LTC3856 Datasheet, PDF (33/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C_OUT , causing a rapid drop in V_OUT . No regulator can
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C_LOAD to C_OUT is greater than 1:50, the switch rise time
should be controlled so that the load rise time is limited
to approximately 25 • C_LOAD. Thus a 10µF capacitor would
require a 250µs rise time, limiting the charging current
to about 200mA.
Design Example (Using Two Phases)
As a design example, assume:
V_IN = 5V (nominal)
V_IN = 5.5V (max),
V_OUT = 1.8V,
I_MAX = 20A
T_A = 70°C
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. Use a 71.5k resistor
from FREQ to ground to set the switching frequency at
about 300kHz. The minimum inductance for 30% ripple
current is:
L
≥
V_OUT
f(∆I)


1−
V_OUT
V_IN


≥
1.8V
(300kHz)(30%)(10A)


1−
1.8V
5.5V


≥ 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 11A. The minimum on-time oc-
curs at maximum V_IN:
( )( ) ^tON(MIN)
=
V_OUT
V_INf
=
1.8V
5.5V 300kHz
= 1.1µs
With the ILIM pin tied to ground, the R_SENSE resistors
value can be calculated by using the minimum current
sense voltage specification with some accommodation
for tolerances:
R_SENSE
=
25mV
11A
≈
0.002Ω
Choosing 1% resistors: R1 = 10k and R2 = 20k yields an
output voltage of 1.80V.
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:
( ) ( )( ) ^P_MAIN
=
1.8V
5.5V
10
2 _{1+}
0.005
110°C − 25°C ^
•
0
.01
3Ω
+
(⁵
.5V₎²


1
0A
2


(^2Ω)(³⁰
0pF
)
( ) ^

5V
1
− 2.6V
+
1
2.6V 
•
300kHz
= 0.606 + 0.022 = 0.628W
The worst-case power dissipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction
temperature rise is:
P_SYNC
=
5.5V − 1.8V
5.5V
(^10A)²
(^1.25)(^0.013Ω)
= 1.09W
3856f
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