Datasheet

LTM4611
14
4611fb
For more information www.linear.com/LTM4611
PLL, Frequency Adjustment and Synchronization
The default switching frequency of the LTM4611–with
PLLFLTR/f
SET
left floating–is 500kHz, nominally. The
PLLFLTR/f
SET
pin is driven to 1.23V through a high im-
pedance (>350kΩ) network. If desired, a resistor (R
fSET
)
can be connected from the PLLFLTR/f
SET
pin to INTV
CC
to increase the switching frequency to as high as 780kHz,
nominally. Alternatively, R
fSET
can instead be connected
from PLLFLTR/f
SET
to signal ground (SGND) to decrease
the switching frequency to as low as the minimum specified
value of 330kHz, nominally. In practical terms, however, be
advised that switching frequencies below 400kHz may be
of limited benefit due to the high inductor ripple currents
associated with that operating condition. See Figure 3.
There exists a fundamental trade-off between switch
mode DC/DC power conversion efficiency and switching
frequency: higher operating module switching frequency
enables the smallest overall solution size (minimized
output capacitance) for a given application; whereas,
lower switching frequency enables the highest efficiency
for a given application (to the extent that peak and RMS
inductor currents can be supported), but requires more
output capacitance to maintain comparable output voltage
ripple and noise characteristics.
The LTM4611 can be synchronized from 360kHz to 710kHz
with an input clock that has a high level above 2V and a
low level below 0.6V. Again in practical terms, be advised
that switching frequencies below 400kHz may be of limited
benefit due to the high inductor ripple currents associated
with that operating condition. See the Typical Applications
section for synchronization examples. The LTM4611 mini-
mum on-time is limited to 90ns. Guardband the on-time
to 130ns. The on-time can be calculated as:
t
ON(MIN)
=
1
FREQ
•
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Output Voltage Tracking and Soft-Start Functions
Output voltage tracking can be programmed externally
using the TRACK/SS 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 LTM4611 uses an
accurate 60.4k resistor internally for the top feedback resis-
tor. Figure 4 shows an example of coincident tracking.
V
OUT _SLAVE
= 1+
60.4k
R
FB2
⎛
⎝
⎜
⎞
⎠
⎟
• V
TRACK
V
TRACK
is the track ramp applied to the slave’s track pin.
V
TRACK
has a control range of 0V to 0.8V, or the internal
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. Volt-
age tracking is disabled when V
TRACK
is more than 0.8V.
R
TA
in Figure 4 will be equal to the R
FB2
for coincident
tracking.
The TRACK/SS pin of the master can be controlled by an
external ramp or the soft-start function of that regulator
can be used to develop that master ramp. The LTM4611 can
be used as a master by setting the ramp rate on its track
pin using a soft-start capacitor. A 1.4µA current source
is used to charge the soft-start capacitor. The following
equation can be used:
t
SOFTSTART
= 0.8V •
C
SS
1.4µA
⎛
⎝
⎜
⎞
⎠
⎟
applicaTions inForMaTion
VALUE OF RESISTOR ON PLLFLTR/f
SET
PIN (MΩ)
0.1
0.96
PLLFLTR/f
SET
PIN VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
1.23
1.50
1 10 100
4611 F03
0.68
0.41
2.05
1.78
0.82
1.09
1.37
0.55
1.91
1.64
400
500
600
300
200
800
700
350
450
550
250
750
650
SWITCHING FREQUENCY
PLLFLTR/f
SET
VOLTAGE
R
fSET
CONNECTED
TO INTV
CC
R
fSET
CONNECTED
TO GND
R
fSET
NOT USED
Figure 3. Relationship Between Oscillator Frequency,
PLLFLTR/f
SET
Voltage, and External R
ISET
Value and
Connection