ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 28

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3858-2
Design Example
As a design example for one channel, assume V
12V(nominal), V
V
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
A 4.7μH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 5.73A. Increasing the ripple
current will also help ensure that the minimum on-time
of 95ns is not violated. The minimum on-time occurs at
maximum V
The equivalent R
using the minimum value for the maximum current sense
threshold (64mV):
Choosing 0.5% resistors: R
an output voltage of 3.296V.
28
APPLICATIONS INFORMATION
SENSE(MAX)
R
ΔI
t
ON(MIN)
SENSE
L
=
( )
V
f
≤
OUT
IN
=
( )
= 75mV and f = 350kHz.
5.73A
L
64mV
:
V
V
IN
OUT
IN
⎛
⎜
⎝
SENSE
1–
( )
= 22V (max), V
f
= 0.011Ω
V
=
V
OUT
resistor value can be calculated by
IN
22V 350kHz
A
⎞
⎟
⎠
(
= 24.9k and R
3.3V
OUT
)
= 3.3V, I
= 429ns
B
= 77.7k yields
MAX
= 5A,
IN
=
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
maximum input voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02Ω for low output ripple volt-
age. The output ripple in continuous mode will be highest
at the maximum input voltage. The output voltage ripple
due to ESR is approximately:
IN
V
I
P
P
SC
is chosen for an RMS current rating of at least 3A at
SYNC
ORIPPLE
MAIN
=
0.015Ω
= 2.98A
=
32mV
⎡
⎣ ⎢
(
DS(ON)
(
3.3V
= R
22V
0.035Ω
5V – 2.3V
ESR
( )
1
–
= 0.035Ω/0.022Ω, C
5A
)
(ΔI
2
2
1
)
(
⎛
⎜
⎝
+ 22V
2
1.125
L
95ns 22V
) = 0.02Ω(1.45A) = 29mV
⎡ ⎣
+
DS(ON)
(
1+ 0.005
2.3V
4.7μH
(
1
(
)
)
(
2
and δ = (0.005/°C)(25°C)
0.022Ω
⎤
⎦ ⎥
5A
(
2
350kHz
)
)
⎞
⎟ = 2.98A
⎠
(
(
2.5Ω
50°C – 25°C
MILLER
)
= 220mW
)
)
= 331mW
(
215pF
= 215pF . At
P-P
OUT
)
)
⎤ ⎦
38582f
•
is