LTC3729_15 Datasheet, PDF (22/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
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_INf
=
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.
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
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