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| LTC3851A_15 Datasheet, PDF (24/30 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller | |||
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LTC3851A
Applications Information
Design Example
As a design example, assume VIN = 12V (nominal), VIN =
22V (maximum), VOUT = 1.8V, IMAX = 5A, and f = 250kHz.
Refer to Figure 13.
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 opÂeration. The minimum inductance
for 30% ripple current is:
ÎIL
=
1
(f)(L)
VOUT

1â
ï£
VOUT
VIN



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 VIN:
tON(MIN)
=
VOUT
VIN(MAX) (f)
=
1.8V
22V (250kHz)
=
327ns
The RSENSE resistor value can be calculated by connect-
ing ILIM to INTVCC and using the maximum current sense
voltage specification with some accommodation for toler-
ances. Tie ILIM to INTVCC.
RSENSE
â¤
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: RDS(ON) = 0.035â¦/0.022â¦, CMILLER = 215pF. At
maximum input voltage with T (estimated) = 50°C:
PMAIN
=
1.8V
22V
(5)2
1+
(0.005) (50°C
â
25°C)
â¢
(0.035Ω)
+
(22V
)2

ï£ï£¬
5A
2
(2Ω)
(215pF
)
â¢

5
1
â 2.3
+
21.3(250kHz)
=
185mW
A short circuit to ground will result in a folded back cur-
rent of:
ISC
=
29mV
0.0125Ω
â
1
2


ï£
90ns (22V )
3.3µH



=
2.02A
with a typical value of RDS(ON) and δ = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
PSYNC
=
22V
22V
(2.02A)2
(1.125) (0.022Ω)
= 101.0mW
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 3A at
temperature. COUT 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:
VORIPPLE = RESR (âIL) = 0.02⦠(2A) = 40mVP-P
3851afa
24
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