, the LTC3850-2 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
voltage and inductor value:
The resulting short-circuit current is:
Phase-Locked Loop and Frequency Synchronization
The LTC3850-2 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (V
detector. This allows the turn-on of the top MOSFET of
controller 1 to be locked to the rising edge of an external
clock signal applied to the MODE/PLLIN pin. The turn-on
I
ΔI
SC
L(SC)
=
1/3 V
FF
ON(MIN)
= t
, may be used. Great care should be taken to
FB
R
ON(MIN)
line away from noise sources, such as the
SENSE(MAX)
Figure 9. Setting Output Voltage
1/2 LTC3850-2
SENSE
of the LTC3850-2 (≈ 90ns), the input
V
•
FB
V
L
IN
–
2
1
V
OUT
ΔI
R
R
38502 F09
L(SC)
B
A
LTC3850-2
C
FF
CO
) and a phase
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