Datasheet

LTC3827-1
16
38271fe
APPLICATIONS INFORMATION
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
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 LTC3827-1, is
also suggested. A 10 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 satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
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 LTC3827-1 output voltages are each set by an exter-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 3. The regulated output voltage
is determined by:
V
OUT
= 0.8V • 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.
SENSE
+
and SENSE
–
Pins
The common mode input range of the current comparator
is from 0V to 10V. Continuous linear operation is provided
throughout this range allowing output voltages from 0.8V
to 10V. The input stage of the current comparator requires
that current either be sourced or sunk from the SENSE pins
depending on the output voltage, as shown in the curve in
Figure 4. If the output voltage is below 1.5V, current will
fl ow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V
OUT
resistor divider to compensate for the current comparator’s
negative input bias current. Since V
FB
is servoed to the
0.8V reference voltage, R
A
in Figure 3 should be chosen
to be less than 0.8V/I
SENSE
, with I
SENSE
determined from
Figure 4 at the specifi ed output voltage.
Figure 3. Setting Output Voltage
Figure 4. SENSE Pins Input Bias Current
vs Common Mode Voltage
1/2 LTC3827-1
V
FB
V
OUT
R
B
C
FF
R
A
3827-1 F03
V
SENSE
COMMON MODE VOLTAGE (V)
0
–700
INPUT CURRENT (µA)
–600
–400
–300
–200
6789
200
38271 F04
–500
12345 10
–100
0
100