LTC3729L-6 Datasheet, PDF (22/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

LTC3729L-6
APPLICATIO S I FOR ATIO
Although the LTC3729L-6 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET BV_DSS.
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3729L-6
3729L-6 F08
Figure 8. Automotive Application Protection
Design Example (Using Two Phases)
Asadesignexample,assumeV_IN =5V(nominal),V_IN␣ =␣ 5.5V
(max), V_OUT = 1.8V, I_MAX = 20A, T_A = 70°C and f␣ =␣ 250kHz.
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 the SGND pin for 260kHz operation. The minimum
inductance for 30% ripple current is:
L
≥
V_OUT
f₍∆I₎
_1−
V_OUT
V_IN


≥
1.8V
(^260kHz)(^30%)(^10A)

1−
1.8V
5.5V

≥ 1.55µH
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)(^260kHz)
= 1.2µs
The R_SENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
R_SENSE
=
60mV
11.5A
≈
0.005Ω
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the sense pin input current for
both channels.
R1_(MIN)
=
20k
2


V_OUT
2.4V − V_OUT


=
10k^{}
1.8V
2.4V − 1.8V

=
30k
Choosing 1% resistors: R1 = 10k and R2 = 20k yields an
output voltage of 1.80V and satisfies the above condition.
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si7840DP for example;
R_DS(ON) = 0.014Ω, C_MILLER = 147pF. At maximum input
voltage with T_j (estimated) = 110°C at an elevated ambient
temperature:
[ ] ^PMAIN
=
1.8V
5.5V
(¹⁰)²
1+
(^0.005)(^110°C
−
25°C₎
0.014Ω
+
(^5.5V)²

10A
2

(^147pF)

5
1
− 2.3
+
21.3^{}
•
(^260kHz)⁼
0.66W
The worst-case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is: (Si7440DP)
P_SYNC
=
5.5V − 1.8V
5.5V
(^10A)²
(^1.25)(^0.008Ω)
= 0.673W
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
22
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