ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 29

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Reduce V
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering V
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
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 regu-
lator, assume V
V
(see Figure 13).
The regulated output voltages are determined by:
Using 20k 1% resistors from both V
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 9).
OUT1
V
OUT
= 1.8V, V
= 0.6V • 1+
IN
from its nominal level to verify operation
OUT2
IN
= 12V(nominal), V
⎛
⎜
⎝
= 1.2V, I
R
R
B
A
⎞
⎟
⎠
MAX1,2
IN
, Schottky and the top
= 15A, and f = 400kHz
IN
FB
= 20V(maximum),
nodes to ground,
IN
while
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:
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:
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
With I
can be calculated by using the minimum value for the
maximum current sense threshold (43mV).
The equivalent required R
nel 1 and 2.5mΩ 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
required to divide down the signal.
IN
L =
R
ΔI
t
, and should not be less than 90ns:
ON(MIN)
SENSE(EQUIV)
L(NOM)
LIM
ƒ • ΔI
floating, the equivalent R
V
=
OUT
=
L(MAX)
V
IN(MAX)
V
ƒ • L
LTC3869/LTC3869-2
V
OUT
=
OUT
I
LOAD(MAX)
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
1−
1−
ƒ
SENSE
=
V
SENSE
V
V
SENSE(MIN)
20V(400kHz)
IN(NOM)
IN(MAX)
V
V
values. Therefore, R2 is not
OUT
OUT
+
1.2V
value is 2.4mΩ for chan-
ΔI
L(NOM)
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
SENSE
2
= 150ns
resistor value
29
3869f