LTC3851A-1_15 Datasheet, PDF (25/30 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851A-1
Applications Information
Design Example
As a design example, assume V_IN = 12V (nominal), V_IN =
22V (maximum), V_OUT = 1.8V, I_MAX = 5A, and f = 250kHz
(refer to Figure 12).
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. Connect a
160k resistor between the FREQ/PLLFLTR and GND pins,
generating 250kHz op­eration. The minimum inductance
for 30% ripple current is:
∆I_L
=
1
(^f)(^L)
V_OUT
⎛
⎜^1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
A 4.7µH inductor will produce 28% ripple current and
a 3.3µH will result in 40%. The peak inductor current
will be the maximum DC value plus one-half the ripple
current, or 6A, for the 3.3µH value. Increasing the ripple
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₎
=
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
≤
50mV
6A
=
0.0083Ω
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
+
21.3^{⎤⎦⎥(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 (∆I_L) = 0.02Ω (2A) = 40mV_P-P
3851a1fa
25