LTC3868 Linear Technology, LTC3868 Datasheet - Page 26

• Manufacturer Part Number: LTC3868
• Description: Low IQ Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

LTC3868
C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250µs rise time, limiting the charging current
to about 200mA.
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 1% resistors: R
an output voltage of 3.299V.

applicaTions inForMaTion
LOAD
SENSE(MAX)
R
∆I
t
ON MIN
SENSE
L(NOM)
to C
(
)
OUT
≤
=
IN
= 75mV and f = 350kHz.
=
5.73A
64mV
:
is greater than 1:50, the switch rise time
V
( )
V
IN MAX
f
IN
SENSE
OUT
(
V
( )
= 22V (max), V
L
OUT
= 0.01Ω




resistor value can be calculated by
)
1–
LOAD
( )
f
A
V
=
= 25k and R
. Thus a 10µF capacitor would
IN(NOM)
V
22 350
OUT
V
(
3 3
OUT
.




V
= 3.3V, I
kHz
B
= 78.1k yields
)
=
429
MAX
ns
= 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 under full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02Ω for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
V
IN
I
P
P
is chosen for an RMS current rating of at least 3A at
SC
MAIN
SYNC
ORIPPLE
=
0.01Ω
32mV
=
= 2.98A
(
= R



(
DS(ON)
3.3V
22V
0.035Ω
5V – 2.3V
ESR
–
( )
1
= 0.035Ω/0.022Ω, C
5A
2
1
(∆I
)



2
)
95ns 22V
(
+ 22V
2
1.125
L


) = 0.02Ω(1.45A) = 29mV
(
+
DS(ON)
4.7µH
1+ 0.005
2.3V
(
(
1
)
)
(
2
and δ = (0.005/°C)(25°C)
0.022Ω

 350kHz

5A
)
www.DataSheet4U.com
2
(

 = 2.98A

)
(
2.5Ω
(
50°C – 25°C
MILLER
)
= 220mW
)
)
(
= 331mW
215pF
= 215pF . At
P-P
)
OUT
)


•
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is