LTC3819 Datasheet, PDF (24/32 Pages) Linear Technology – 2-Phase, High Efficiency, Step-Down Controller for Sun Server CPUs

LTC3819
APPLICATIO S I FOR ATIO
Design Example
Asadesignexample,assumeV_IN =5V(nominal),V_IN = 5.5V
(max), V_OUT = 1.2V, I_MAX = 20A, T_A = 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 INTV_CC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
( ) ^{L ≥}
V_OUT
f ∆L
⎛
⎝⎜^1−
V_OUT
V_IN
⎞
⎠⎟
( )( )( ) ^≥
1.2V
300kHz 30%
10A
⎛
⎝⎜^1−
1.2V
5.5V
⎞
⎠⎟
≥ 1.04µH
A 1µH inductor will produce 31% 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 V_IN:
( )( ) ^{( ) tON MIN}
= ^VOUT =
V_IN f
1.2V
5.5V 300kHz
= 0.73µs
The R_SENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
R_SENSE
=
50mV
11.5A
≈
0.004Ω
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R_DS(ON) = 0.013Ω, C_RSS = 300pF. At maximum input
voltage with T_J (estimated) = 110°C at an elevated ambient
temperature:
( ) _[ ( )( )_] ^PMAIN
=
1.2V
5.5V
2
10 1+
0.005
110°C − 25°C
( ) ( )( ) ²
0.013Ω + 1.7 5.5V 10A 300pF
( ) ^{300kHz = 0.45W}
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 − 1.2V ₂
5.5V
10A
2
1.48
0.013Ω
= 1.5W
A short-circuit to ground will result in a folded back current
of about:
( ) _ISC
=
25mV
0.004Ω
+
1
⎡ _200ns
⎢
5.5V
2
⎢
⎣
1µH
⎤
⎥ _{= 6.8A}
⎥
⎦
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 − 1.2V
5.5V
2
6.8A 1.48
0.013Ω
= 696mW
which is less than normal, full-load conditions. Inciden-
tally, since the load no longer dissipates power in the
shorted condition, total system power dissipation is de-
creased by over 99%.
The duty factor for this application is:
DF = ^VO = ^1.2V = 0.24
V_IN 5V
3819f
24