LTC3869-2_15 Datasheet, PDF (30/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Applications Information
Reduce V_IN from its nominal level to verify operation of
the regulator in dropout. Check the operation of the un-
dervoltage 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 regu-
lator, 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 13).
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 9).
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
=
ƒ
•
V_OUT
∆ I_{L (MAX )}
⎛
⎝⎜⎜^1−
V_OUT
V_{IN (MAX )}
⎞
⎠⎟⎟
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_L(NOM)
=
V_OUT
ĥL
⎛
⎝⎜⎜^1−
V_OUT
V_IN(NOM)
⎞
⎠⎟⎟
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) ƒ
=
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 (43mV).
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 2.5mΩ 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_SENSE values. Therefore, R2 is not
required to divide down the signal.
30
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