LTC3851_15 Datasheet, PDF (24/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851
APPLICATIONS INFORMATION
current will also help ensure that the minimum on-time
of 90ns is not violated. The minimum on-time occurs at
maximum V_IN:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )}
(
f
)
=
22V
1.8V
(250kHz
)
=
327ns
The R_SENSE resistor value can be calculated by connect-
ing I_LIM to INTV_CC and using the maximum current sense
voltage specification with some accommodation for toler-
ances. Tie I_LIM to INTV_CC.
R_SENSE
≤
75mV
6A
=
0.0125Ω,
so
0.01Ω
is
selected
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the topside 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
⎞
⎠⎟
(2Ω
)
(215pF
)
•
⎡
⎣⎢
5
1
− 2.3
+
1
2.3
⎤
⎦⎥
(250kHz)
=
185mW
A short-circuit to ground will result in a folded back cur-
rent of:
I_SC
=
29mV
0.0125Ω
–
1
2
⎛
⎝⎜
90ns(22V)
3.3µH
⎞
⎠⎟
=
2.02A
with a typical value of R_DS(ON) and δ = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P_SYNC
=
22V
22V
(2.02A
)²
(1.125)(0.022Ω)
= 101.0mW
which is less than under full-load conditions.
C_IN is chosen for an RMS current rating of at least 3A at
temperature. 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 (ΔI_L) = 0.02Ω (2A) = 40mV_P-P
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