LTC1629/LTC1629-PG
U
W U U
APPLICATIO S I FOR ATIO
Although the LTC1629 has a maximum input voltage of
36V, most applications will be limited to 30V by the
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the sense pin input current for
both channels.
MOSFET BVDSS
.
50A I RATING
PK
V
IN
12V
20k
2
VOUT
2.4V − VOUT
LTC1629
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
R1MIN
=
(
)
1.8V
2.4V − 1.8V
= 10k
= 30k
1629 F08
Figure 8. Automotive Application Protection
Choosing 1% resistors: R1=13.2k and R2=16.5k yields an
output voltage of 1.80V and satisfies the above condition.
Design Example (Using Two Phases)
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
RDS(ON) = 0.013Ω, CRSS = 300pF. At maximum input
voltage with Tj (estimated) = 110°C at an elevated ambient
temperature:
Asadesignexample,assumeVIN=5V(nominal),VIN = 5.5V
(max), VOUT =1.8V, IMAX =20A, TA =70°Candf = 300kHz.
Theinductancevalueischosenfirstbasedona30%ripple
current assumption. The highest value of ripple current
occursatthemaximuminputvoltage.TietheFREQSETpin
to the INTVCC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
2
1.8V
5.5V
PMAIN
=
10 1+ 0.005 110°C − 25°C
( )
(
)(
)
[
]
2
) (
0.013Ω + 1.7 5.5V 10A 300pF
(
)(
)
VOUT
f ∆I
( )
VOUT
V
IN
L ≥
1−
310kHz = 0.61W
(
)
1.8V
1.8V
5.5V
≥
1−
The worst-case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is:
300kHz 30% 10A
(
)( )(
)
≥ 1.35µH
A 2µH inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on-time
occurs at maximum VIN:
2
) (
5.5V − 1.8V
5.5V
= 1.29W
PSYNC
=
10A 1.48 0.013Ω
(
)(
)
VOUT
V f
IN
1.8V
Ashort-circuittogroundwillresultinafoldedbackcurrent
of:
tON MIN
=
=
= 1.1µs
(
)
5.5V 300kHz
(
)(
)
The RSENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
200ns 5.5V
(
)
25mV
0.005Ω
1
2
ISC
=
+
= 5.28A
2µH
60mV
11.5A
RSENSE
=
≈ 0.005Ω
22
Linear [ Linear ]
