LTM4616

14

4616fa

APPLICATIONS INFORMATION

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.

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

coincident tracking. The LTM4616 uses an accurate 10k

resistor internally for the top feedback resistor. Figure 5

shows an example of coincident tracking:

Slave

= 1+

10k

reference voltage. When the master’s output is divided

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

its final value from the slave’s regulation point. Voltage

The track pin of the master can be controlled by an external

RC ramp time can be programmed using equation:

t

= –ln 1–

0.596V

Ratiometric tracking can be achieved by a few simple

calculations and the slew rate value applied to the master’s

track pin. As mentioned above, the TRACK pin has a control

range from 0V to 0.596V. The master’s TRACK pin slew

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

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)