LTC3890
applicaTions inForMaTion
Design Example
ThepowerdissipationonthetopsideMOSFETcanbeeasily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
As a design example for one channel, assume V = 12V
IN
MAX
results in: R
= 0.035Ω/0.022Ω, C
= 215pF. At
DS(ON)
MILLER
(nominal), V = 22V (max), V
SENSE(MAX)
= 3.3V, I
= 5A,
IN
OUT
maximum input voltage with T(estimated) = 50°C:
V
= 75mV and f = 350kHz.
3.3V
22V
2
Theinductancevalueischosenfirstbasedona30%ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
PMAIN
=
5A 1+ 0.005 50°C – 25°C
(
)
(
)
(
)
2 5A
0.035Ω + 22V
) (
2.5Ω 215pF •
) (
(
)
(
)
2
1
1
+
350kHz = 331mW
(
)
5V – 2.3V 2.3V
VOUT
f L
( ) ( )
VOUT
∆IL =
1–
IN(NOM)
V
A short-circuit to ground will result in a folded back cur-
rent of:
A 4.7µH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 5.73A. Increasing the ripple
current will also help ensure that the minimum on-time
of 95ns is not violated. The minimum on-time occurs at
95ns 22V
34mV
0.01Ω 2
1
(
)
ISC =
–
= 3.18A
4.7µH
with a typical value of R
and δ = (0.005/°C)(25°C)
DS(ON)
maximum V :
IN
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
VOUT
3.3V
tON(MIN)
=
=
= 429ns
V
f
22V 350kHz
2
IN(MAX) ( )
(
)
P
= 3.28A 1.125 0.022Ω
(
) (
) (
)
SYNC
= 250mW
The equivalent R
resistor value can be calculated by
SENSE
using the minimum value for the maximum current sense
threshold (64mV):
which is less than under full-load conditions.
C is chosen for an RMS current rating of at least 3A at
64mV
5.73A
IN
RSENSE
≤
≈ 0.01Ω
temperature assuming only this channel is on. C
is
OUT
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:
Choosing 1% resistors: RA = 25k and RB = 78.7k yields
an output voltage of 3.32V.
V
= R (∆I ) = 0.02Ω(1.45A) = 29mV
ESR L P-P
ORIPPLE
3890fb
27
Linear Systems [ Linear Systems ]
