LTC1929C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1929C' PDF : 28 Pages View PDF
LTC1929/LTC1929-PG
APPLICATIO S I FOR ATIO
Design Example
Asadesignexample,assumeVIN =5V(nominal),VIN␣ =␣ 5.5V
(max), VOUT = 1.8V, IMAX = 20A, TA = 70°C and f␣ =␣ 300kHz.
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. Tie the FREQSET pin
to the INTVCC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
( ) L
≥
VOUT
f ∆I
1−
VOUT
VIN
( )( )( ) ≥
1.8V
300kHz 30%
10A
1−
1.8V
5.5V
≥ 1.35µH
A 1.5µH inductor will produce 27% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 11.4A. The minimum on-
time occurs at maximum VIN:
tON(MIN)
=
VOUT
VINf
=
1.8V
(5.5V)(300kHz)
=
1.1µs
The RSENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
RSENSE
=
50mV
11.4A
≈
0.004Ω
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:
( ) [ ( )( )] PMAIN
=
1.8V
5.5V
2
10 1+
0.005
110°C − 25°C
( ) ( )( ) 2
0.013Ω + 1.7 5.5V 10A 300pF
( ) 300kHz = 0.65W
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:
( ) ( )( ) PSYNC
=
5.5V − 1.8V
5.5V
2
10A 1.48
0.013Ω
= 1.29W
A short-circuit to ground will result in a folded back current
of about:
( ) ISC
=
25mV
+
1
200ns
5.5V
0.004Ω 2 1.5µH
= 6.6A
The worst-case power disipated by the synchronous
MOSFET under short-circuit conditions at elevated ambi-
ent temperature and estimated 50°C junction temperature
rise is:
( ) ( )( ) PSYNC
=
5.5V − 1.8V
5.5V
2
6.6A 1.48
0.013Ω
= 564mW
which is less than half of the normal, full-load dissipation.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
The duty factor for this application is:
DF = VO = 1.8V = 0.36
VIN 5V
Using Figure 4, the RMS ripple current will be:
IINRMS = (20A)(0.23) = 4.6ARMS
An input capacitor(s) with a 4.6ARMS ripple current rating
is required.
The output capacitor ripple current is calculated by using
the inductor ripple already calculated for each inductor
and multiplying by the factor obtained from Figure␣ 3
along with the calculated duty factor. The output ripple in
22
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