LTC3856 Datasheet, PDF (34/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
A short-circuit to ground will result in a folded back
current of:
25mV
( ) _ISC
=
3
0.002Ω
−
1 ^{ 90ns 5.5V}
2


2µH



=
4.04A
The worst-case power dissipated by the synchronous
MOSFET under short-circuit conditions at elevated ambi-
ent temperature and estimated 50°C junction temperature
rise is:
( ) ( )( ) ^P_SYNC
=
5.5V − 1.8V
5.5V
4.04A
2
1.25
0.013Ω
= 0.18W
which is much less than normal, full-load conditions.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
The duty cycles when the peak RMS input current occurs is
at D = 0.25 and D = 0.75 according to Figure 10. Calculate
the worst-case required RMS input current rating at the
input voltage, which is 5.5V, that provides a duty cycle
nearest to the peak.
From Figure 10, C_IN will require an RMS current rating of:
( )( ) ^CIN ^{required I}RMS ^{= 20A 0.23}
= 4.6A_RMS
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 8 along with
the calculated duty factor. The output ripple in continuous
mode will be highest at the maximum input voltage. From
Figure 8, the maximum output current ripple is:
∆I_COUT
=
V_OUT
fL
(0.34)
∆I_COUTMAX
=
1.8(0.34)
(300kHz)(2µH)
=
1A
Note that the PolyPhase technique will have its maximum
benefit for input and output ripple currents when the number
of phases times the output voltage is approximately equal
to or greater than the input voltage.
3856f
34