Datasheet

LTM4616
15
4616ff
For more information www.linear.com/LTM4616
applications inForMation
Spread Spectrum Operation
Switching regulators can be particularly troublesome
where electromagnetic interference (EMI) is concerned.
Switching regulators operate on a cycle-by-cycle basis to
transfer power to an output. In most cases, the frequency
of operation is fixed based on the output load. This method
of conversion creates large components of noise at the
frequency of operation (fundamental) and multiples of the
operating frequency (harmonics).
To reduce this noise, the LTM4616 can run in spread
spectrum operation by tying the CLKIN pin to SV
IN
.
In spread spectrum operation, the LTM4616’s internal
oscillator is
designed to produce a clock pulse whose
period
is random on a cycle-by-cycle basis but fixed
between 70% and 130% of the nominal frequency. This
has the benefit of spreading the switching noise over a
range of frequencies, thus significantly reducing the peak
noise. Spread spectrum operation is disabled if CLKIN is
tied to ground or if it’s driven by an external frequency
synchronization signal. A capacitor value of 0.01µF to
0.1µF be placed from the PLLLPF pin to ground to control
the slew rate of the spread spectrum frequency change.
To ensure proper start-up, add a control ramp
on the
TRACK
pin with a resistor, R
SR
, from TRACK to SV
IN
and
a capacitor, C
SR
, from TRACK to ground:
R
SR
≥
1
– In 1–
0.592
V
IN






• 500 • C
SR








Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK pin. The output can be tracked up and
down with another regulator. The master regulator’s output
is divided down with an external resistor divider that is the
same as the slave regulator’s feedback divider to implement
DUTY FACTOR (V
O
/V
IN
)
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
0.85 0.9
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
4616 F04
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
Figure 4. Normalized Input RMS Ripple Current vs Duty Factor for One to Six Channels (Phases)