Datasheet

LTC3855
32
3855f
This occurs around 50% duty cycle on either channel due
to the phasing of the internal clocks and may cause minor
duty cycle jitter.
Reduce V
IN
from its nominal level to verify operation
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering V
IN
while
monitoring the outputs to verify operation.
Investigate whether any problems exist only at higher out-
put currents or only at higher input voltages. If problems
coincide with high input voltages and low output currents,
look for capacitive coupling between the BOOST, SW, TG,
and possibly BG connections and the sensitive voltage
and current pins. The capacitor placed across the current
sensing pins needs to be placed immediately adjacent to
the pins of the IC. This capacitor helps to minimize the
effects of differential noise injection due to high frequency
capacitive coupling. If problems are encountered with
high current output loading at lower input voltages, look
for inductive coupling between C
IN
, Schottky and the top
MOSFET components to the sensitive current and voltage
sensing traces. In addition, investigate common ground
path voltage pickup between these components and the
SGND pin of the IC.
Design Example
As a design example for a two channel high current regula-
tor, assume V
IN
= 12V(nominal), V
IN
= 20V(maximum),
V
OUT1
= 1.8V, V
OUT2
= 1.2V, I
MAX1,2
= 15A, and f = 400kHz
(see Figure 16).
The regulated output voltages are determined by:
V
OUT
= 0.6V • 1+
R
B
R
A






Using 20k 1% resistors from both V
FB
nodes to ground,
the top feedback resistors are (to the nearest 1% standard
value) 40.2k and 20k.
The frequency is set by biasing the FREQ pin to 1V (see
Figure 12).
The inductance values are based on a 35% maximum
ripple current assumption (5.25A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
L
V
I
V
V
OUT
L MAX
OUT
IN MAX
= −






f •
( ) ( )
∆
1
Channel 1 will require 0.78µH, and channel 2 will require
0.54µH. The Vishay IHLP4040DZ-01, 0.56µH inductor is
chosen for both rails. At the nominal input voltage (12V),
the ripple current will be:
∆I
V
L
V
V
L NOM
OUT OUT
IN NOM
( )
( )
•
= −






f
1
Channel 1 will have 6.8A (46%) ripple, and channel 2 will
have 4.8A (32%) ripple. The peak inductor current will be
the maximum DC value plus one-half the ripple current,
or 18.4A for channel 1 and 17.4A for channel 2.
The minimum on-time occurs on channel 2 at the maximum
V
IN
, and should not be less than 90ns:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
f
=
1.2V
20V(400kHz)
=150ns
With I
LIM
floating, the equivalent R
SENSE
resistor value
can be calculated by using the minimum value for the
maximum current sense threshold (45mV).
R
SENSE(EQUIV)
=
V
SENSE(MIN)
I
LOAD(MAX)
+
∆I
L(NOM)
2
The equivalent required R
SENSE
value is 2.4mΩ for chan-
nel 1 and 2.6mΩ for channel 2. The DCR of the 0.56µH
inductor is 1.7mΩ typical and 1.8mΩ maximum for a
25°C ambient. At 100°C, the estimated maximum DCR
value is 2.3mΩ. The maximum DCR value is just slightly
under the equivalent R
SENSE
values. Therefore, R2 is not
required to divide down the signal.
For each channel, 0.1µF is selected for C1.
R1=
L
(DCR
MAX
at 25°C)•C1
=
0.56µH
1.8mΩ • 0.1µF
= 3.11k
Choose R1 = 3.09k
applicaTions inForMaTion