ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 28

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LTC3850-2
APPLICATIONS INFORMATION
Design Example
As a design example for a two channel medium current regu-
lator, assume V
V
(see Figure 14).
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) 63.4k and 25.5k.
The frequency is set by biasing the FREQ/PLLFLTR pin to
1.2V (see Figure 10), using a divider from INTV
voltage will decrease as V
switching frequency. If a separate 5V supply is connected to
EXTV
The inductance values are based on a 35% maximum
ripple current assumption (1.75A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
Channel 1 will require 3.2μH, and Channel 2 will require
1.9μH. The next highest standard values are 3.3μH and 2.2μH.
At the nominal input voltage (12V), the ripple will be:
Channel 1 will have 1.45A (29%) ripple, and Channel 2 will
have 1.4A (28%) ripple. The peak inductor current will be
the maximum DC value plus one-half the ripple current,
or 5.725A for Channel 1 and 5.7A for Channel 2.
28
OUT1
L =
V
ΔI
OUT
CC
L(NOM)
= 3.3V, V
ƒ • ΔI
, INTV
= 0.8V • 1+
V
OUT
=
L(MAX)
CC
V
ƒ • L
IN
OUT2
OUT
will remain at 5V even if V
⎛
⎝ ⎜
= 12V(nominal), V
⎛
⎜
⎝
= 1.8V, I
⎛
⎜
⎝
1−
R
R
1−
B
A
⎞
⎠ ⎟
V
IN
V
IN(MAX)
IN(NOM)
V
V
approaches 5V, lowering the
MAX1,2
OUT
OUT
⎞
⎟
⎠
⎞
⎟
⎠
= 5A, and f = 500kHz
IN
FB
= 20V(maximum),
nodes to ground,
IN
decreases.
CC
. This
The minimum on-time occurs on Channel 1 at the maximum
V
The equivalent R
using the minimum value for the maximum current sense
threshold (40mV).
The equivalent R
The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCR
at 20°C) and HCP0703-3R3 (30mΩ DCR
chosen. At 100°C, the estimated maximum DCR values are
26.4mΩ and 39.6mΩ. The divider ratios are:
For each channel, 0.1μF is selected for C1.
For channel 1, the DCR
IN
R
R
R1||R2 =
R1=
R2 =
t
and
= 1.1k ; and
, and should not be less than 90ns:
5A +
ON(MIN)
D
SENSE(EQUIV)
40mV
=
R1||R2
R1 • R
DCR
1.5A
1− R
39.6mΩ
R
2
R
7mΩ
D
=
(DCR
SENSE(EQUIV)
MAX
D
V
≅ 7mΩ
D
IN(MAX)
=
V
SENSE
=
SENSE
MAX
=
OUT
0.18
at T
1.1k
6.19k • 0.18
30mΩ • 0.1µF
≅ 0.18
I
LOAD(MAX)
1− 0.18
L(MAX)
at 20°C) • C1
L
ƒ
resistor value can be calculated by
is the same for Channel 2.
3.3µH
SENSE
≅ 6.19k;
=
V
SENSE(MIN)
20V(500kHz)
=
fi lter/divider values are:
+
1.8V
26.4mΩ
≅ 1.33k
ΔI
7mΩ
= 1.1k
=
L(NOM)
2
20mΩ • 0.1µF
MAX
= 0.26;
= 180ns
2.2µH
=
at 20°C) are
38502f
MAX