SC415 Datasheet, PDF (15/30 Pages) Semtech Corporation – Dual Synchronous Buck Controller

SC415
Applications Information (continued)
Design example:
VIN = 10V min, 20V max
VOUT1 = 1.8V +/- 4%
Load = 10A maximum
Side1 will be used as an example.
Figure 8
Design Procedure
Prior to designing a switch mode supply, the input
voltage, load current, and switching frequency must
be specified. For notebook systems the maximum
input voltage (VINMAX) is determined by the highest
AC adaptor voltage, and the minimum input voltage
(VINMIN) is determined by the lowest battery voltage
after accounting for voltage drops due to connectors,
fuses and battery selector switches.
In general, four parameters are needed to define the
design:
1. Nominal output voltage (VOUT)
2. Static or DC output tolerance
3. Transient response
4. Maximum load current (IOUT)
There are two values of load current to consider: continuous
load current and peak load current. Continuous load
current is concerned with thermal stresses which drive
the selection of input capacitors, MOSFETs and diodes.
Peak load current determines instantaneous component
stresses and filtering requirements such as inductor
saturation, output capacitors and design of the current
limit circuit.
Inductor Selection
Low inductor values result in smaller size but create
higher ripple current. Higher inductor values will reduce
the ripple current but are larger and more costly. Because
wire resistance varies widely for different inductors and
because magnetic core losses vary widely with operating
conditions, it is often difficult to choose which inductor
will optimize efficiency. The general rule is that higher
inductor values have better efficiency at light loads due
to lower core losses and lower peak currents, but at high
load the smaller inductors are better because of lower
resistance. The inductor selection is generally based on
the ripple current which is typically set between 20% to
50% of the maximum load current. Cost, size, output ripple
and efficiency all play a part in the selection process.
The first step is to select the switching frequency. In this
case VOUT1 will be used at a nominal 270kHz.
For 15V input and 1.8V output, the typical on-time is:
TONtyp = VOUT/VIN/Freq
TONtyp = 444nsec.
The timing resistor RTON must be selected to provide
TONtyp:
RTON = (TONtyp – 35) × (VIN/(3.3 × VOUT) – 37
RTON = 976k.
We will use RTON = 1Meg.
Note that side2 will run typically 20% faster than side1, in
this case 320kHz.
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