LTC3834-1 Datasheet, PDF (21/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834-1
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume V_IN = 12V(nominal), V_IN =
22V(max), V_OUT = 1.8V, I_MAX = 5A, 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 PLLLPF pin
to GND, generating 250kHz operation. The minimum
inductance for 30% ripple current is:
ΔI_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 180ns
is not violated. The minimum on-time occurs at maxi-
mum V_IN:
t_ON(MIN)
=
V_OUT
V_IN(MAX)f
=
1.8V
22V(250kHz)
=
327ns
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R_SENSE
≤
80mV
5.84A
≈
0.012Ω
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) = 0.035 /0.022 , 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)
(2
15pF
)
•
⎡
⎣⎢
5
1
– 2.3
+
1
2.3
⎤
⎦⎥
(300kHz
)
=
332mW
A short-circuit to ground will result in a folded back cur-
rent of:
I_SC
=
25mV
0.01Ω
–
1⎛ 120ns(22V)⎞
2 ⎝⎜ 3.3μH ⎠⎟
=
2.1A
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
=
22V – 1.8V
22V
(2.1A)²
(1.125)(0.022Ω)
= 100mW
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 0.02 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) = 0.02 (1.67A) = 33mV _P–P
38341f
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