LTC3774
31
3774fc
For more information www.linear.com/LTC3774
8. Are the signal and power grounds kept separate? The
IC ground pin and the ground return of C
INTVCC
must
return to the combined C
OUT
(–) terminals. The V
OSNS
+
and I
TH
traces should be as short as possible. The path
formed by the top N-channel MOSFET, Schottky diode
and the C
IN
capacitor should have short leads and PC
trace lengths. The output capacitor (–) terminals should
be connected as close as possible to the (–) terminals
of the input capacitor by placing the capacitors next to
each other and away from the Schottky loop described
above.
9. Use a modified “star ground” technique: a low imped
-
ance, large copper area central grounding point on
the same side of the PC board as the input and output
capacitors with tie-ins for the bottom of the INTV
CC
decoupling capacitor, the bottom of the voltage feedback
resistive divider and the GND pin of the IC.
Design Example
As a design example of the front page circuit for a two-
channel high current regulator, assume V
IN
= 12V(nominal),
V
IN
= 20V(maximum), V
OUT
= 1.5V, I
MAX
= 60A, and
f = 400kHz (see front page schematic).
The regulated output voltage is determined by:
V
OUT
= 0.6V • 1+
R
B
R
A
⎛
⎝
⎜
⎞
⎠
⎟
Using a 10k 1% resistor from the V
FB
node to ground, the
top feedback resistor is 15k.
The frequency is set by biasing the FREQ pin to 0.75V
(see Figure 12).
The inductance value is based on a 35% maximum ripple
current assumption (10.5A per phase). The highest value
of ripple current occurs at the maximum input voltage:
L =
V
OUT
f • ∆I
L(MAX)
1−
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
This design will require 0.33µH. The Würth 744301033,
0.32µH inductor is chosen. At the nominal input voltage
(12V), the ripple current will be:
∆I
L(NOM)
=
V
OUT
f • L
1−
V
OUT
V
IN(NOM)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
It will have 10A (33%) ripple. The peak inductor current
will be the maximum DC value plus one-half the ripple
current, or 35A per phase.
The minimum on-time occurs at the maximum V
IN
, and
should not be less than 100ns (includes margin):
t
ON(MIN)
=
OUT
V
IN(MAX)
f
=
20V(400kHz)
= 187ns
DCR sensing is used in this circuit. If C1 and C2 are chosen
to be 220nF, based on the chosen 0.33µH inductor with
0.32mΩ DCR, R1 and R2 can be calculated as:
R1=
DCR •C1
= 4.69k
R2 =
L
= 937Ω
Choose R1 = 4.64k and R2 = 931Ω.
APPLICATIONS INFORMATION
