LTC3727LX-1_15 Datasheet, PDF (23/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
APPLICATIO S I FOR ATIO
can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse-battery is
just what it says, while double-battery is a consequence of
tow-truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 9 is the most straight forward
approach to protect a DC/DC converter from the ravages
of an automotive power line. The series diode prevents
current from flowing during reverse-battery, while the
transient suppressor clamps the input voltage during
load-dump. Note that the transient suppressor should not
conduct during double-battery operation, but must still
clamp the input voltage below breakdown of the converter.
Although the LTC3727LX-1 has a maximum input voltage
of 32V, most applications will be limited to 30V by the
MOSFET BVDSS.
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3727LX-1
3727LX1 F09
Figure 9. Automotive Application Protection
Design Example
As a design example for one channel, assume V_IN =
24V(nominal), V_IN = 30V(max), V_OUT = 12V, I_MAX = 5A and
f = 250kHz.
The inductance value is chosen first based on a 40% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR pin
to the SGND pin for 250kHz operation. The minimum
inductance for 40% ripple current is:
∆I_L
=
V_OUT
(f)(L)
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
A 14µH inductor will result in 40% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 6A, for the 14µH value.
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
90mV
R_SENSE ≤ _6A ≈ 0.015Ω
Choosing 1% resistors; R1 = 20k and R2 = 280k yields an
output voltage of 12V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in: R_DS(ON) = 0.042Ω, C_RSS = 100pF. At maximum input
voltage with T(estimated) = 50°C:
P_MAIN
=
12V
30V
(5)²
[¹⁺
(0.005)(50°C
–
25°C)_]
(0.042Ω) + 1.7(30V)² (5A)(100pF)(250kHz)
= 664mW
A short-circuit to ground will result in a folded back
current of:
I_SC
=
45mV
0.015Ω
+
1⎛
2 ⎝⎜
200ns(30V)⎞
14µH ⎠⎟
=
3.2A
with a typical value of R_DS(ON) and δ = (0.005/°C)(20) =
0.1. The resulting power dissipated in the bottom
MOSFET is:
P_SYNC
=
30V – 12V
30V
(3.2A)²
(1.1)(0.042Ω)
= 284mW
which is less than under full-load conditions.
C_IN is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. C_OUT is
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_ORIPPLE = R_ESR (∆I_L) = 0.02Ω(2A) = 40mV_P–P
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