Datasheet

LTC3729
22
3729fb
Design Example (Using Two Phases)
As a design example, assume V
IN
= 5V (nominal), V
IN
= 5.5V
(max), V
OUT
= 1.8V, I
MAX
= 20A, T
A
= 70°C and f = 300kHz.
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 PLLFLTR
pin to a resistive divider using the INTV
CC
pin to generate
1V for 300kHz operation. The minimum inductance for
30% ripple current is:
L ≥
V
OUT
f ∆I
( )
1−
V
OUT
V
IN






≥
1.8V
300kHz
( )
30%
( )
10A
( )
1−
1.8V
5.5V






≥ 1.35µH
A 2µH inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on‑time
occurs at maximum V
IN
:
t
ON MIN
( )
=
V
OUT
V
IN
f
=
1.8V
5.5V
( )
300kHz
( )
= 1.1µs
The R
SENSE
resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
R
SENSE
=
50mV
11.5A
≈ 0.005Ω
Choosing 1% resistors: R1 = 16.5k and R2 = 13.2k yields
an output voltage of 1.80V.
APPLICATIONS INFORMATION
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
DS(ON)
= 0.013Ω, C
RSS
= 300pF. At maximum input
voltage with T
j
(estimated) = 110°C at an elevated ambient
temperature:
P
MAIN
=
1.8V
5.5V
10
( )
2
1+ 0.005
( )
110°C − 25°C
( )




0.013Ω+ 1.7 5.5V
( )
2
10A
( )
300pF
( )
310kHz
( )
= 0.61W
The worst‑case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction
temperature rise is:
P
SYNC
=
5.5V
−
1.8V
5.5V
10A
( )
2
1.48
( )
0.013Ω
( )
= 1.29W
A short‑circuit to ground will result in a folded back
current of:
I
SC
=
25mV
0.005Ω
+
1
2
200ns 5.5V
( )
2µH






= 5.28A
The worst‑case power disipated by the synchronous
MOSFET under short‑circuit conditions at elevated ambi‑
ent temperature and estimated 50°C junction temperature
rise is:
P
SYNC
=
5.5V
−
1.8V
5.5V
5.28A
( )
2
1.48
( )
0.013Ω
( )
= 360mW