SC415 Semtech Corporation, SC415 Datasheet - Page 16

• Manufacturer Part Number: SC415
• Description: Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

w w w . D a t a S h e e t 4 U . c o m
Applications Information (continued)
During the DH on-time, voltage across the inductor is
(VIN - VOUT). To determine the inductance, the ripple
current must be defi ned. Smaller ripple current will
give smaller output ripple voltage but will require larger
inductors. The ripple current will also set the boundary
for PSAVE operation. The switcher will typically enter
PSAVE operation when the load current decreases to ½ of
the ripple current; (i.e. if ripple current is 4A then PSAVE
operation will typically start for loads less than 2A. If ripple
current is set at 40% of maximum load current, then PSAVE
will occur for loads less than 20% of maximum).
The equation for inductance is:
Use the maximum value for VIN, and for TON use the value
associated with maximum VIN, and that side’s TON using
the RTON value selected. For selecting the inductor, we
start with the highest VOUT setting and a maximum ripple
current of 4A.
We will use 1.5μH which will slightly increase the maximum
IRIPPLE to 4.2A.
Note: the inductor must be rated for the maximum DC
load current plus ½ of the ripple current.
The minimum ripple current is also calculated. This occurs
when VIN is at the minimum value of 10V.
L = (VIN - VOUT) × TON / I
TON1 = 343 nsec at 20VIN, 1.8VOUT
L = (20 - 1.8) × 343 nsec / 4A = 1.56μH
TON
TON
I
I
RIPPLE
RIPPLE_VINMIN
VINMIN
VINMIN
= (VIN - VOUT) × TON / L
= 3.3 × (RTON+37) × (VOUT/VIN) + 35
= 651nsec
= (10 – 1.8) × 651 nsec / 1.5μH
= 3.55A
RIPPLE
Capacitor Selection
The output capacitors are chosen based on required ESR
and capacitance. The ESR requirement is driven by the
output ripple requirement and the DC tolerance. The
output voltage has a DC value that is equal to the valley
of the output ripple, plus ½ of the peak-to-peak ripple.
Changing the ripple voltage will lead to a change in DC
output voltage.
The design goal is +/-4% output regulation. The internal
750mV reference tolerance is 1%, and assuming 1%
tolerance for the FB resistor divider, this allows 2%
tolerance due to VOUT ripple. Since this 2% error comes
from ½ of the ripple voltage, the allowable ripple is 4%, or
72mV for a 1.8V output. Although this is acceptable from
a regulation standpoint, 72mV ripple is high for a 1.8V
output and therefore more realistic ripple value of 36mV
will be used (2% of VOUT).
The maximum ripple current of 4.2A creates a ripple
voltage across the ESR. The maximum ESR value allowed
would create 36mV ripple:
While the ESR is chosen to meet ripple requirements,
the output capacitance (μF) is typically chosen based on
transient requirements. A worst-case load release, from
maximum load to no load at the exact moment when
inductor current is at the peak, defi nes the required
capacitance. If the load release is instantaneous (load
changes from maximum to zero in a very small time), the
output capacitor must absorb all the inductor’s stored
energy. This will cause a peak voltage on the capacitor
according to the equation:
ESRMAX = V
ESRMAX = 8.6 mΩ
C
OUTMIN
= L × (IOUT + 1/2 × I
_______________________
RIPPLE
/I
(V
RIPPLEMAX
PEAK
2
- V
= 36mV / 4.2A
OUT
RIPPLEMAX
2
)
)
2
SC415
16