LTC3828 Datasheet, PDF (24/32 Pages) Linear Technology – Dual 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIO S I FOR ATIO
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 LTC3828 have a maximum input
voltage of 30V, most applications will also be limited to
30V by the MOSFET BVD_SS.
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3828
3828 F11
Figure 11. Automotive Application Protection
Design Example
As a design example for one channel, assume V_IN =
12V(nominal), V_IN = 22V(max), V_OUT = 1.8V, I_MAX = 5A,
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 from the INTV_CC pin, generating 0.7V
for 300kHz operation. The minimum inductance for 30%
ripple current is:
∆I_L
=
V_OUT
(f)(L)
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
A 4.7µH inductor will produce 23% ripple current and a
3.3µH will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3µH value. Increasing the ripple current
will also help ensure that the minimum on-time of 100ns
is not violated. The minimum on-time occurs at maximum
V_IN:
t_ON(MIN)
=
V_OUT
V_IN(MAX)f
=
1.8V
22V(300kHz)
=
273ns
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R_SENSE
≤
60mV
5.84A
≈
0.01Ω
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’s specified input
current.
R1_(MAX)
=
⎛
24k_⎝⎜
0.8V
2.4V – V_OUT
⎞
⎠⎟
=
⎛
24k_⎝⎜
0.8V ^⎞
2.4V – 1.8V⎠⎟
=
32k
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Fairchild FDS6982S dual
MOSFET results in: R_DS(ON) = 0.035Ω/0.022Ω, C_MILLER =
215pF. At maximum input voltage with T(estimated) =
50°C:
P_MAIN
=
1.8V
22V
(⁵)²[¹⁺
(0.005)(50°C
–
25°C
)_]
•
(^0.035Ω)
+
(^22V)²
⎛⎝⎜
5A
2
⎞⎠⎟
(^4Ω)(^215pF)
•
⎡
⎣⎢⁵
1
– 2.3
+
1
2.3
⎤⎦⎥_(300kHz)
=
332mW
A short-circuit to ground will result in a folded back current
of:
I_SC
=
25mV
0.01Ω
–
1⎛ 120ns(22V)⎞
2 ⎝⎜ 3.3µH ⎠⎟
=
2.1A
3828f
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